arXiv daily: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Authors:Mohammad Reza Safari, Frank Matthes, Claus M. Schneider, Karl-Heinz Ernst, Daniel E. Bürgler

Abstract: The interplay between chirality and magnetism has been a source of fascination among scientists for over a century. In recent years, chirality-induced spin selectivity (CISS) has attracted renewed interest. It has been observed that electron transport through layers of homochiral molecules leads to a significant spin polarization of several tens of percent. Despite the abundant experimental evidence gathered through mesoscopic transport measurements, the exact mechanism behind CISS remains elusive. In this study, we report spin-selective electron transport through single helical aromatic hydrocarbons that were sublimed in vacuo onto ferromagnetic cobalt surfaces and examined with spin-polarized scanning tunneling microscopy (SP-STM) at a temperature of 5 K. Direct comparison of two enantiomers under otherwise identical conditions revealed magnetochiral conductance asymmetries of up to 50% when either the molecular handedness was exchanged or the magnetization direction of the STM tip or Co substrate was reversed. Importantly, our results rule out electron-phonon coupling and ensemble effects as primary mechanisms responsible for CISS.

Authors:Rachid El Aitouni, Miloud Mekkaoui, Ahmed Jellal

Abstract: In this work, we will study the transmission probability of Dirac fermions through a double laser barrier. As part of the Floquet approximation, we will determine the spinors in the five regions. Due to the continuity of the wave function at the barrier edges, we find eight equations, each with infinity modes. To simplify, we use the matrix formalism and limit our study to the first three bands, the central band, and the first two side bands. From the continuity equation and the spinors in the five regions, we will determine the current density in each region, which makes it possible to determine the expression of the transmission probability corresponding to each energy band. The time-dependent laser fields generate several transmission modes, which give two transmission processes: transmission with zero photon exchange corresponds to the central band $\varepsilon$, and transmission with emission or absorption of photons corresponds to the first two sidebands $\varepsilon\pm\varpi$. One of the two modes can be suppressed by varying the distance between the two barriers or the barrier width. The transmission is not permitted if the incoming energy is below an energy threshold $\varepsilon>k_y+2\varpi$. Increasing the intensity of the laser fields makes it possible to modify the sharpness and amplitude of the transmission.

Authors:S. D. Balika

Abstract: We analyze the possibility of evaluation of the thickness and refractive index of the stagnant layer in concentrated suspensions of nanoparticles through the transport characteristics of scattered light photons. The analysis is based on a physically-transparent generalization of the concept of the single scattering intensity off systems in which the number of particles within regions with linear sizes of order of the wavelength in the medium greatly exceeds unity. This generalization is carried out within the notion of compact groups of particles, makes it possible to go beyond the traditional Born approximation, and take into account many-particle effects contributed from those ranges of integration variables in the terms of the iteration series for the scattered field where the internal propagators have delta-function-type behavior. The evaluation of the photon transport characteristics becomes possible without a detailed modeling of many-particle scattering and correlation processes in the system. The photon mean-free-path length, $l$, is investigated as a function of the stagnant refractive index and that of the layer thickness to show a noticeable effect of both parameters on it. As the layer refractive index is increased at a fixed layer thickness, $l$ decreases because the suspension optical density increases. As a function of the layer thickness, $l$ reveals different types of behavior, depending on the relation between refractive indices of the stagnant layer and base liquid. If the former is smaller than the latter, this behavior is increasing; in the opposite case, it is decreasing. An experimentally observed increase of $l$ with the particle concentration is explained as a manifestation of higher correlation effects. Our theory reveals that the stagnant layer make the situation more complicated, for both factors may either enhance or diminish each other.

Authors:Chang-An Li, Junsong Sun, Song-Bo Zhang, Huaiming Guo, Björn Trauzettel

Abstract: The Benalcazar-Bernevig-Hughes (BBH) quadrupole insulator model is a cornerstone model for higher-order topological phases. It requires \pi flux threading through each plaquette of the two-dimensional Su-Schrieffer-Heeger model. Recent studies show that particular \pi-flux patterns can modify the fundamental Brillouin zone from the shape of a torus to a Klein-bottle with emerging topological phases. By designing different \pi-flux patterns, we propose two types of Klein-bottle BBH models. These models show rich topological phases including Klein-bottle quadrupole insulators and Dirac semimetals. The phase with nontrivial Klein-bottle topology shows twined edge modes at open boundaries. These edge modes can further support second-order topology yielding a quadrupole insulator. Remarkably, both models are robust against flux perturbations. Moreover, we show that different \pi-flux patterns dramatically affect the phase diagram of the Klein-bottle BBH models. Going beyond the original BBH model, Dirac semimetal phases emerge in Klein-bottle BBH models featured by the coexistence of twined edge modes and bulk Dirac points.

Authors:Zhuocheng Lu, Ji Feng

Abstract: A Weyl node is characterized by its chirality and tilt. We develop a theory of how $n$th order nonlinear optical conductivity behaves under transformations of anisotropic tensor and tilt, which clarify how chirality-dependent and -independent parts of optical conductivity transform under the reversal of tilt and chirality. Built on this theory, we propose ferromagnetic MnBi2Te4 as a magnetoelectrically regulated, terahertz optical device, by magnetoelectrically switching the chirality-dependent and -independent dc photocurrents. These results are useful for creating nonlinear optical devices based on topological Weyl semimetals.

Authors:Ignacio A. Martinez, Artyom Petrosyan, Sergio Ciliberto

Abstract: Critical Casimir interactions represent a perfect example of bath-induced forces at mesoscales. These forces may have a relevant role in the living systems as well as a role in the design of nanomachines fueled by environmental fluctuations. Since the thermal fluctuations are enhanced in the vicinity of a demixing point of a second-order phase transition, we can modulate the magnitude and range of these Casimir-like forces by slight changes in the temperature. Here, we consider two optical trapped colloidal beads inside a binary mixture. The Casimir interaction is controlled by warming the mixture by laser-induced heating, whose local application ensures high reproducibility. Once this two-particle system is warmed, the critical behavior of different observables allows the system to become its self-thermometer. We use this experimental scheme for analyzing the energetics of a critical colloidal system under a non-equilibrium-driven protocol. We quantify how the injected work can be dissipated to the environment as heat or stored as free energy. Indeed, our system allows us to use the fluctuation theorems framework for analyzing the performance of this critically driven toy model. Our work paves the way for future experimental studies on the non-equilibrium features of bath-induced forces and the design of critically driven nanosystems.

Authors:Mu-Kun Lee, Masahito Mochizuki

Abstract: By performing numerical simulations for the handwritten digit recognition task, we demonstrate that a magnetic skyrmion lattice confined in a thin-plate magnet possesses high capability of reservoir computing. We obtain a high recognition rate of more than 88%, higher by about 10% than a baseline taken as the echo state network model. We find that this excellent performance arises from enhanced nonlinearity in the transformation which maps the input data onto an information space with higher dimensions, carried by interferences of spin waves in the skyrmion lattice. Because the skyrmions require only application of static magnetic field instead of nanofabrication for their creation in contrast to other spintronics reservoirs, our result consolidates the high potential of skyrmions for application to reservoir computing devices.

Authors:Konrad Merkel, Frank Ortmann

Abstract: We present a theoretical method for calculating optical absorption spectra based on maximally localized Wannier functions, which is suitable for large periodic systems. For this purpose, we calculate the exciton Hamiltonian, which determines the Bethe-Salpeter equation for the macroscopic polarization function and optical absorption characteristics. The Wannier functions are specific to each material and provide a minimal and therefore computationally convenient basis. Furthermore, their strong localization greatly improves the computational performance in two ways: first, the resulting Hamiltonian becomes very sparse and, second, the electron-hole interaction terms can be evaluated efficiently in real space, where large electron-hole distances are handled by a multipole expansion. For the calculation of optical spectra we employ the sparse exciton Hamiltonian in a time-domain approach, which scales linearly with system size. We demonstrate the method for bulk silicon - one of the most frequently studied benchmark systems - and envision calculating optical properties of systems with much larger and more complex unit cells, which are presently computationally prohibitive.

Authors:H. Ness, M. van Schilfgaarde

Abstract: Applying the Bogoliubov-de Gennes equations with density-functional theory, it is possible to formulate first-principles description of current-phase relationships in superconducting/normal (magnetic)/superconducting trilayers. Such structures are the basis for the superconducting analog of Magnetoresistive random access memory devices (JMRAM). In a recent paper [1] we presented results from the first attempt to formulate such a theory, applied to the Nb/Ni/Nb trilayers. In the present work we provide computational details, explaining how to construct key ingredient (scattering matrices $S_N$) in a framework of linear muffin-tin orbitals (LMTO).

Authors:Rui Chen, Z. Z. Du, Hai-Peng Sun, Hai-Zhou Lu, X. C. Xie

Abstract: The nonlinear Hall effect has recently attracted significant interest due to its potentials as a promising spectral tool and device applications. A theory of the nonlinear Hall effect on a disordered lattice is a crucial step towards explorations in realistic devices, but has not been addressed. We study the nonlinear Hall response on a lattice, which allows us to introduce disorder numerically and reveal a mechanism that was not discovered in the previous momentum-space theories. In the mechanism, disorder induces an increasing fluctuation of the nonlinear Hall conductance as the Fermi energy moves from the band edges to higher energies. This fluctuation is a surprise, because it is opposite to the disorder-free distribution of the Berry curvature. More importantly, the fluctuation may explain those unexpected observations in the recent experiments. We also discover an "Anderson localization" of the nonlinear Hall effect. This work shows an emergent territory of the nonlinear Hall effect yet to be explored.

Authors:Lakpa Tamang, Sonu Verma, Tutul Biswas

Abstract: The $\alpha$-$T_3$ system undergoes a topological phase transition(TPT) between two distinct quantum spin-Hall phases across $\alpha=0.5$ when the spin-orbit interaction of Kane-Mele type is taken into consideration. As a hallmark of such a TPT, we find that the Berry curvature and the orbital magnetic moment change their respective signs across the TPT. The trails of the TPT found in another physical observable e.g. the orbital magnetization(OM) are understood in terms of valley and spin physics. The valley-resolved OM(VROM) and the spin-resolved OM(SROM) exhibit interesting characteristics related to the valley and the spin Chern number when the chemical potential is tuned in the forbidden gap(s) of the energy spectrum. In particular, we find that the slope of the VROM vs the chemical potential in the forbidden gap changes its sign abruptly across the TPT which is also consistent with the corresponding change in the valley Chern number. Moreover, the slope of the SROM demonstrates a sudden jump by one unit of $e/h$, (where $e$ is the electronic charge and $h$ is the Planck's constant), across the TPT which is also in agreement with the corresponding change in the spin Chern number.

Authors:Yuhao Wang, Fangting Chen, Wenyu Song, Zuhan Geng, Zehao Yu, Lining Yang, Yichun Gao, Ruidong Li, Shuai Yang, Wentao Miao, Wei Xu, Zhaoyu Wang, Zezhou Xia, Huading Song, Xiao Feng, Yunyi Zang, Lin Li, Runan Shang, Qi-Kun Xue, Ke He, Hao Zhang

Abstract: Disorder is the primary obstacle in current Majorana nanowire experiments. Reducing disorder or achieving ballistic transport is thus of paramount importance. In clean and ballistic nanowire devices, quantized conductance is expected with plateau quality serving as a benchmark for disorder assessment. Here, we introduce ballistic PbTe nanowire devices grown using the selective-area-growth (SAG) technique. Quantized conductance plateaus in units of $2e^2/h$ are observed at zero magnetic field. This observation represents an advancement in diminishing disorder within SAG nanowires, as none of the previously studied SAG nanowires (InSb or InAs) exhibit zero-field ballistic transport. Notably, the plateau values indicate that the ubiquitous valley degeneracy in PbTe is lifted in nanowire devices. This degeneracy lifting addresses an additional concern in the pursuit of Majorana realization. Moreover, these ballistic PbTe nanowires may enable the search for clean signatures of the spin-orbit helical gap in future devices.

Authors:Therese Frostad, Philipp Pirro, Alexander A. Serga, Burkard Hillebrands, Arne Brataas, Alireza Qaiumzadeh

Abstract: We theoretically demonstrate that adding an easy-axis magnetic anisotropy facilitates magnon condensation in thin yttrium iron garnet (YIG) films. Dipolar interactions in a quasi-equilibrium state stabilize room-temperature magnon condensation in YIG. Even though the out-of-plane easy-axis anisotropy generally competes with the dipolar interactions, we show that adding such magnetic anisotropy may assist the generation of the magnon condensation electrically, via the spin transfer torque mechanism. We use analytical calculations and micromagnetic simulations to illustrate this effect. Our results may explain the recent experiment on Bi-doped YIG and open a new pathway toward application of current-driven magnon condensation in quantum spintronics.

Authors:Jiong-Hao Wang, Yong Xu

Abstract: Weyl semimetals have been theoretically predicted to become topological metals with anomalous Hall conductivity in amorphous systems. However, measuring the anomalous Hall conductivity in realistic materials, particularly those with multiple pairs of Weyl points, is a significant challenge. If a system respects time-reversal symmetry, then the anomalous Hall conductivity even vanishes. As such, it remains an open question how to probe the Weyl band like topology in amorphous materials. Here, we theoretically demonstrate that, under magnetic fields, a topological metal slab in amorphous systems exhibits three-dimensional quantum Hall effect, even in time-reversal invariant systems, thereby providing a feasible approach to exploring Weyl band like topology in amorphous materials. We unveil the topological origin of the quantized Hall conductance by calculating the Bott index. The index is carried by broadened Landau levels with bulk states spatially localized except at critical transition energies. The topological property also results in edge states localized at distinct hinges on two opposite surfaces.

Authors:Adrian Pena

Abstract: We theoretically analyze the scattering process of an electron on a graphene quantum dot (GQD) exposed to an external light irradiation. We prove that for suitable choices of the light polarization state, there emerge scattering resonances, characterized by electron trapping effects inside the GQD.

Authors:Adriano Di Pietro, Felipe García Sánchez, Gianfranco Durin

Abstract: The influence of different Dzyaloshinskii-Moriya interaction (DMI) tensor components on the static and dynamic properties of domain walls (DWs) in magnetic nanowires is investigated using one dimensional collective coordinates models and micromagnetic simulations. It is shown how the different contributions of the DMI can be compactly treated by separating the symmetric traceless, antisymmetric and diagonal components of the DMI tensor. First, we investigate the effect of all different DMI components on the static DW tilting in the presence and absence of in plane (IP) fields. We discuss the possibilities and limitations of this measurement approach for arbitrary DMI tensors. Secondly, the interplay of different DMI tensor components and their effect on the field driven dynamics of the DWs are studied and reveal a non-trivial effect of the Walker breakdown field of the material. It is shown how DMI tensors combining diagonal and off-diagonal elements can lead to a non-linear enhancement of the Walker field, in contrast with the linear enhancement obtainable in the usual cases (interface DMI or bulk DMI).

Authors:Chang Niu, Shouyuan Huang, Neil Ghosh, Pukun Tan, Mingyi Wang, Wenzhuo Wu, Xianfan Xu, Peide D. Ye

Abstract: Chirality arises from the asymmetry of matters, where two counterparts are the mirror image of each other. The interaction between circular-polarization light and quantum materials is enhanced in chiral space groups due to the structural chirality. Tellurium (Te) possesses the simplest chiral crystal structure, with Te atoms covalently bonded into a spiral atomic chain (left- or right-handed) with a periodicity of three. Here, we investigate the tunable circular photo-electric responses in 2D Te field-effect transistor with different chirality, including the longitudinal circular photogalvanic effect induced by the radial spin texture (electron-spin polarization parallel to the electron momentum direction) and the circular photovoltaic induced by the chiral crystal structure (helical Te atomic chains). Our work demonstrates the controllable manipulation of the chirality degree of freedom in materials.

Authors:Cynthia I. Osuala, Zitao Tang, Stefan Strauf, Eui-Hyeok Yang, Chunlei Qu

Abstract: We investigate the quantum transport dynamics of electrons in a multi-path Aharonov-Bohm interferometer comprising several parallel graphene nanoribbons. At low magnetic field strengths, the conductance displays a complex oscillatory behavior stemming from the interference of electron wave functions from different paths, reminiscent of the diffraction grating in optics. With increasing magnetic field strength, certain nanoribbons experience transport blockade, leading to conventional Aharonov-Bohm oscillations arising from two-path interference. We also discuss the impact of edge effects and the influence of finite temperature. Our findings offer valuable insights for experimental investigations of quantum transport in multi-path devices and their potential application for interferometry and quantum sensing.

Authors:I. A. Ado, M. Titov, Rembert A. Duine, Arne Brataas

Abstract: We revise the Kubo formula for the electric dc conductivity in the presence of spin-orbit coupling (SOC). We discover that each velocity operator that enters this formula differs from $\partial H/\partial \boldsymbol p$, where $H$ is the Hamiltonian and $\boldsymbol p$ is the canonical momentum. Moreover, we find an additional contribution to the Hall dc conductivity from noncommuting coordinates that is missing in the conventional Kubo-Streda formula. We show that the widely used Rashba model does in fact provide a finite anomalous Hall dc conductivity in the metallic regime (in the noncrossing approximation). In addition to the Kubo formula, the Berry-phase theory of the anomalous Hall effect should also be revised for systems where the velocity operator differs from $\partial H/\partial \boldsymbol p$. While we focus on the Hall response of the charge current to the electric field, linear response theories of other SOC-related effects should be modified similarly.

Authors:Yu-Ping Lin, Giandomenico Palumbo

Abstract: Topological semimetals, such as the Weyl and Dirac semimetals, represent one of the most active research fields in modern condensed matter physics. The peculiar physical behavior of these systems mainly originates from their underlying symmetries, emergent relativistic dispersion, and band topology. In this paper, we present a novel class of $\textit{geometric semimetals}$ in three dimensions. These semimetals are protected by the generalized chiral and rotation symmetries, but are topologically trivial. Nevertheless, we show that their band geometry is nontrivial, as evidenced by the corresponding quantum metric trace that can give rise to a quantized geometric invariant. The possible realization in synthetic matter is also discussed.

Authors:Mauricio Gómez Viloria, Philippe Ben-Abdallah, Riccardo Messina

Abstract: Two metals at different temperatures separated by large gaps exchange heat under the form of electromagnetic radiation. When the separation distance is reduced and they approach contact (nanometer and sub-nanometer gaps), electrons and phonons can tunnel between the bodies, competing and eventually going beyond the flux mediated by thermal photons. In this transition regime the accurate modeling of electronic current and heat flux is of major importance. Here we show that, in order to quantitatively model this transfer, a careful description of the tunneling barrier between two metals is needed and going beyond the traditional WKB approximation is also essential. We employ analytical and numerical approaches to model the electronic potential between two semi-infinite jellium planar substrates separated by a vacuum gap in order to calculate the electronic heat flow and compare it with its radiative counterpart described by near-field radiative heat transfer. We demonstrate that the results for heat flux and electronic current density are extremely sensitive to both the shape and height of the barrier, as well as the calculation scheme for the tunneling probability, with variations up to several orders of magnitude. Using the proximity force approximation, we also provide estimates for tip-plane geometries. The present work provides realistic models to describe the electronic heat flux, in the scanning-thermal-microscopy experiments.

Authors:K. K. Grigoryan, D. S. Zohrabyan, M. M. Glazov

Abstract: We develop a theory of anomalous Hall effect in ultraclean channels with two-dimensional electron gas. In ultraclean systems, the electron mean free path due to static disorder and phonons is considered to be much larger compared to the channel width. The electron momentum relaxation and conductivity are mainly determined by the diffusive scattering at the channel edges and electron-electron collisions making electron magnetotransport highly nontrivial. The anomalous Hall electric field and Hall voltage are generated owing to the skew scattering of electrons, side-jump, and anomalous velocity effects that appear as a result of the spin-orbit coupling. We study both ballistic and hydrodynamic transport regimes which are realized depending on the relation between the electron-electron mean free path and the channel width. Compact analytical expressions for the anomalous Hall field and voltage are derived. We demonstrate that both in the ballistic and hydrodynamic regime the electron momentum relaxation in the bulk of the channel due to impurities or phonons is required for the anomalous Hall effect in contrast to the normal Hall effect and conductivity that can appear due to the electron-electron collisions combined with momentum relaxation at the channels edge scattering.

Authors:Ritesh Das, Colin Benjamin

Abstract: Violation of the Wiedemann-Franz (WF) law in a 2D topological insulator due to Majorana bound states (MBS) is studied via the Lorenz ratio in the single-particle picture. We study the scaling of the Lorenz ratio in the presence and absence of MBS with inelastic scattering modeled using a B\"uttiker voltage-temperature probe. We compare our results with that seen in a quantum dot junction in the Luttinger liquid picture operating in the topological Kondo regime. We find that the scaling of the Lorenz ratio in our setup corresponds to the scaling in the Luttinger-liquid setup only when both phase and momentum relaxation occur, but not when only phase relaxation occurs. This suggests that the interplay between the presence of Majorana bound states and the type of inelastic scattering process, can have a significant impact on the violation of the Wiedemann-Franz law in 2D topological insulators.

Authors:Loic Temdie, Vincent Castel, Vincent Vlaminck, Matthias Benjamin Jungfleisch, Romain Bernard, Hicham Majjad, Daniel Stoeffler, Yves Henry, Matthieu Bailleul

Abstract: We report on the dependence of curvilinear shaped coplanar waveguides on the near-field diffraction patterns of spin waves propagating in perpendicularly magnetized thin films. Implementing the propagating spin waves spectroscopy techniques on either concentrically or eccentrically shaped antennas, we show how the link budget is directly affected by the spin wave interference, in good agreement with near-field diffraction simulations. This work demonstrates the feasibility to inductively probe a magnon interference pattern with a resolution down to 1$\mu$m$^2$, and provides a methodology for shaping spin wave beams from an antenna design. This methodology is successfully implemented in the case study of a spin wave Young's interference experiment.

Authors:Raj N. Patel, Rebecca E. K. Fishman, Tzu-Yung Huang, Jordan A. Gusdorff, David A. Fehr, David A. Hopper, S. Alex Breitweiser, Benjamin Porat, Michael E. Flatté, Lee C. Bassett

Abstract: Hexagonal boron nitride (h-BN), a wide bandgap, two-dimensional solid-state material, hosts pure single-photon emitters that have shown signatures of optically-addressable electronic spins. Here, we report on a single emitter in h-BN exhibiting optically detected magnetic resonance at room temperature, and we propose a model for its electronic structure and optical dynamics. Using photon emission correlation spectroscopy in conjunction with time-domain optical and microwave experiments, we establish key features of the emitter's electronic structure. Specifically, we propose a model that includes a spinless optical ground and excited state, a metastable spin-1/2 configuration, and an emission modulation mechanism. Using optical and spin dynamics simulations, we constrain and quantify transition rates in the model, and we design protocols that optimize the signal-to-noise ratio for spin readout. This constitutes a necessary step toward quantum control of spin states in h-BN.

Authors:Dimitris Kechrakos, Vito Puliafito, Alejandro Riveros, Jiahao Liu, Wanjun Jiang, Mario Carpentieri, Riccardo Tomasello, Giovanni Finocchio

Abstract: The dynamical properties of skyrmions can be exploited to build devices with new functionalities. Here, we first investigate a skyrmion-based ring-shaped device by means of micromagnetic simulations and Thiele equation. We subsequently show three applications scenarios: (1) a clock with tunable frequency that is biased with an electrical current having a radial spatial distribution, (2) an alternator, where the skyrmion circular motion driven by an engineered anisotropy gradient is converted into an electrical signal, and (3) an energy harvester, where the skyrmion motion driven by a thermal gradient is converted into an electrical signal, thus providing a heat recovery operation. We also show how to precisely tune the frequency and amplitude of the output electrical signals by varying material parameters, geometrical parameters, number and velocity of skyrmions, and we further prove the correct device functionality under realistic conditions given by room temperature and internal material defects. Our results open a new route for the realization of energy efficient nanoscale clocks, generators, and energy harvesters.

Authors:C. Adambukulam, B. C. Johnson, A. Morello, A. Laucht

Abstract: A high-spin nucleus coupled to a color center can act as a long-lived memory qudit in a spin-photon interface. The germanium vacancy (GeV) in diamond has attracted recent attention due to its excellent spectral properties and provides access to the 10-dimensional Hilbert space of the $I = 9/2$ ${}^{73}$Ge nucleus. Here, we observe the ${}^{73}$GeV hyperfine structure, perform nuclear spin readout, and optically initialize the ${}^{73}$Ge spin into any eigenstate on a $\mu$s-timescale and with a fidelity of up to $97 \pm 3\%$. Our results establish ${}^{73}$GeV as an optically addressable high-spin quantum platform for a high-efficiency spin-photon interface as well as for foundational quantum physics and metrology.

Authors:Wenlu Lin, Xing Fan, Lili Zhao, Yoon Jang Chung, Adbhut Gupta, Kirk W. Baldwin, Loren Pfeiffer, Hong Lu, Yang Liu

Abstract: We study two dimensional electron systems confined in wide quantum wells whose subband separation is comparable with the Zeeman energy. Two N = 0 Landau levels from different subbands and with opposite spins are pinned in energy when they cross each other and electrons can freely transfer between them. When the disorder is strong, we observe clear hysteresis in our data corresponding to instability of the electron distribution in the two crossing levels. When the intra-layer interaction dominates, multiple minima appear when a Landau level is 1/3 or 2/3 filled and fractional quantum hall effect can be stabilized.

Authors:C. Han, M. Wang, B. Zhang, M. I. Dykman, H. B. Chan

Abstract: Asymmetric Ising model, in which coupled spins affect each other differently, plays an important role in diverse fields, from physics to biology to artificial intelligence. We show that coupled parametric oscillators provide a well-controlled and fully characterizable physical system to implement the model. Such oscillators are bistable. The coupling changes the rate of interstate switching of an oscillator depending on the state of other oscillators. Our experiment on two coupled micromechanical resonators reveals unusual features of asymmetric Ising systems, including the onset of a probability current that circulates in the stationary state. We relate the asymmetry to the exponentially strong effect of a periodic force on the switching rates of an individual parametric oscillator, which we measure. Our findings open the possibilities of constructing and exploring asymmetric Ising systems with controlled parameters and connectivity.

Authors:Moojune Song, Tomas Polakovic, Jinho Lim, Thomas W. Cecil, John Pearson, Ralu Divan, Wai-Kwong Kwok, Ulrich Welp, Axel Hoffmann, Kab-Jin Kim, Valentine Novosad, Yi Li

Abstract: Magnon interference is a signature of coherent magnon interactions for coherent information processing. In this work, we demonstrate programmable real-time magnon interference, with examples of nearly perfect constructive and destructive interference, between two remotely coupled yttrium iron garnet spheres mediated by a coplanar superconducting resonator. Exciting one of the coupled resonators by injecting single- and double-microwave pulse leads to the coherent energy exchange between the remote magnonic resonators and allows us to realize a programmable magnon interference that can define an arbitrary state of coupled magnon oscillation. The demonstration of time-domain coherent control of remotely coupled magnon dynamics offers new avenues for advancing coherent information processing with circuit-integrated hybrid magnonic networks.

Authors:Nezhat Pournaghavi, Banasree Sadhukhan, Anna Delin

Abstract: Nontrivial band topology along with magnetism leads to different novel quantum phases. When time-reversal-symmetry is broken in three-dimensional topological insulators (TIs) by applying high enough magnetic field or proximity effect, different phases such as quantum Hall or quantum anomalous Hall(QAH) emerge and display interesting transport properties for spintronic applications. The QAH phase displays sidewall chiral edge states which leads to the QAH effect. In a finite slab, contribution of the surface states depends on both the cross-section and thickness of the system. Having a small cross-section and a thin thickness leads to direct coupling of the surfaces, on the other hand, a thicker slab results in a higher contribution of the non-trivial sidewall states which connect top and bottom surfaces. In this regard, we have considered a heterostructure consisting of a TI, namely Bi2Se3, which is sandwiched between two-dimensional magnetic monolayers of CrI3 to study its topological and transport properties. Combining DFT and tight-binding calculations along with non-equilibrium Green's function formalism, we show that a well-defined exchange gap appears in the band structure in which spin polarised edge states flow. We also study the width and finite-size effect on the transmission and topological properties of this magnetised TI nanoribbon.

Authors:Fangyuan Yang, Ruiheng Bai, Alexander A. Zibrov, Sandeep Joy, Takashi Taniguchi, Kenji Watanabe, Brian Skinner, Mark O. Goerbig, Andrea F. Young

Abstract: The phase diagram of an interacting two-dimensional electron system in a high magnetic field is enriched by the varying form of the effective Coulomb interaction, which depends strongly on the Landau level index. While the fractional quantum Hall states that dominate in the lower energy Landau levels have been explored experimentally in a variety of two-dimensional systems, much less work has been done to explore electron solids owing to their subtle transport signatures and extreme sensitivity to disorder. Here we use chemical potential measurements to map the phase diagram of electron solid states in $N=2$, $N=3$, and $N=4$ Landau levels in monolayer graphene. Direct comparison between our data and theoretical calculations reveals a cascade of density-tuned phase transitions between electron bubble phases up to two, three or four electrons per bubble in the N=2, 3 and 4 Landau levels respectively. Finite temperature measurements are consistent with melting of the solids for T$\approx$1K.

Authors:Jiří Doležal, Amandeep Sagwal, Rodrigo Cezar de Campos Ferreira, Martin Švec

Abstract: The spontaneous fluorescence rates of single-molecule emitters are typically on the order of nanoseconds. However coupling them with plasmonic nanostructures can substantially increase their fluorescence yields. The confinement between the tip and sample of a scanning tunneling microscope creates a tunable nanocavity, an ideal platform for exploring the yields and excitation decay rates of single-molecule emitters depending on the coupling strength to the nanocavity. With this setup we estimate the excitation lifetimes from the direct time-resolved measurements of the fluorescence decays of phthalocyanine adsorbates, decoupled from the metal substrates by ultrathin NaCl layers. It is found that nanosecond-range lifetimes prevail for the emitters away from the nanocavity, whereas for the tip approached to a molecule, we find a substantial effect of the nanocavity coupling, which reduces the lifetimes to a few picoseconds. An analysis is performed to investigate the crossover between the far-field and tip-enhanced photoluminescence regimes. This approach overcomes the drawbacks associated with the estimation of lifetimes for single molecules from their respective emission linewidths.

Authors:Ankit Purohit, Akhilesh Kumar Mishra

Abstract: In this article, we theoretically investigate a spaser (surface plasmon amplification by stimulated emission of radiation), which consists of a spherical silver nanoparticle surrounded by four-level gain medium of quantum dots (QDs). The spaser system is pumped coherently and incoherently with the same excitation rate, and the characteristics of coherent localized surface plasmon (LSP) mode, thus produced, are compared for the two pumping scenarios. We provide a detailed analytical expression for the steady state and show that the incoherent pump is more suitable for the continuous spaser mode. The reason is better understood by studying the temporal evolution of number of LSP (N_n ), where the oscillation of LSP starts early for incoherent drive and relaxes to steady state with a large value of N_n. At a large pump rate, spaser curve shows saturation. In addition, we have found that the resonance peak of spaser field is independent of coherent as well as incoherent pumping, while the peak amplitude of field depends on the pump rate.

Authors:Lauro B. Braz, George B. Martins, Luis G. G. V. Dias da Silva

Abstract: Magic-angle twisted bilayer graphene (MATBG) has been extensively explored both theoretically and experimentally as a suitable platform for a rich and tunable phase diagram that includes ferromagnetism, charge order, broken symmetries, and unconventional superconductivity. In this work, we investigate the intricate interplay between long-range electron-electron interactions, spin fluctuations, and superconductivity in MATBG. By employing a low-energy model for MATBG that captures the correct shape of the flat bands, we explore the effects of short- and long-range interactions on spin fluctuations and their impact on the superconducting (SC) pairing vertex in the Random Phase Approximation (RPA). We find that the SC state is notably influenced by the strength of long-range Coulomb interactions. Interestingly, our RPA calculations indicate that there is a regime where the system can traverse from a magnetic phase to the SC phase by \emph{increasing} the relative strength of long-range interactions compared to the on-site ones. These findings underscore the relevance of electron-electron interactions in shaping the intriguing properties of MATBG and offer a pathway for designing and controlling its SC phase.

Authors:Eugene B. Kolomeisky, Joseph P. Straley

Abstract: Coulomb impurity of charge $Ze$ is known to destabilize the ground state of undoped graphene with respect to creation of screening space charge if $Z$ exceeds a critical value of $1/2\alpha$ set by material's fine structure constant $\alpha$. Recent experimental advances made it possible to explore this transition in a controlled manner by tuning $Z$ across the critical point. Combined with relatively large value of $\alpha$ this opens a possibility to study graphene's screening response to a supercritical impurity $Z\alpha\gg1$ when the screening charge is large, and the Thomas-Fermi analysis, that we revisit, is adequate. The character of screening in this regime is controlled by the dimensionless screening parameter $Z\alpha^{2}$. Specifically, for circular impurity cluster most of the screening charge in the weak-screening regime $Z\alpha^{2}\ll1$ is found to reside outside the cluster. The strong-screening regime $Z\alpha^{2}\gg1$ provides a realization of the Thomson atom: most of the screening charge is inside the cluster nearly perfectly neutralizing the source charge with the exception of a transition layer near cluster's edge where the rest of the space charge is localized.

Authors:Andrii M Sokolov, Tero T. Heikkilä

Abstract: We consider a model of two harmonically driven damped harmonic oscillators that are coupled linearly and with a cross-Kerr coupling. We show how to distinguish this combination of coupling types from the case where a coupling of optomechanical type is present. This can be useful for the characterization of various nonlinear systems, such as mechanical oscillators, qubits, and hybrid systems. We then consider a hybrid system with linear and cross-Kerr interactions and a relatively high damping in one of the modes. We derive a quantum Hamiltonian of a doubly clamped magnetic beam, showing that the cross-Kerr coupling is prominent there. We discuss, in the classical limit, measurements of its linear response as well as the specific higher-harmonic responses. These frequency-domain measurements can allow estimating the magnitude of the cross-Kerr coupling or its magnon population.

Authors:Chenjiang Qian, Viviana Villafañe, Pedro Soubelet, Peirui Ji, Andreas V. Stier, Jonathan J. Finley

Abstract: We report resonant second harmonic generation (SHG) spectroscopy of an hBN-encapsulated monolayer of MoS$_2$. By tuning the energy of the excitation laser, we identify a dark state transition (D) that is blue detuned by +25 meV from the neutral exciton X$^0$. We observe a splitting of the SHG spectrum into two distinct peaks and a clear anticrossing between them as the SHG resonance is tuned through the energy of the dark exciton D. This observation is indicative of quantum interference arising from the strong two-photon light-matter interaction. We further probe the incoherent relaxation from the dark state to the bright excitons, including X$^0$ and localized excitons LX, by the resonant enhancement of their intensities at the SHG-D resonance. The relaxation of D to bright excitons is strongly suppressed on the bare substrate whilst enabled when the hBN/MoS$_2$/hBN heterostructure is integrated in a nanobeam cavity. The relaxation enabled by the cavity is explained by the phonon scattering enhanced by the cavity phononic effects. Our work reveals the two-photon quantum interference with long-lived dark states and enables the control through nanostructuring of the substrate. These results indicate the great potential of dark excitons in 2D-material based nonlinear quantum devices.

Authors:Danny Fainozzi, Sara Catalini, Renato torre, Claudio Masciovecchio, Cristian Svetina

Abstract: We used optical laser pulses to create transient gratings (TGs) with sub-10 {\mu}m spatial periodicity in a Bismuth Germanate (310) (Bi4Ge3O12) single crystal at room temperature. The TG launches phonon modes, whose dynamics were revealed via forward diffraction of a third, time-delayed, heterodyne-detected optical pulse. Acoustic oscillations have been clearly identified in a time-frequency window not covered by previous spectroscopic studies and their characteristic dynamic parameters have been measured as a function of transferred momenta magnitude and direction.

Authors:Stefano Reale, Jiyoon Hwang, Jeongmin Oh, Harald Brune, Andreas J. Heinrich, Fabio Donati, Yujeong Bae

Abstract: A pivotal challenge in present quantum technologies lies in reconciling long coherence times with efficient manipulation of the quantum states of a system. Lanthanide atoms, with their well-localized 4f electrons, emerge as a promising solution to this dilemma if provided with a rational design of the manipulation and detection schemes. Here we utilize a scanning tunneling microscope to construct tailored spin structures and perform electron spin resonance on a single lanthanide atom in such a structure. A magnetically coupled structure made of an Erbium and a Titanium atom at sub-nanometer distance enables us to both drive Erbium's 4f electron spins and indirectly probe them through the Titanium's 3d electrons. In this coupled configuration, the Erbium spin states exhibit a four-fold increase in the spin relaxation time and a two-fold increase in the driving efficiency compared to the 3d electron counterparts. Our work provides a new approach to accessing highly protected spin states, enabling us to control them in an all-electric fashion.

Authors:Igor Mazin, Rafael González-Hernández, Libor Šmejkal

Abstract: Altermagnets (AM) are a recently discovered third class of collinear magnets, distinctly different from conventional ferromagnets (FM) and antiferromagnets (AF). AM have been actively researched in the last few years, but two aspects so far remain unaddressed: (1) Are there realistic 2D single-layer altermagnets? And (2) is it possible to functionalize a conventional AF into AM by external stimuli? In this paper we address both issues by demonstrating how a well-known 2D AF, MnP(S,Se)$_3$ can be functionalized into strong AM by applying out-of-plane electric field. Of particular interest is that the induced altermagnetism is of a higher even-parity wave symmetry than expected in 3D AM with similar crystal symmetries. We confirm our finding by first-principles calculations of the electronic structure and magnetooptical response. We also propose that recent observations of the time-reversal symmetry breaking in the famous Fe-based superconducting chalchogenides, either in monolayer form or in the surface layer, may be related not to an FM, as previously assumed, but to the induced 2D AM order. Finally, we show that monolayer FeSe can simultaneously exhibit unconventional altermagnetic time-reversal symmetry breaking and quantized spin Hall conductivity indicating possibility to research an intriquing interplay of 2D altermagnetism with topological and superconducting states within a common crystal-potential environment.

Authors:Mushita M. Munia, Serajum Monir, Edyta N. Osika, Michelle Y. Simmons, Rajib Rahman

Abstract: Atomic engineering in a solid-state material has the potential to functionalize the host with novel phenomena. STM-based lithographic techniques have enabled the placement of individual phosphorus atoms at selective lattice sites of silicon with atomic precision. Here, we show that by placing four phosphorus donors spaced 10-15 nm apart from their neighbours in a linear chain, it is possible to realize coherent spin coupling between the end dopants of the chain, analogous to the superexchange interaction in magnetic materials. Since phosphorus atoms are a promising building block of a silicon quantum computer, this enables spin coupling between their bound electrons beyond nearest neighbours, allowing the qubits to be spaced out by 30-45 nm. The added flexibility in architecture brought about by this long-range coupling not only reduces gate densities but can also reduce correlated noise between qubits from local noise sources that are detrimental to error correction codes. We base our calculations on a full configuration interaction technique in the atomistic tight-binding basis, solving the 4-electron problem exactly, over a domain of a million silicon atoms. Our calculations show that superexchange can be tuned electrically through gate voltages where it is less sensitive to charge noise and donor placement errors.

Authors:R. A. Ng, M. E. Portnoi, R. R. Hartmann

Abstract: We employ a tight-binding model to calculate the optical selection rules of gold-terminated carbyne chains in the presence of an applied electric field. We show that both the magnitude of the edge-state gap and the strength of optical transitions across it can be tuned via the Stark effect. In the case of sufficiently long carbyne chains, the dipole transitions between edge states occur within the THz frequency range.

Authors:Junren Shi

Abstract: The theory of composite fermions consists of two complementary parts: a standard ansatz for constructing many-body wave-functions of various fractional quantum Hall states, and an effective theory (the HLR theory) for predicting responses of these states to external perturbations. Conventionally, both the ansatz and the HLR theory are justified by Lopez-Fradkin's theory based on the singular Chern-Simons transformation. In this work, we aim to provide an alternative basis and unify the two parts into a coherent theory by developing quantum mechanics of composite fermions based on the dipole picture. We argue that states of a composite fermion in the dipole picture are naturally described by bivariate wave functions which are holomorphic (anti-holomorphic) in the coordinate of its constituent electron (vortex), defined in a Bergman space with its weight determined by the spatial profiles of the physical and the emergent Chern-Simons magnetic fields. Based on a semi-classical phenomenological model and the quantization rules of the Bergman space, we establish general wave equations for composite fermions. The wave equations resemble the ordinary Schr\"odinger equation but have drift velocity corrections not present in the HLR theory. Using Pasquier-Haldane's interpretation of the dipole picture, we develop a general wave-function ansatz for constructing many-body wave functions of electrons by projecting states of composite fermions solved from the wave equation into a half-filled bosonic Laughlin state of vortices. It turns out that for ideal fractional quantum Hall states the general ansatz and the standard ansatz are equivalent, albeit using different wave-function representations for composite fermions. To justify the phenomenological model, we derive it from the microscopic Hamiltonian and the general variational principle of quantum mechanics.

Authors:Kostas Vilkelis, Antonio Manesco, Juan Daniel Torres Luna, Sebastian Miles, Michael Wimmer, Anton Akhmerov

Abstract: We propose a practical implementation of a universal quantum computer that uses local fermionic modes (LFM) rather than qubits. Our design consists of quantum dots tunnel coupled by a hybrid superconducting island together with a tunable capacitive coupling between the dots. We show that coherent control of Cooper pair splitting, elastic cotunneling, and Coulomb interactions allows us to implement the universal set of quantum gates defined by Bravyi and Kitaev. Finally, we discuss possible limitations of the device and list necessary experimental efforts to overcome them.

Authors:Teresa Tschirner, Philipp Keßler, Ruben Dario Gonzalez Betancourt, Tommy Kotte, Dominik Kriegner, Bernd Buechner, Joseph Dufouleur, Martin Kamp, Vedran Jovic, Libor Smejkal, Jairo Sinova, Ralph Claessen, Tomas Jungwirth, Simon Moser, Helena Reichlova, Louis Veyrat

Abstract: Observations of the anomalous Hall effect in RuO$_2$ and MnTe have demonstrated unconventional time-reversal symmetry breaking in the electronic structure of a recently identified new class of compensated collinear magnets, dubbed altermagnets. While in MnTe the unconventional anomalous Hall signal accompanied by a vanishing magnetization is observable at remanence, the anomalous Hall effect in RuO$_2$ is excluded by symmetry for the N\'eel vector pointing along the zero-field [001] easy-axis. Guided by a symmetry analysis and ab initio calculations, a field-induced reorientation of the N\'eel vector from the easy-axis towards the [110] hard-axis was used to demonstrate the anomalous Hall signal in this altermagnet. We confirm the existence of an anomalous Hall effect in our RuO$_2$ thin-film samples whose set of magnetic and magneto-transport characteristics is consistent with the earlier report. By performing our measurements at extreme magnetic fields up to 68 T, we reach saturation of the anomalous Hall signal at a field $H_{\rm c} \simeq$ 55 T that was inaccessible in earlier studies, but is consistent with the expected N\'eel-vector reorientation field.

Authors:Søren Ulstrup, Yann in 't Veld, Jill A. Miwa, Alfred J. H. Jones, Kathleen M. McCreary, Jeremy T. Robinson, Berend T. Jonker, Simranjeet Singh, Roland J. Koch, Eli Rotenberg, Aaron Bostwick, Chris Jozwiak, Malte Rösner, Jyoti Katoch

Abstract: Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials [1]. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed". This leads to the formation of polaronic quasiparticles that dramatically impact charge transport, surface reactivity, thermoelectric and optical properties, as observed in a variety of crystals and interfaces composed of polar materials [2-6]. Similarly, when oscillations of the charge density couple to conduction electrons the more elusive plasmon polaron emerges [7], which has been detected in electron-doped semiconductors [8-10]. However, the exploration of polaronic effects on low energy excitations is still in its infancy in two-dimensional (2D) materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of SL WS$_2$. By using micro-focused angle-resolved photoemission spectroscopy (microARPES) during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the SL WS$_2$ conduction band minimum (CBM). Our results are explained by an effective many-body model in terms of a coupling between SL WS$_2$ conduction electrons and graphene plasmon modes. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides (TMDs) [11-15].

Authors:Mathieu Lamblin, Bhavishya Chowrira, Victor Da Costa, Bertrand Vileno, Loic Joly, Samy Boukari, Wolfgang Weber, Romain Bernard, Benoit Gobaut, Michel Hehn, Daniel Lacour, Martin Bowen

Abstract: An electrical current that flows across individual atoms or molecules can generate exotic quantum-based behavior, from memristive effects to Coulomb blockade and the promotion of quantum excited states. These fundamental effects typically appear one at a time in model junctions built using atomic tip or lateral techniques. So far, however, a viable industrial pathway for such discrete state devices has been lacking. Here, we demonstrate that a commercialized device platform can serve as this industrial pathway for quantum technologies. We have studied magnetic tunnel junctions with a MgO barrier containing C atoms. The paramagnetic localized electrons due to individual C atoms generate parallel nanotransport paths across the micronic device as deduced from magnetotransport experiments. Coulomb blockade effects linked to tunnelling magnetoresistance peaks can be electrically controlled, leading to a persistent memory effect. Our results position MgO tunneling spintronics as a promising platform to industrially implement quantum technologies.

Authors:XinYu Zhou, H Wang, Q M Liu, S J Zhang, S X Xu, Q Wu, R S Li, L Yue, T C Hu, J Y Yuan, S S Han, T Dong, D Wu, N L Wang

Abstract: Ultrafast light-matter interaction has emerged as a powerful tool to control and probe the macroscopic properties of functional materials, especially two-dimensional transition metal dichalcogenides which can form different structural phases with distinct physical properties. However, it is often difficult to accurately determine the transient optical constants. In this work, we developed a near-infrared pump - terahertz to midinfrared (12-22 THz) probe system in transmission geometry to measure the transient optical conductivity in 2H-MoTe2 layered material. By performing separate measurements on bulk and thin-film samples, we are able to overcome issues related to nonuniform substrate thickness and penetration depth mismatch and to extract the transient optical constants reliably. Our results show that photoexcitation at 690 nm induces a transient insulator-metal transition, while photoexcitation at 2 um has a much smaller effect due to the photon energy being smaller than the band gap of the material. Combining this with a single-color pump-probe measurement, we show that the transient response evolves towards 1T' phase at higher flunece. Our work provides a comprehensive understanding of the photoinduced phase transition in the 2H-MoTe2 system.

Authors:S. S. Babkin, I. S. Burmistrov

Abstract: Generalized multifractality characterizes system size dependence of pure scaling local observables at Anderson transitions in all ten symmetry classes of disordered systems. Recently, the concept of generalized multifractality has been extended to boundaries of critical disordered noninteracting systems. Here we study the generalized boundary multifractality in the presence of electron-electron interaction, focusing on the spin quantum Hall symmetry class (class C). Employing the two-loop renormalization group analysis within Finkel'stein nonlinear sigma model we compute the anomalous dimensions of the pure scaling operators located at the boundary of the system. We find that generalized boundary multifractal exponents are twice larger than their bulk counterparts. Exact symmetry relations between generalized boundary multifractal exponents in the case of noninteracting systems are explicitly broken by the interaction.

Authors:Sanjib Kumar Das, Bitan Roy

Abstract: Nontrivial bulk topological invariants of quantum materials can leave their signatures on charge, thermal and spin transports. In two dimensions, their imprints can be experimentally measured from well-developed multi-terminal Hall bar arrangements. Here, we numerically compute the low temperature ($T$) thermal ($\kappa_{xy}$) and zero temperature spin ($\sigma^{sp}_{xy}$) Hall conductivities, and longitudinal thermal conductance ($G^{th}_{xx}$) of various paradigmatic two-dimensional fully gapped topological superconductors, belonging to distinct Altland-Zirnbauer symmetry classes, namely $p+ip$ (class D), $d+id$ (class C) and $p \pm ip$ (class DIII) paired states, in mesoscopic six-terminal Hall bar setups from the scattering matrix formalism using Kwant. In both clean and weak disorder limits, the time-reversal symmetry breaking $p+ip$ and $d+id$ pairings show half-quantized and quantized $\kappa_{xy}$ [in units of $\kappa_0=\pi^2 k^2_B T/(3h)$], respectively, while the latter one in addition accommodates a quantized $\sigma^{sp}_{xy}$ [in units of $\sigma^{sp}_0=\hbar/(8 \pi)$]. By contrast, the time-reversal invariant $p \pm ip$ pairing only displays a quantized $G^{th}_{xx}$ at low $T$ up to a moderate strength of disorder. In the strong disorder regime, all these topological responses ($\kappa_{xy}$, $\sigma^{sp}_{xy}$ and $G^{th}_{xx}$) vanish. Possible material platforms hosting such paired states and manifesting these robust topological thermal and spin responses are highlighted.

Authors:Luis Brey, H. A. Fertig

Abstract: Plasmons are fundamental excitations of metals which can be described in terms of electron dynamics, or in terms of the electromagnetic fields associated with them. In this work we develop a quantum description of plasmons in a double layer structure, treating them as confined electromagnetic modes of the structure. The structure of the resulting bosonic Hamiltonian indicates the presence of virtual plasmons of the individual layers which appear as quantum fluctuations in the ground state. For momenta smaller than the inverse separation between layers, these modes are in the ultrastrong coupling regime. Coherence terms in the Hamiltonian indicate that modes with equal and opposite momenta are entangled. We consider how in principle these entangled modes might be accessed, by analyzing a situation in which the conductivity of one of the two layers suddenly drops to zero. The resulting density matrix has a large entanglement entropy at small momenta, and modes at $\pm \mathbf{q}$ that are inseparable. More practical routes to releasing and detecting entangled plasmons from this system are considered.

Authors:Yadong Jiang, Huan Wang, Kejie Bao, Jing Wang

Abstract: Intrinsic magnetic topological insulators offers an ideal platform to explore exotic topological phenomena, such as axion electrodynamics, quantum anomalous Hall (QAH) effect and Majorana edge modes. However, these emerging new physical effects have rarely been experimentally observed due to the limited choice of suitable materials. Here, we predict the van der Waals layered V$_2$WS$_4$ and its related materials show intralayer ferromagnetic and interlayer antiferromagnetic exchange interactions. We find extremely rich magnetic topological states in V$_2$WS$_4$, including an antiferromagnetic topological insulator, the axion state with the long-sought quantized topological magnetoelectric effect, three-dimensional QAH state, as well as a collection of QAH insulators and intrinsic axion insulators in odd- and even-layer films, respectively. Remarkably, the N\'eel temperature of V$_2$WS$_4$ is predicted to be much higher than that of MnBi$_2$Te$_4$. These interesting predictions, if realized experimentally, could greatly promote the topological quantum physics research and application.

Authors:D. S. Smirnov, E. L. Ivchenko

Abstract: In this work, we theoretically investigate the optical orientation and alignment of excitons in quantum dots with weak electron-hole exchange interaction and long exciton radiative lifetimes. This particular regime is realized in semiconductor heterosystems where excitons are indirect in the $\boldsymbol r$ or $\boldsymbol k$ space. The main role in the fine structure of excitonic levels in these systems is played by the hyperfine interaction of the electron in the confined exciton and fluctuations of the Overhauser field. Along with it, the effects of nonradiative recombination and exchange interaction are considered. We start with the model of vanishing exchange interaction and nonradiative exciton recombination and then include them into consideration in addition to the strong Overhauser field. In the nanoobjects under study, the polarization properties of the resonant photoluminescence are shown to vary with the external magnetic filed in completely different way as compared with the behaviour of the conventional quantum dot structures.

Authors:Ghulam Hussain, Kinga Warda, Giuseppe Cuono, Carmine Autieri

Abstract: We investigate the electronic, structural and topological properties of the SnTe and PbTe cubic nanowires using ab-initio calculations. Using standard and linear-scale density functional theory, we go from the ultrathin limit up to the nanowires thicknesses observed experimentally. Finite-size effects in the ultra-thin limit produce an electric quadrupole and associated structural distortions, these distortions increase the band gap but they get reduced with the size of the nanowires and become less and less relevant. Ultrathin SnTe cubic nanowires are trivial band gap insulators, we demonstrate that by increasing the thickness there is an electronic transition to a spin-orbit insulating phase due to trivial surface states in the regime of thin nanowires. These trivial surface states with a spin-orbit gap of a few meV appear at the same k-point of the topological surface states. Going to the limit of thick nanowires, we should observe the transition to the topological crystalline insulating phase with the presence of two massive surface Dirac fermions hybridized with the persisting trivial surface states. Therefore, we have the co-presence of massive Dirac surface states and trivial surface states close to the Fermi level in the same region of the k-space. According to our estimation, the cubic SnTe nanowires are trivial insulators below the critical thickness tc1=10 nm, and they become spin-orbit insulators between tc1=10 nm and tc2=17 nm, while they transit to the topological phase above the critical thickness of tc2=17 nm. These critical thickness values are in the range of the typical experimental thicknesses, making the thickness a relevant parameter for the synthesis of topological cubic nanowires. Pb(1-x)Sn(x)Te nanowires would have both these critical thicknesses tc1 and tc2 at larger values depending on the doping concentration.

Authors:Mengqi Huang, Jazmine C. Green, Jingcheng Zhou, Violet Williams, Senlei Li, Hanyi Lu, Dziga Djugba, Hailong Wang, Benedetta Flebus, Ni Ni, Chunhui Rita Du

Abstract: van der Waals (vdW) magnets, an emerging family of two-dimensional (2D) materials, have received tremendous attention due to their rich fundamental physics and significant potential for cutting-edge technological applications. In contrast to the conventional bulk counterparts, vdW magnets exhibit significant tunability of local material properties, such as stacking engineered interlayer coupling and layer-number dependent magnetic and electronic interactions, which promise to deliver previously unavailable merits to develop multifunctional microelectronic devices. As a further ingredient of this emerging topic, here we report nanoscale quantum sensing and imaging of atomically thin vdW magnet chromium thiophosphate CrPS4, revealing its characteristic layer-dependent 2D static magnetism and dynamic spin fluctuations. We also show a large tunneling magnetoresistance in CrPS4-based spin filter vdW heterostructures. The excellent material stability, robust strategy against environmental degradation, in combination with tailored magnetic properties highlight the potential of CrPS4 in developing state-of-the-art 2D spintronic devices for next-generation information technologies.

Authors:Lucie Leguay, Abhiroop Chellu, Joonas Hilska, Esperanza Luna, Andrei Schliwa, Mircea Guina, Teemu Hakkarainen

Abstract: Epitaxially-grown semiconductor quantum dots (QDs) provide an attractive platform for the development of deterministic sources of high-quality quantum states of light. Such non-classical light sources are essential for quantum information processing and quantum communication. QDs emitting in the telecom wavelengths are especially important for ensuring compatibility with optical fiber systems required to implement quantum communication networks. To this end, GaSb QDs fabricated by filling local-droplet etched nanoholes are emerging as a viable approach, yet the electronic properties of such nanostructures have not been studied in detail. In this article, an insight into the electronic structure and carrier dynamics in GaSb/AlGaSb QDs is provided through a systematic experimental analysis of their temperature-dependent photoluminescence behavior. A steady-state rate equation model is used to reveal the relevant energy barriers for thermally activated carrier capture and escape processes. Furthermore, results of detailed theoretical simulations of quantum-confined energy states using the multi-band k.p model and the effective mass method are presented. The purpose of the simulations is to reveal the direct and indirect energy states, carrier wavefunctions, and allowed optical transitions for GaSb QDs with different physical dimensions.

Authors:Zheyu Cheng, Yi-jun Guan, Haoran Xue, Yong Ge, Ding Jia, Yang Long, Shou-qi Yuan, Hong-xiang Sun, Yidong Chong, Baile Zhang

Abstract: When electrons moving in two-dimensions (2D) are subjected to a strong uniform magnetic field, they form flat bands called Landau levels, which are the basis for the quantum Hall effect. Landau levels can also arise from pseudomagnetic fields (PMFs) induced by lattice distortions; for example, mechanically straining graphene causes its Dirac quasiparticles to form a characteristic set of unequally-spaced Landau levels, including a zeroth Landau level. In three-dimensional (3D) systems, there has thus far been no experimental demonstration of Landau levels or any other type of flat band. For instance, applying a uniform magnetic field to materials hosting Weyl quasiparticles, the 3D generalizations of Dirac quasiparticles, yields bands that are non-flat in the direction of the field. Here, we report on the experimental realization of a flat 3D Landau level in an acoustic crystal. Starting from a lattice whose bandstructure exhibits a nodal ring, we design an inhomogeneous distortion corresponding to a specific pseudomagnetic vector potential (PVP) that causes the nodal ring states to break up into Landau levels, with a zeroth Landau level that is flat along all three directions. These findings point to the possibility of using nodal ring materials to generate 3D flat bands, to access strong interactions and other interesting physical regimes in 3D.

Authors:Takuya Taniguchi Tohoku University, Japan Technical University of Munich, Germany, Jan Sahliger Technical University of Munich, Germany, Christian H. Back Technical University of Munich, Germany

Abstract: As one of the fundamental magnonic devices, a magnonic splitter device has been proposed and spin wave propagation in this device has been studied numerically and experimentally. In the present work, we fabricated a T-shaped magnonic splitter with 6 $\mu$m-wide three arms using a 100 nm-thick yttrium iron garnet film and, using time-resolved magneto-optic Kerr microscopy, observed that spin waves split into both, the vertical and the horizontal direction at the junction. Analyzing the results, we found that spin wave modes are converted into another during the splitting process and the splitting efficiency is dominantly dependent on the 1st order of incoming spin waves.

Authors:Zhao Gong, Qing-Qing Zhang, Hui-Ying Mu, Xing-Tao An, Jian-Jun Liu

Abstract: Since the discovery of the fascinating properties in magic-angle graphene, the exploration of moir\'e systems in other two-dimensional materials has garnered significant attention and given rise to a field known as 'moir\'e physics'. Within this realm, magnetic van der Waals heterostructure and the magnetic proximity effect in moir\'e superlattices have also become subjects of great interest. However, the spin-polarized transport property in this moir\'e structures is still a problem to be explored. Here, we investigate the spin-polarized transport properties in a moir\'e superlattices formed by a two-dimensional ferromagnet CrI_3 stacked on a monolayer BAs, where the spin degeneracy is lifted because of the magnetic proximity effect associated with the moir\'e superlattices. We find that the conductance exhibits spin-resolved miniband transport properties at a small twist angle because of the periodic moir\'e superlattices. When the incident energy is in the spin-resolved minigaps, the available states are spin polarized, thus providing a spin-polarized current from the superlattice. Moreover, only a finite number of moir\'e period is required to obtain a net spin polarization of 100\%. In addition, the interlayer distance of the heterojunction is also moir\'e modifiable, so a perpendicular electric field can be applied to modulate the intensity and direction of the spin polarization. Our finding points to an opportunity to realize spin functionalities in magnetic moir\'e superlattices.

Authors:Bo Hu, Zong-Kai Xie, Jie Lu, Wei He

Abstract: We investigate magnon magnon hybrid states using a non Hermitian two band Hamiltonian and the concept of wavefunction hybridization. By comparing our model with micromagnetic simulations conducted on a synthetic antiferromagnet with strong magnon magnon coupling, we successfully reproduce not only the resonance frequencies and linewidths but also the phases and amplitudes of the magnon wavefunction. The hybridization effect influences the dissipation rate, leading to the crossing of linewidths. Additionally, we quantify the magnon hybridization within a magnonic Bloch sphere, which enhances the ability to manipulate hybrid magnons for coherent information processing.

Authors:Reyne Dowling, Mikhail Kostylev

Abstract: Magnetic nanoparticles (MNPs) have many applications which require MNPs to be captured and immobilized for their manipulation and sensing. For example, MNP sensors based on detecting changes to the ferromagnetic resonances of an antidot nanostructure exhibit better performance when the nanoparticles are captured within the antidot inclusions. This study investigates the influence of microfluidics upon the capture of MNPs by four geometries of antidot array nanostructures hollowed into 30 nm-thick Permalloy films. The nanostructures were exposed to a dispersion of 130 nm MNP clusters which passed through PDMS microfluidic channels with a 400 {\mu}m circular cross-section fabricated from wire molds. With the microfluidic flow of MNPs, the capture efficiency - the ratio between the number of nanoparticles captured inside of the antidot inclusions to the number outside the inclusions - decreased for all four geometries compared to previous results introducing the particles via droplets on the film surface. This indicates that most MNPs were passing over the nanostructures, since there were no significant magnetophoretic forces acting upon the particles. However, when a static magnetic field is applied, the magnetophoretic forces generated by the nanostructure are stronger and the capture efficiencies are significantly higher than those obtained using droplets. In particular, circular antidots demonstrated the highest capture efficiency among the four geometries of almost 83.1% when the magnetic field is parallel to the film plane. In a magnetic field perpendicular to the film, the circle antidots again show the highest capture efficiency of about 77%. These results suggest that the proportion of nanoparticles captured inside antidot inclusions is highest under a parallel magnetic field. Clearly, the geometry of the nanostructure has a strong influence on the capture of MNPs.

Authors:Yuanjun Jin, XingYu Yue, Yong Xu, Xiang-Long Yu, Guoqing Chang

Abstract: The one-dimensional (1D) Su-Schrieffer-Heeger (SSH) model is central to band topology in condensed matter physics, which allows us to understand and design topological states. The Su-Schrieffer-Heeger (SSH) model serves as a basis for topological insulators and provides insights into various topological states. In this letter, we find another mechanism to analogize the SSHmodel by introducing intrinsic spin-orbital coupling (SOC) and in-plane Zeeman field instead of relying on alternating hopping integrals. In our model, the bound states are protected by a quantizedhidden polarization andcharacterized by a weak Z2 index (0;01) due to the inversion symmetry I. When the I symmetry is broken, charge pumping is achieved by tuning the polarization. Moreover, by introducing the p + ip superconductor pairing potential, a new topological phase called weak topological superconductor (TSC) is identified. The new TSC is characterized by a weak Z2 index (0;01) and nonchiral bound states. More interestingly, these nonchiral bound states give rise to a chiral nonlocal conductance, which is different from the traditional chiral TSC. Our findings not only innovate the SSH model, but also predict the existence of weak TSC, providing an alternative avenue for further exploration of its transport properties.

Authors:Joakim Hagel, Samuel Brem, Johannes Abelardo Pineiro, Ermin Malic

Abstract: Twisted bilayers of transition metal dichalcogenides (TMDs) have revealed a rich exciton landscape including hybrid excitons and spatially trapped moir\'e excitons that dominate the optical response of the material. Recent studies have revealed that in the low-twist-angle regime, the lattice undergoes a significant relaxation in order to minimize local stacking energies. Here, large domains of low energy stacking configurations emerge, deforming the crystal lattices via strain and consequently impacting the electronic band structure. However, so far the direct impact of atomic reconstruction on the exciton energy landscape and the optical properties has not been well understood. Here, we apply a microscopic and material-specific approach and predict a significant change in the potential depth for moir\'e excitons in a reconstructed lattice, with the most drastic change occurring in TMD homobilayers. We reveal the appearance of multiple flat bands and a significant change in the position of trapping sites compared to the rigid lattice. Most importantly, we predict a multi-peak structure emerging in optical absorption of WSe$_2$ homobilayers - in stark contrast to the single peak that dominates the rigid lattice. This finding can be exploited as an unambiguous signature of atomic reconstruction in optical spectra of moir\'e excitons in twisted homobilayers.

Authors:Sebastián Filipini, Mauro Cambiaso

Abstract: In the context of $\theta$ electrodynamics we find transverse electromagnetic wave solutions forbidden in Maxwell electrodynamics. Our results attest to new evidence of the topological magnetoelectric effect in topological insulators, resulting from a polarization rotation of an external electromagnetic field. Unlike Faraday and Kerr rotations, the effect does not rely on a longitudinal magnetic field, the reflected field, or birefringence. The rotation occurs due to transversal discontinuities of the topological magnetoelectric parameter in cylindrical geometries. The dispersion relation is linear, and birefringence is absent. One solution behaves as an optical fiber confining exact transverse electromagnetic fields with omnidirectional reflectivity. These results may open new possibilities in optics and photonics by utilizing topological insulators to manipulate light.

Authors:Mateusz Gołębiewski, Hanna Reshetniak, Uladzislau Makartsou, Maciej Krawczyk, Arjen van den Berg, Sam Ladak, Anjan Barman

Abstract: The research on the properties of spin waves (SWs) in three-dimensional nanosystems is an innovative idea in the field of magnonics. Mastering and understanding the nature of magnetization dynamics and binding of SWs at surfaces, edges, and in-volume parts of three-dimensional magnetic systems enables the discovery of new phenomena and suggests new possibilities for their use in magnonic and spintronic devices. In this work, we use numerical methods to study the effect of geometry and external magnetic field manipulations on the localization and dynamics of SWs in crescent-shaped (CS) waveguides. It is shown that changing the magnetic field direction in these waveguides breaks the symmetry and affects the localization of eigenmodes with respect to the static demagnetizing field. This in turn has a direct effect on their frequency. Furthermore, CS structures were found to be characterized by significant saturation at certain field orientations, resulting in a cylindrical magnetization distribution. Thus, we present chirality-based nonreciprocal dispersion relations for high-frequency SWs, which can be controlled by the field direction (shape symmetry) and its amplitude (saturation).

Authors:Rubén Seoane Souto, Athanasios Tsintzis, Martin Leijnse, Jeroen Danon

Abstract: Artificial Kitaev chains, formed by quantum dots coupled via superconductors, have emerged as a promising platform for realizing Majorana bound states. Even a minimal Kitaev chain (a quantum dot--superconductor--quantum dot setup) can host Majorana states at discrete sweet spots. However, unambiguously identifying Majorana sweet spots in such a system is still challenging. In this work, we propose an additional dot coupled to one side of the chain as a tool to identify good sweet spots in minimal Kitaev chains. When the two Majorana states in the chain overlap, the extra dot couples to both and thus splits an even--odd ground-state degeneracy when its level is on resonance. In contrast, a ground-state degeneracy will persist for well-separated Majorana states. This difference can be used to identify points in parameter space with spatially separated Majorana states, using tunneling spectroscopy measurements. We perform a systematic analysis of different relevant situations. We show that the additional dot can help distinguishing between Majorana sweet spots and other trivial zero-energy crossings. We also characterize the different conductance patterns, which can serve as a guide for future experiments aiming to study Majorana states in minimal Kitaev chains.

Authors:F. Mesple, N. R. Walet, G. Trambly de Laissardière, F. Guinea, D. Dosenovic, H. Okuno, C. Paillet, A. Michon, C. Chapelier, V. T. Renard

Abstract: The study of moir\'e engineering started with the advent of van der Waals heterostructures in which stacking two-dimensional layers with different lattice constants leads to a moir\'e pattern controlling their electronic properties. The field entered a new era when it was found that adjusting the twist between two graphene layers led to strongly-correlated-electron physics and topological effects associated with atomic relaxation. Twist is now used routinely to adjust the properties of two-dimensional materials. Here, we investigate a new type of moir\'e superlattice in bilayer graphene when one layer is biaxially strained with respect to the other - so-called biaxial heterostrain. Scanning tunneling microscopy measurements uncover spiraling electronic states associated with a novel symmetry-breaking atomic reconstruction at small biaxial heterostrain. Atomistic calculations using experimental parameters as inputs reveal that a giant atomic swirl forms around regions of aligned stacking to reduce the mechanical energy of the bilayer. Tight-binding calculations performed on the relaxed structure show that the observed electronic states decorate spiraling domain wall solitons as required by topology. This study establishes biaxial heterostrain as an important parameter to be harnessed for the next step of moir\'e engineering in van der Waals multilayers.

Authors:Michele Masseroni, Tingyu Qu, Takashi Taniguchi, Kenji Watanabe, Thomas Ihn, Klaus Ensslin

Abstract: $\mathrm{MoS_2}$ is an emergent van der Waals material that shows promising prospects in semiconductor industry and optoelectronic applications. However, its electronic properties are not yet fully understood. In particular, the nature of the insulating state at low carrier density deserves further investigation, as it is important for fundamental research and applications. In this study, we investigate the insulating state of a dual-gated exfoliated bilayer $\mathrm{MoS_2}$ field-effect transistor by performing magnetotransport experiments. We observe positive and non-saturating magnetoresistance, in a regime where only one band contributes to electron transport. At low electron density ($\sim 1.4\times 10^{12}~\mathrm{cm^{-2}}$) and a perpendicular magnetic field of 7 Tesla, the resistance exceeds by more than one order of magnitude the zero field resistance and exponentially drops with increasing temperature. We attribute this observation to strong electron localization. Both temperature and magnetic field dependence can, at least qualitatively, be described by the Efros-Shklovskii law, predicting the formation of a Coulomb gap in the density of states due to Coulomb interactions. However, the localization length obtained from fitting the temperature dependence exceeds by more than one order of magnitude the one obtained from the magnetic field dependence. We attribute this discrepancy to the presence of a nearby metallic gate, which provides electrostatic screening and thus reduces long-range Coulomb interactions. The result of our study suggests that the insulating state of $\mathrm{MoS_2}$ originates from a combination of disorder-driven electron localization and Coulomb interactions.

Authors:Gaëtan J. Percebois, Antonio Lacerda-Santos, Boris Brun, Benoit Hackens, Xavier Waintal, Dietmar Weinmann

Abstract: The weak disorder potential seen by the electrons of a two-dimensional electron gas in high-mobility semiconductor heterostructures leads to fluctuations in the physical properties and can be an issue for nanodevices. In this paper, we show that a scanning gate microscopy (SGM) image contains information about the disorder potential, and that a machine learning approach based on SGM data can be used to determine the disorder. We reconstruct the electric potential of a sample from its experimental SGM data and validate the result through an estimate of its accuracy.

Authors:Mathias Zambach, Miriam Varón, Matti Knaapila, Ziwei Ouyang, Marco Beleggia, Cathrine Frandsen

Abstract: According to the laws of magnetism, the shape of magnetically soft objects limits the effective susceptibility. For example, spherical soft magnets cannot display an effective susceptibility larger than 3. Although true for macroscopic multi-domain magnetic materials, we show that magnetic nanoparticles in a single-domain state do not suffer from this limitation. This is a consequence of the particle moment being forced to saturation by the predominance of exchange forces, and only allowed to rotate coherently in response to thermal and/or applied fields. We apply statistical mechanics to determine the static and dynamic susceptibility of single-domain particles as a function of size, temperature and material parameters. Our calculations reveal that spherical single-domain particles with large saturation magnetisation and small magneto-crystalline anisotropy, e.g. FeNi particles, can have very a large susceptibility of 200 or more. We further show that susceptibility and losses can be tuned by particle easy axis alignment with the applied field in case of uniaxial anisotropy, but not for particles with cubic anisotropy. Our model is validated experimentally by comparison with measurements on nanocomposites containing spherical 11$\pm$3 nm $\gamma$-Fe$_2$O$_3$ particles up to 45 vol% finely dispersed in a polymer matrix. In agreement with the calculations for this specific material, the measured susceptibility of the composites is up to 17 ($\gg$3) and depends linearly on the volume fraction of particles. Based on these results, we predict that nanocomposites of 30 vol% of superparamagnetic FeNi particles in an insulating non-magnetic matrix can have a sufficiently large susceptibility to be used as micro-inductor core materials in the MHz frequency range, while maintaining losses below state-of-the-art ferrites.

Authors:Bing Zhao, Lakhan Bainsla, Roselle Ngaloy, Peter Svedlindh, Saroj P. Dash

Abstract: The discovery of van der Waals (vdW) materials exhibiting non-trivial and tunable magnetic interactions at room temperature can give rise to exotic magnetic states, which are not readily attainable with conventional materials. Such vdW magnets can provide a unique platform for studying new magnetic phenomena and realising magnetization dynamics for energy-efficient and non-volatile spintronic memory and logic technologies. Recent developments in vdW magnets have revealed their potential to enable spin-orbit torque (SOT) induced magnetization dynamics. However, the deterministic and field-free SOT switching of vdW magnets at room temperature has been lacking, prohibiting their potential applications. Here, we demonstrate magnetic field-free and deterministic SOT switching of a vdW magnet (Co0.5Fe0.5)5GeTe2 (CFGT) at room temperature, capitalizing on its non-trivial intrinsic magnetic ordering. We discover a coexistence of ferromagnetic and antiferromagnetic orders in CFGT at room temperature, inducing an intrinsic exchange bias and canted perpendicular magnetism. The resulting canted perpendicular magnetization of CFGT introduces symmetry breaking, facilitating successful magnetic field-free magnetization switching in the CFGT/Pt heterostructure devices. Furthermore, the SOT-induced magnetization dynamics and their efficiency are evaluated using 2nd harmonic Hall measurements. This advancement opens new avenues for investigating tunable magnetic phenomena in vdW material heterostructures and realizing field-free SOT-based spintronic technologies.

Authors:S. S. Krishtopenko, A. V. Ikonnikov, B. Jouault, F. Teppe

Abstract: By using the self-consistent Born approximation, we investigate disorder effect induced by short-range impurities on the band-gap of a seminal two-dimensional (2D) system, whose phase diagram contains trivial, single-band-inverted and double-band-inverted states. Following the density-of-states (DOS) evolution, we demonstrate multiple closings and openings of the band-gap with the increase of the disorder strength. Calculations of the spectral function describing the quasiparticles at the $\Gamma$ point of the Brillouin zone evidence that the observed band-gap behavior is unambiguously caused by the topological phase transitions due to the mutual inversions between the first and second electron-like and hole-like subbands. We also find that an increase in the disorder strength in the double-inverted state always leads to the band-gap closing due to the overlap of the tails of DOS from conduction and valence subbands.

Authors:Christoph Adelsberger, Stefano Bosco, Jelena Klinovaja, Daniel Loss

Abstract: Electron spins confined in silicon quantum dots are promising candidates for large-scale quantum computers. However, the degeneracy of the conduction band of bulk silicon introduces additional levels dangerously close to the window of computational energies, where the quantum information can leak. The energy of the valley states - typically 0.1 meV - depends on hardly controllable atomistic disorder and still constitutes a fundamental limit to the scalability of these architectures. In this work, we introduce designs of CMOS-compatible silicon fin field-effect transistors that enhance the energy gap to non-computational states by more than one order of magnitude. Our devices comprise realistic silicon-germanium nanostructures with a large shear strain, where troublesome valley degrees of freedom are completely removed. The energy of non-computational states is therefore not affected by unavoidable atomistic disorder and can further be tuned in-situ by applied electric fields. Our design ideas are directly applicable to a variety of setups and will offer a blueprint towards silicon-based large-scale quantum processors.

Authors:Kevin S. Guo, MengKe Feng, Jonathan Y. Huang, Will Gilbert, Kohei M. Itoh, Fay E. Hudson, Kok Wai Chan, Wee Han Lim, Andrew S. Dzurak, Andre Saraiva

Abstract: In a full-scale quantum computer with a fault-tolerant architecture, having scalable, long-range interaction between qubits is expected to be a highly valuable resource. One promising method of achieving this is through the light-matter interaction between spins in semiconductors and photons in superconducting cavities. This paper examines the theory of both transverse and longitudinal spin-photon coupling and their applications in the silicon metal-oxide-semiconductor (SiMOS) platform. We propose a method of coupling which uses the intrinsic spin-orbit interaction arising from orbital degeneracies in SiMOS qubits. Using theoretical analysis and experimental data, we show that the strong coupling regime is achievable in the transverse scheme. We also evaluate the feasibility of a longitudinal coupling driven by an AC modulation on the qubit. These coupling methods eschew the requirement for an external micromagnet, enhancing prospects for scalability and integration into a large-scale quantum computer.

Authors:Pritam Chatterjee, Arnob Kumar Ghosh, Ashis K. Nandy, Arijit Saha

Abstract: We put forth a theoretical framework for engineering a two-dimensional (2D) second-order topological superconductor (SOTSC) by utilizing a heterostructure: incorporating noncollinear magnetic textures between an s-wave superconductor and a 2D quantum spin Hall insulator. It stabilizes the higher order topological superconducting phase, resulting in Majorana corner modes (MCMs) at four corners of a 2D domain. The calculated non-zero quadrupole moment characterizes the bulk topology. Subsequently, through a unitary transformation, an effective low-energy Hamiltonian reveals the effects of magnetic textures, resulting in an effective in-plane Zeeman field and spin-orbit coupling. This approach provides a qualitative depiction of the topological phase, substantiated by numerical validation within exact real-space model. Analytically calculated effective pairings in the bulk illuminate the microscopic behavior of the SOTSC. The comprehension of MCM emergence is aided by a low-energy edge theory, which is attributed to the interplay between effective pairings of (px + py )-type and (px + ipy )-type. Our extensive study paves the way for practically attaining the SOTSC phase by integrating noncollinear magnetic textures.

Authors:M. D. Moldavskaya, L. E. Golub, S. N. Danilov, V. V. Bel'kov, D. Weiss, S. D. Ganichev

Abstract: We report a comprehensive study of polarized infrared/terahertz photocurrents in bulk tellurium crystals. We observe different photocurrent contributions and show that, depending on the experimental conditions, they are caused by the trigonal photogalvanic effect, the transverse linear photon drag effect, and the magnetic field induced linear and circular photogalvanic effects. All observed photocurrents have not been reported before and are well explained by the developed phenomenological and microscopic theory. We show that the effects can be unambiguously distinguished by studying the polarization, magnetic field, and radiation frequency dependence of the photocurrent. At frequencies around 30 THz, the photocurrents are shown to be caused by the direct optical transitions between subbands in the valence band. At lower frequencies of 1 to 3 THz, used in our experiment, these transitions become impossible and the detected photocurrents are caused by the indirect optical transitions (Drude-like radiation absorption).

Authors:Victor Champain, Vivien Schmitt, Benoit Bertrand, Heimanu Niebojewski, Romain Maurand, Xavier Jehl, Clemens Winkelmann, Silvano De Franceschi, Boris Brun

Abstract: We report local time-resolved thermometry in a silicon nanowire quantum dot device designed to host a linear array of spin qubits. Using two alternative measurement schemes based on rf reflectometry, we are able to probe either local electron or phonon temperatures with $\mu$s-scale time resolution and a noise equivalent temperature of $3$ $\rm mK/\sqrt{\rm Hz}$. Following the application of short microwave pulses, causing local periodic heating, time-dependent thermometry can track the dynamics of thermal excitation and relaxation, revealing clearly different characteristic time scales. This work opens important prospects to investigate the out-of-equilibrium thermal properties of semiconductor quantum electronic devices operating at very low temperature. In particular, it may provide a powerful handle to understand heating effects recently observed in semiconductor spin-qubit systems.

Authors:Mohammed El Azar, Ahmed Bouhlal, Abdulaziz D. Alhaidari, Ahmed Jellal

Abstract: It is known that the appearance of Klein tunneling in graphene makes it hard to keep or localize electrons in a graphene-based quantum dot (GQD). However, a magnetic field can be used to temporarily confine an electron that is traveling into a GQD. The electronic states investigated here are resonances with a finite trapping time, also referred to as quasi-bound states. By subjecting the GDQ to a magnetic flux, we study the scattering phenomenon and the Aharonov-Bohm effect on the lifetime of quasi-bound states existing in a GQD. We demonstrate that the trapping time increases with the magnetic flux sustaining the trapped states for a long time even after the flux is turned off. Furthermore, we discover that the probability density within the GQD is also clearly improved. We demonstrate that the trapping time of an electron inside a GQD can be successfully extended by adjusting the magnetic flux parameters.

Authors:Alexander Altland, Piet W. Brouwer, Johannes Dieplinger, Matthew S. Foster, Mateo Moreno-Gonzalez, Luka Trifunovic

Abstract: Topological insulators and superconductors support extended surface states protected against the otherwise localizing effects of static disorder. Specifically, in the Wigner-Dyson insulators belonging to the symmetry classes A, AI, and AII, a band of extended surface states is continuously connected to a likewise extended set of bulk states forming a ``bridge'' between different surfaces via the mechanism of spectral flow. In this work we show that this principle becomes \emph{fragile} in the majority of non-Wigner-Dyson topological superconductors and chiral topological insulators. In these systems, there is precisely one point with granted extended states, the center of the band, $E=0$. Away from it, states are spatially localized, or can be made so by the addition of spatially local potentials. Considering the three-dimensional insulator in class AIII and winding number $\nu=1$ as a paradigmatic case study, we discuss the physical principles behind this phenomenon, and its methodological and applied consequences. In particular, we show that low-energy Dirac approximations in the description of surface states can be treacherous in that they tend to conceal the localizability phenomenon. We also identify markers defined in terms of Berry curvature as measures for the degree of state localization in lattice models, and back our analytical predictions by extensive numerical simulations. A main conclusion of this work is that the surface phenomenology of non-Wigner-Dyson topological insulators is a lot richer than that of their Wigner-Dyson siblings, extreme limits being spectrum wide quantum critical delocalization of all states vs. full localization except at the $E=0$ critical point. As part of our study we identify possible experimental signatures distinguishing between these different alternatives in transport or tunnel spectroscopy.

Authors:Nojoon Myoung, Taegeun Song, Hee Chul Park

Abstract: We investigate the effect of elastic strain on a Mach-Zehnder (MZ) interferometer created by graphene p-n junction in quantum Hall regime. We demonstrate that a Gaussian-shaped nanobubble causes detuning of the quantum Hall conductance oscillations across the p-n junction, due to the strain-induced local pseudo-magnetic fields. By performing a machine-learning-based Fourier analysis, we differentiate the nanobubble-induced Fourier component from the conductance oscillations originating from the external magnetic fields. We show that the detuning of the conductance oscillations is due to the altered pathway of quantum Hall interface channels caused by the strain-induced pseudo-magnetic fields. In the presence of the nanobubble, a new Fourier component for a magnetic flux $\Phi_{0}/2$ appears, and the corresponding MZ interferometry indicates that the enclosed area is reduced by half due to the strain-mediated pathway between two quantum Hall interface channels. Our findings suggest the potential of using graphene as a strain sensor for developments in graphene-based device fabrications and measurements technologies.

Authors:Andy Knoll, Carsten Timm

Abstract: Materials that break time-reversal or inversion symmetry possess nondegenerate electronic bands, which can touch at so-called Weyl points. The spinor eigenstates in the vicinity of a Weyl point exhibit a well-defined chirality $\pm 1$. Numerous works have studied the consequences of this chirality, for example in unconventional magnetoelectric transport. However, even a Weyl point with isotropic dispersion is not only characterized by its chirality but also by the momentum dependence of the spinor eigenstates. For a single Weyl point, this momentum-space spin structure can be brought into standard "hedgehog" form by a unitary transformation, but for two or more Weyl points, this is not possible. In this work, we show that the relative spin structure of a pair of Weyl points has strong qualitative signatures in the electromagnetic response. Specifically, we investigate the Friedel oscillations in the induced charge density due to a test charge for a centrosymmetric system consisting of two Weyl points with isotropic dispersion. The most pronounced signature is that the amplitude of the Friedel oscillations falls off as $1/r^4$ in directions in which both Weyl points exhibit the same spin structure, while for directions with inverted spin structures, the amplitude of the Friedel oscillations decreases as $1/r^3$.

Authors:Guo-Liang Guo, Han-Bing Leng, Xin Liu

Abstract: We propose a novel architecture that utilizes two 0-$\pi$ qubits based on topological Josephson junctions to implement a parity-protected superconducting qubit. The topological Josephson junctions provides protection against fabrication variations, which ensures the identical Josephson junctions required to implement the0-$\pi$ qubit. By viewing the even and odd parity ground states of a 0-$\pi$ qubit as spin-$\frac{1}{2}$ states, we construct the logic qubit states using the total parity odd subspace of two 0-$\pi$ qubits. This parity-protected qubit exhibits robustness against charge noise, similar to a singlet-triplet qubit's immunity to global magnetic field fluctuations. Meanwhile, the flux noise cannot directly couple two states with the same total parity and therefore is greatly suppressed. Benefiting from the simultaneous protection from both charge and flux noise, we demonstrate a dramatic enhancement of both $T_1$ and $T_2$ coherence times. Our work presents a new approach to engineer symmetry-protected superconducting qubits.

Authors:Jakob Kjærulff Svaneborg, Aleksander Bach Lorentzen, Fei Gao, Antti-Pekka Jauho, Mads Brandbyge

Abstract: Terahertz (THz) field pulses can now be applied in Scanning Tunnelling Microscopy (THz-STM) junction experiments to study time resolved dynamics. The relatively slow pulse compared to the typical electronic time-scale calls for approximations based on a time-scale separation. Here, we contrast three methods based on non-equilibrium Green's functions (NEGF): (i) the steady-state, adiabatic results, (ii) the lowest order dynamic expansion in the time-variation (DE), and (iii) the auxiliary mode (AM) propagation method without approximations in the time-variation. We consider a concrete THz-STM junction setup involving a hydrogen adsorbate on graphene where the localized spin polarization can be manipulated on/off by a local field from the tip electrode and/or a back-gate affecting the in-plane transport. We use steady-state NEGF combined with Density Functional Theory (DFT-NEGF) to obtain a Hubbard model for the study of the junction dynamics. Solving the Hubbard model in a mean-field approximation, we find that the near-adiabatic first order dynamical expansion provides a good description for STM voltage pulses up to the 1 V range.

Authors:Lucas Winter, Oded Zilberberg

Abstract: It is commonly believed that light cannot couple to the collective excitations of the fractional quantum Hall effect (FQHE). This assumption relies on Kohn's theorem that states that electron-electron interactions decouple from homogeneous electromagnetic fields due to galilean invariance. Here, we demonstrate that the existence of an edge breaks Kohn's theorem, and enables coupling of cavity light to the plasmonic edge modes of the FQHE. We derive the coupling using the FQHE bulk-boundary correspondence and predict the formation of experimentally detectable plasmon polaritons. We find that a single cavity mode leaves the system's topological protection intact. Interestingly, however, a multimode cavity mediates plasmon backscattering, and effectively transforms the edges of the 2D FQHE into a 1D wire. Such cavity-meditated nonlocal backscattering bodes the breakdown of the topological protection in the regime of ultra-strong photon-plasmon coupling. Our analytical framework and photoelectric findings pave the way for investigating the topological order of the FQHE via optical spectroscopic probes and provide new opportunities to control FQHE edge excitations using light.

Authors:Galina L. Klimchitskaya, Constantine C. Korikov, Vladimir M. Mostepanenko, Oleg Yu. Tsybin

Abstract: The out-of-thermal-equilibrium Casimir-Polder force between nanoparticles and dielectric substrates coated with gapped graphene is considered in the framework of the Dirac model using the formalism of the polarization tensor. This is an example of physical phenomena violating the time-reversal symmetry. After presenting the main points of the used formalism, we calculate two contributions to the Casimir-Polder force acting on a nanoparticle on the source side of a fused silica glass substrate coated with gapped graphene, which is either cooler or hotter than the environment. The total nonequilibrium force magnitudes are computed as a function of separation for different values of the energy gap and compared with those from an uncoated plate and with the equilibrium force in the presence of graphene coating. According to our results, the presence of a substrate increases the magnitude of the nonequlibrium force. The force magnitude becomes larger with higher and smaller with lower temperature of the graphene-coated substrate as compared to the equilibrium force at the environmental temperature. It is shown that with increasing energy gap the magnitude of the nonequilibrium force becomes smaller, and the graphene coating makes a lesser impact on the force acting on a nanoparticle from the uncoated substrate. Possible applications of the obtained results are discussed.

Authors:Ismael Ribeiro de Assis, Ingrid Mertig, Börge Göbel

Abstract: Magnetic skyrmions are nano-sized topologically non-trivial spin textures that can be moved by external stimuli such as spin currents and internal stimuli such as spatial gradients of a material parameter. Since the total energy of a skyrmion depends linearly on most of these parameters, like the perpendicular magnetic anisotropy, the exchange constant, or the Dzyaloshinskii-Moriya interaction strength, a skyrmion will move uniformly in a weak parameter gradient. In this paper, we show that the linear behavior changes once the gradients are strong enough so that the magnetic profile of a skyrmion is significantly altered throughout the propagation. In that case, the skyrmion experiences acceleration and moves along a curved trajectory. Furthermore, we show that when spin-orbit torques and material parameter gradients trigger a skyrmion motion, it can move on a straight path along the current or gradient direction. We discuss the significance of suppressing the skyrmion Hall effect for spintronic and neuromorphic applications of skyrmions. Lastly, we extend our discussion and compare it to a gradient generated by the Dzyaloshinskii-Moriya interaction.

Authors:Viktor Könye, Kyrylo Ochkan, Anastasiia Chyzhykova, Jan Carl Budich, Jeroen van den Brink, Ion Cosma Fulga, Joseph Dufouleur

Abstract: Measuring large electrical resistances forms an essential part of common applications such as insulation testing, but suffers from a fundamental problem: the larger the resistance, the less sensitive a canonical ohmmeter is. Here we develop a conceptually different electronic sensor by exploiting the topological properties of non-Hermitian matrices, whose eigenvalues can show an exponential sensitivity to perturbations. The ohmmeter is realized in an multi-terminal, linear electric circuit with a non-Hermitian conductance matrix, where the target resistance plays the role of the perturbation. We inject multiple currents and measure a single voltage in order to directly obtain the value of the resistance. The relative accuracy of the device increases exponentially with the number of terminals, and for large resistances outperforms a standard measurement by over an order of magnitude. Our work paves the way towards leveraging non-Hermitian conductance matrices in high-precision sensing.

Authors:Maryam Khosravian, Rouven Koch, Jose L. Lado

Abstract: Extracting Hamiltonian parameters from available experimental data is a challenge in quantum materials. In particular, real-space spectroscopy methods such as scanning tunneling spectroscopy allow probing electronic states with atomic resolution, yet even in those instances extracting effective Hamiltonian is an open challenge. Here we show that impurity states in modulated systems provide a promising approach to extracting non-trivial Hamiltonian parameters of a quantum material. We show that by combining the real-space spectroscopy of different impurity locations in a moire topological superconductor, modulations of exchange and superconducting parameters can be inferred via machine learning. We demonstrate our strategy with a physically-inspired harmonic expansion combined with a fully-connected neural network that we benchmark against a conventional convolutional architecture. We show that while both approaches allow extracting exchange modulations, only the former approach allows inferring the features of the superconducting order. Our results demonstrate the potential of machine learning methods to extract Hamiltonian parameters by real-space impurity spectroscopy as local probes of a topological state.

Authors:Shusen Liao, Yunxuan Zhu, Qian Ye, Stephen Sanders, Jiawei Yang, Alessandro Alabastri, Douglas Natelson

Abstract: Surface-enhanced Raman spectroscopy (SERS) is enabled by local surface plasmon resonances (LSPRs) in metallic nanogaps. When SERS is excited by direct illumination of the nanogap, the background heating of lattice and electrons can prevent further manipulation of the molecules. To overcome this issue, we report SERS in electromigrated gold molecular junctions excited remotely: surface plasmon polaritons (SPPs) are excited at nearby gratings, propagate to the junction, and couple to the local nanogap plasmon modes. Like direct excitation, remote excitation of the nanogap can generate both SERS emission and an open-circuit photovoltage (OCPV). We compare SERS intensity and OCPV in both direct and remote illumination configurations. SERS spectra obtained by remote excitation are much more stable than those obtained through direct excitation when photon count rates are comparable. By statistical analysis of 33 devices, coupling efficiency of remote excitation is calculated to be around 10%, consistent with the simulated energy flow.

Authors:Sergii Morozov, Torgom Yezekyan, Christian Wolff, Sergey I. Bozhevolnyi, N. Asger Mortensen

Abstract: The lowest energy states in transition metal dichalcogenide (TMD) monolayers follow valley selection rules, which have attracted vast interest due to the possibility of encoding and processing of quantum information. However, these quantum states are strongly affected by the temperature-dependent intervalley scattering causing complete valley depolarization, which is hampering any practical applications of TMD monolayers at room temperature. Therefore, for achieving clear and robust valley polarization in TMD monolayers one needs to suppress parasitic depolarization processes, which is the central challenge in the growing field of valleytronics. Here, in electron-doping experiments on TMD monolayers, we demonstrate that strong doping levels beyond $10^{13}$~cm$^{-2}$ can induce 61\% and 37\% valley contrast at room temperature in tungsten diselenide and molybdenum diselenide monolayers, respectively. Our results indicate that charged excitons in TMD monolayers can be utilized as quantum units in designing of practical valleytronic devices operating at 300 K.

Authors:Jun-Xiao Lin, Michel Hehn, Thomas Hauet, Yi Peng, Junta Igarashi, Yann Le Guen, Quentin Remy, Jon Gorchon, Gregory Malinowski, Stéphane Mangin, Julius Hohlfeld

Abstract: The discovery of all-optical ultra-fast deterministic magnetization switching has opened up new possibilities for manipulating magnetization in devices using femtosecond laser pulses. Previous studies on single pulse all-optical helicity-independent switching (AO-HIS) have mainly focused on perpendicularly magnetized thin films. This work presents a comprehensive study on AO-HIS for in-plane magnetized GdxCo100-x thin films. Deterministic single femtosecond laser pulse toggle magnetization switching is demonstrated in a wider concentration range (x=10% to 25%) compared to the perpendicularly magnetized counterparts with GdCo thicknesses up to 30 nm. The switching time strongly depends on the GdxCo100-x concentration, with lower Gd concentration exhibiting shorter switching times (less than 500 fs). Our findings in this geometry provide insights into the underlying mechanisms governing single pulse AO-HIS, which challenge existing theoretical predictions. Moreover, in-plane magnetized GdxCo100-x thin films offer extended potential for opto-spintronic applications compared to their perpendicular magnetized counterparts.

Authors:Harley D. Scammell, Mathias S. Scheurer

Abstract: We investigate the superconducting properties of inversion-symmetric twisted trilayer graphene by considering different parent states, including spin-singlet, triplet, and SO(4) degenerate states, with or without nodal points. By placing transition metal dichalcogenide layers above and below twisted trilayer graphene, spin-orbit coupling is induced in TTLG and, due to inversion symmetry, the spin-orbit coupling does not spin-split the bands. The application of a displacement field ($D_0$) breaks the inversion symmetry and creates spin-splitting. We analyze the evolution of the superconducting order parameters in response to the combined spin-orbit coupling and $D_0$-induced spin-splitting. Utilizing symmetry analysis combined with both a direct numerical evaluation and a complementary analytical study of the gap equation, we provide a comprehensive understanding of the influence of spin-orbit coupling and $D_0$ on superconductivity. These results contribute to a better understanding of the superconducting order in twisted trilayer graphene.

Authors:Jens U. Neurohr, Friederike Nolle, Thomas Faidt, Samuel Grandthyll, Anton Wittig, Michael A. Klatt, Karin Jacobs, Frank Müller

Abstract: For a smooth surface, the chemical composition can be readily evaluated by a variety of spectroscopy techniques; a prominent example is X-ray photoelectron spectroscopy (XPS), where the relative proportions of the elements are mainly determined by the intensity ratio of the element-specific photoelectrons. This deduction, however, is more intricate for a nanorough surface, such as black silicon, since the steep slopes of the geometry mimic local variations of the local emission angle. Here, we explicitly quantify this effect via an integral geometric analysis, by using so-called Minkowski tensors. Thus, we match the chemical information from XPS with topographical information from atomic force microscopy (AFM). Our method provides reliable estimates of layer thicknesses for nanorough surfaces. For our black silicon samples, we found that the oxide layer thickness is on average comparable to that of a native oxide layer. Our study highlights the impact of complex geometries at the nanoscale on the analysis of chemical properties with implications for a broad class of spectroscopy techniques.

Authors:Henry F. Legg, Richard Hess, Daniel Loss, Jelena Klinovaja

Abstract: In this Reply we respond to the comment by Antipov et al. from Microsoft Quantum on Hess et al., PRL 130, 207001 (2023). Antipov et al. reported only a single simulation and claimed it did not pass the Microsoft Quantum topological gap protocol (TGP). They have provided no parameters or data for this simulation (despite request). Regardless, in this reply we demonstrate that the trivial bulk gap reopening mechanism outlined in Hess et al., in combination with trivial ZBPs, passes the TGP and therefore can result in TGP false positives.

Authors:Danxuan Chen, Jin Jiang, Thomas F. K. Weatherley, Jean-François Carlin, Mitali Banerjee, Nicolas Grandjean

Abstract: III-nitride wide bandgap semiconductors exhibit large exciton binding energies, preserving strong excitonic effects at room temperature. On the other hand, semiconducting two-dimensional (2D) materials, including MoS$_2$, also exhibit strong excitonic effects, attributed to enhanced Coulomb interactions. This study investigates excitonic interactions between surface GaN quantum well (QW) and 2D MoS$_2$ in van der Waals heterostructures by varying the spacing between these two excitonic systems. Optical property investigation first demonstrates the effective passivation of defect states at the GaN surface through MoS$_2$ coating. Furthermore, a strong interplay is observed between MoS$_2$ monolayers and GaN QW excitonic transitions. This highlights the interest of the 2D material/III-nitride QW system to study near-field interactions, such as F\"orster resonance energy transfer, which could open up novel optoelectronic devices based on such hybrid excitonic structures.

Authors:Zhen Lian, Yuze Meng, Lei Ma, Indrajit Maity, Li Yan, Qiran Wu, Xiong Huang, Dongxue Chen, Xiaotong Chen, Xinyue Chen, Mark Blei, Takashi Taniguchi, Kenji Watanabe, Sefaattin Tongay, Johannes Lischner, Yong-Tao Cui, Su-Fei Shi

Abstract: Strongly enhanced electron-electron interaction in semiconducting moir\'e superlattices formed by transition metal dichalcogenides (TMDCs) heterobilayers has led to a plethora of intriguing fermionic correlated states. Meanwhile, interlayer excitons in a type-II aligned TMDC heterobilayer moir\'e superlattice, with electrons and holes separated in different layers, inherit this enhanced interaction and strongly interact with each other, promising for realizing tunable correlated bosonic quasiparticles with valley degree of freedom. We employ photoluminescence spectroscopy to investigate the strong repulsion between interlayer excitons and correlated electrons in a WS2/WSe2 moir\'e superlattice and combine with theoretical calculations to reveal the spatial extent of interlayer excitons and the band hierarchy of correlated states. We further find that an excitonic Mott insulator state emerges when one interlayer exciton occupies one moir\'e cell, evidenced by emerging photoluminescence peaks under increased optical excitation power. Double occupancy of excitons in one unit cell requires overcoming the energy cost of exciton-exciton repulsion of about 30-40 meV, depending on the stacking configuration of the WS2/WSe2 heterobilayer. Further, the valley polarization of the excitonic Mott insulator state is enhanced by nearly one order of magnitude. Our study demonstrates the WS2/WSe2 moir\'e superlattice as a promising platform for engineering and exploring new correlated states of fermion, bosons, and a mixture of both.

Authors:G. H. R. Bittencourt, M. Castro, A. S. Nunez, D. Altbir, S. Allende, V. L. Carvalho-Santos

Abstract: This work analyzes the propagation of a transverse domain wall (DW) motion under the action of an electric current along a nanowire (NW) with a curvature gradient. Our results evidence that the curvature gradient induces a chiral spin-transfer torque (CSTT) whose effect on the DW motion depends on the direction along which the DW points. The origin of the CSTT is explained in terms of a position and phase-dependent effective field associated with the DW profile and the electric current direction. Finally, our results reveal that this chiral mechanism can also affect the behavior of other magnetization collective modes, such as spin waves. This work shows the emergence of curvature-induced chiral spin transport and highlights a new phenomenon to be considered for designing spintronic devices.

Authors:Perry T. Mahon, Chao Lei, Allan H. MacDonald

Abstract: A delicate tension complicates the relationship between the topological magnetoelectric effect in three-dimensional $\mathbb{Z}_2$ topological insulators (TIs) and time-reversal symmetry (TRS). TRS underlies a particular $\mathbb{Z}_2$ topological classification of the electronic ground state of a bulk insulator and the associated quantization of the magnetoelectric coefficient calculated using linear response theory, but according to standard symmetry arguments simultaneously forbids any physically meaningful magnetoelectric response. This tension between theories of magnetoelectric response in bulk and finite-sized materials originates from the distinct approaches required to introduce notions of polarization and orbital magnetization in those fundamentally different environments. In this work we argue for a modified interpretation of the bulk linear response calculations in non-magnetic TIs that is more plainly consistent with TRS, and use this interpretation to discuss the effect's observation - still absent over a decade after its prediction. Our analysis is reinforced by microscopic bulk and thin film calculations carried out using a simplified but still realistic model for the well established V$_2$VI$_3$ (V $=$ (Sb,Bi) and VI $=$ (Se,Te)) family of non-magnetic $\mathbb{Z}_2$ TIs. We conclude that the topological magnetoelectric effect in non-magnetic $\mathbb{Z}_2$ TIs is activated by magnetic surface dopants, and that the charge density response to magnetic fields and the orbital magnetization response to electric fields in a given sample are controlled in part by the configuration of those dopants.

Authors:Fan Yang, Xingyu Li, Hui Zhai

Abstract: Topological charge pumping occurs in the adiabatic limit, and the non-adiabatic effect due to finite ramping velocity reduces the pumping efficiency and leads to deviation from quantized charge pumping. In this work, we discuss the relation between this deviation from quantized charge pumping and the entanglement generation after a pumping circle. In this simplest setting, we show that purity $\mathcal{P}$ of the half system reduced density matrix equals to $\mathcal{R}$ defined as $(1-\kappa)^2+\kappa^2$, where $\kappa$ denotes the pumping efficiency. In generic situations, we argue $\mathcal{P}<\mathcal{R}$ and the pumping efficiency can provide an upper bound for purity and, therefore, a lower bound for generated entanglement. To support this conjecture, we propose a solvable pumping scheme in the Rice--Mele--Hubbard model, which can be represented as brick-wall type quantum circuit model. With this pumping scheme, numerical calculation of charge pumping only needs to include at most six sites, and therefore, the interaction and the finite temperature effects can be both included reliably in the exact diagonalization calculation. The numerical results using the solvable pumping circle identify two regimes where the pumping efficiency is sensitive to ramping velocity and support the conjecture $\mathcal{P}<\mathcal{R}$ when both interaction and finite temperature effects are present.

Authors:Ivan Gnusov, Stella Harrison, Sergey Alyatkin, Kirill Sitnik, Helgi Sigurdsson, Pavlos G. Lagoudakis

Abstract: The response of superfluids to the external rotation, evidenced by emergence of quantised vortices, distinguishes them from conventional fluids. In this work, we demonstrate that the number of vortices in a stirred polariton condensate depends on the characteristic size of the employed rotating potential induced by the nonresonant laser excitation. For smaller sizes, a single vortex with a topological charge of +-1 corresponding to the stirring direction is formed. However, for larger optical traps, clusters of two or three co-rotating vortices appear in the narrow range of GHz stirring speed.

Authors:W. Callum Wareham, E. J. Nicol

Abstract: We investigate the finite-frequency optical response of systems described at low energies by Dirac-Weyl Hamiltonians with higher pseudospin $\mathcal{S}$ values. In particular, we examine the situation where a tilting term is applied in the Hamiltonian, which results in tilting of the Dirac electronic band structure. We calculate and discuss the optical conductivity for the cases $\mathcal{S}=1$, $3/2$, and $2$, in both two and three dimensions in order to demonstrate the expected signatures in the optical response. We examine both undertilted (type I) and overtilted (type II) as well as the critically-tilted case (type III). Along with the well-known case of $\mathcal{S} =1/2$, a pattern emerges for any $\mathcal{S}$. We note that in situations with multiple nested cones, such as happens for $\mathcal{S}>1$, the possibility of having one cone being type I while the other is type II allows for more rich variations in the optical signature, which we will label as type IV behavior. We also comment on the presence of optical sum rules in the presence of tilting. Finally, we discuss tilting in the $\alpha$-T$_3$ model in two dimensions, which is a hybrid of the $\mathcal{S}=1/2$ (honeycomb lattice) and $\mathcal{S}=1$ (dice or T$_3$ lattice) model with a variable Berry's phase. We contrast this model's conductivity with that of $\mathcal{S}=3/2$ and $\mathcal{S}=2$ as the resultant optical response has some similarities, although there are clear distinguishing features between the these cases.

Authors:Abigail Timmel, Xiao-Gang Wen

Abstract: It is known that $n$-degenerate Landau levels with the same spin-valley quantum number can be realized by $n$-layer graphene with rhombohedral stacking under magnetic field $B$. We find that the wave functions of degenerate Landau levels are concentrated at the surface layers of the multi-layer graphene if the dimensionless ratio $\eta = \gamma_1/(v_F\sqrt{2e\hbar B/c}) \approx 9/ \sqrt{B[\text{Tesla}]} \gg 1$, where $\gamma_1$ is the interlayer hopping energy and $v_F$ the Fermi velocity of single-layer graphene. This allows us to suggest that: 1) filling fraction $\nu=\frac12$ (or $\nu_n = 5\frac12$) non-Abelian state with Ising anyon can be realized in three-layer graphene for magnetic field $ B \lesssim 9$ Tesla; 2) filling fraction $\nu=\frac23$ (or $\nu_n = 7\frac13$) non-Abelian state with Fibonacci anyon can be realized in four-layer graphene for magnetic field $ B \sim 2 - 9 $ Tesla. Here, $\nu$ is the total filling fraction in the degenerate Landau levels, and $\nu_n$ is the filling fraction measured from charge neutrality point which determines the measured Hall conductance. We have assumed the following conditions to obtain the above results: the exchange effective of Coulomb interaction polarizes the $SU(4)$ spin-valley quantum number and effective dielectric constant $\epsilon \gtrsim 10$ to reduce the Coulomb interaction. The high density of states of multi-layer graphene helps to reduce the Coulomb interaction via screening.

Authors:Qiang Xu, Mauro Del Ben, Mahmut Sait Okyay, Min Choi, Khaled Z. Ibrahim, Bryan M. Wong

Abstract: We present a new velocity-gauge real-time, time-dependent density functional tight-binding (VG-rtTDDFTB) implementation in the open-source DFTB+ software package (https://dftbplus.org) for probing electronic excitations in large, condensed matter systems. Our VG-rtTDDFTB approach enables real-time electron dynamics simulations of large, periodic, condensed matter systems containing thousands of atoms with a favorable computational scaling as a function of system size. We provide computational details and benchmark calculations to demonstrate its accuracy and computational parallelizability on a variety of large material systems. As a representative example, we calculate laser-induced electron dynamics in a 512-atom amorphous silicon supercell to highlight the large periodic systems that can be examined with our implementation. Taken together, our VG-rtTDDFTB approach enables new electron dynamics simulations of complex systems that require large periodic supercells, such as crystal defects, complex surfaces, nanowires, and amorphous materials.

Authors:Tonghang Han, Zhengguang Lu, Giovanni Scuri, Jiho Sung, Jue Wang, Tianyi Han, Kenji Watanabe, Takashi Taniguchi, Liang Fu, Hongkun Park, Long Ju

Abstract: Ferroic orders describe spontaneous polarization of spin, charge, and lattice degrees of freedom in materials. Materials featuring multiple ferroic orders, known as multiferroics, play important roles in multi-functional electrical and magnetic device applications. 2D materials with honeycomb lattices offer exciting opportunities to engineer unconventional multiferroicity, where the ferroic orders are driven purely by the orbital degrees of freedom but not electron spin. These include ferro-valleytricity corresponding to the electron valley and ferro-orbital-magnetism supported by quantum geometric effects. Such orbital multiferroics could offer strong valley-magnetic couplings and large responses to external fields-enabling device applications such as multiple-state memory elements, and electric control of valley and magnetic states. Here we report orbital multiferroicity in pentalayer rhombohedral graphene using low temperature magneto-transport measurements. We observed anomalous Hall signals Rxy with an exceptionally large Hall angle (tan{\Theta}H > 0.6) and orbital magnetic hysteresis at hole doping. There are four such states with different valley polarizations and orbital magnetizations, forming a valley-magnetic quartet. By sweeping the gate electric field E we observed a butterfly-shaped hysteresis of Rxy connecting the quartet. This hysteresis indicates a ferro-valleytronic order that couples to the composite field E\cdot B, but not the individual fields. Tuning E would switch each ferroic order independently, and achieve non-volatile switching of them together. Our observations demonstrate a new type of multiferroics and point to electrically tunable ultra-low power valleytronic and magnetic devices.

Authors:Tomohiro Ichinose, Tatsuya Yamamoto, Takayuki Nozaki, Kay Yakushiji, Shingo Tamaru, Shinji Yuasa

Abstract: We investigated the effect of Fe segregated from partially Fe-substituted MgO (MgFeO) on the magnetic properties of CoFeB/MgFeO multilayers. X-ray photoelectron spectroscopy (XPS) as well as magnetic measurements revealed that the segregated Fe was reduced to metal and exhibited ferromagnetism at the CoFeB/MgFeO interface. The CoFeB/MgFeO multilayer showed more than 2-fold enhancement in perpendicular magnetic anisotropy (PMA) energy density compared with a standard CoFeB/MgO multilayer. The PMA energy density was further enhanced by inserting an ultrathin MgO layer in between CoFeB and MgFeO layers. Ferromagnetic resonance measurement also revealed a remarkable reduction of magnetic damping in the CoFeB/MgFeO multilayers.

Authors:Caiyun Chen, Jiangchang Zheng, Ruopeng Yu, Soumya Sankar, Hoi Chun Po, Kam Tuen Law, Berthold Jäck

Abstract: Destructive interference between electron wavefunctions on the two-dimensional (2D) kagome lattice induces an electronic flat band, which could host a variety of interesting many-body quantum states. Key to realize these proposals is to demonstrate the real space localization of kagome flat band electrons. In particular, the extent to which the often more complex lattice structure and orbital composition of realistic materials counteract the localizing effect of destructive interference, described by the 2D kagome lattice model, is hitherto unknown. We used scanning tunneling microscopy (STM) to visualize the non-trivial Wannier states of a kagome flat band at the surface of CoSn, a kagome metal. We find that the local density of states associated with the flat bands of CoSn is localized at the center of the kagome lattice, consistent with theoretical expectations for their corresponding Wannier states. Our results show that these states exhibit an extremely small localization length of two to three angstroms concomitant with a strongly renormalized quasiparticle velocity, which is comparable to that of moir\'e superlattices. Hence, interaction effects in the flat bands of CoSn could be much more significant than previously thought. Our findings provide fundamental insight into the electronic properties of kagome metals and are a key step for future research on emergent many-body states in transition metal based kagome materials.

Authors:Jannis Krumland, Michele Guerrini, Antonietta De Sio, Christoph Lienau, Caterina Cocchi

Abstract: The development of multidimensional, ultrafast spectroscopy techniques calls for the development of efficient computational schemes that allow for the simulation of such experiments and thus for the interpretation of the corresponding results. In this work, we present the development of a fully first-principles scheme to compute two-dimensional electron spectroscopy maps based on real-time time-dependent density-functional theory. The interface of this approach with the Ehrenfest scheme for molecular dynamics enables the inclusion of vibronic effects in the calculations. We demonstrate the effectiveness of this method by applying it to prototypical molecules such as benzene, pyridine, and pyrene. We discuss the role of the approximations that inevitably enter the adopted theoretical framework and set the stage for further extensions of the proposed method.

Authors:P. M. Thibado, J. C. Neu, Pradeep Kumar, Surendra Singh, L. L. Bonilla

Abstract: We theoretically consider a graphene ripple as a Brownian particle coupled to an energy storage circuit. When circuit and particle are at the same temperature, the second law forbids harvesting energy from the thermal motion of the Brownian particle, even if the circuit contains a rectifying diode. However, when the circuit contains a junction followed by two diodes wired in opposition, the approach to equilibrium may become ultraslow. Detailed balance is temporarily broken as current flows between the two diodes and charges storage capacitors. The energy harvested by each capacitor comes from the thermal bath of the diodes while the system obeys the first and second laws of thermodynamics.

Authors:Eirik Holm Fyhn, Hendrik Bentmann, Jacob Linder

Abstract: The interplay between magnetic and topological order can give rise to phenomena such as the quantum anomalous Hall effect. The extension of topological surface states into magnetic insulators (MIs) has been proposed as an alternative to using intrinsically magnetic topological insulators (TIs). Here, we theoretically study how this extension of surface states into a magnetic insulator are influenced both by the interface barrier potential separating a topological insulator and a magnetic insulator and by finite size effects in such structures. We find that the the gap in the surface states depends non-monotonically on the barrier strength. A small, but finite, barrier potential turns out to be advantageous as it permits the surface states to penetrate even further into the MI. Moreover, we find that due to finite size effects in thin samples, increasing the spin-splitting in the MI can actually decrease the gap of the surface states, in contrast to the usual expectation that the gap opens as the spin-splitting increases.

Authors:Mikkel Have Eriksen, Christos Tserkezis, N. Asger Mortensen, Joel D. Cox

Abstract: Nonlocal and quantum mechanical phenomena in noble metal nanostructures become increasingly crucial when the relevant length scales in hybrid nanostructures reach the few-nanometer regime. In practice, such mesoscopic effects at metal-dielectric interfaces can be described using exemplary surface-response functions (SRFs) embodied by the Feibelman $d$-parameters. Here we show that SRFs dramatically influence quantum electrodynamic phenomena -- such as the Purcell enhancement and Lamb shift -- for quantum emitters close to a diverse range of noble metal nanostructures interfacing different homogeneous media. Dielectric environments with higher permittivities are shown to increase the magnitude of SRFs calculated within the specular-reflection model. In parallel, the role of SRFs is enhanced in nanostructures characterized by large surface-to-volume ratios, such as thin planar metallic films or shells of core-shell nanoparticles. By investigating emitter quantum dynamics close to such plasmonic architectures, we show that decreasing the width of the metal region, or increasing the permittivity of the interfacing dielectric, leads to a significant change in the Purcell enhancement, Lamb shift, and visible far-field spontaneous emission spectrum, as an immediate consequence of SRFs. We anticipate that fitting the theoretically modelled spectra to experiments could allow for experimental determination of the $d$-parameters.

Authors:R. Binder, J. S. Schaibley, N. H. Kwong

Abstract: In a recent publication [Wurdack et al., Nat. Comm. 14:1026 (2023)], it was shown that in microcavities containing atomically thin semiconductors non-Hermitian quantum mechanics can lead to negative exciton polariton masses. We show that mass-sign reversal can occur generally in radiative resonances in two dimensions (without cavity) and derive conditions for it (critical dephasing threshold etc.). In monolayer transition-metal dichalcogenides, this phenomenon is not invalidated by the strong electron-hole exchange interaction, which is known to make the exciton massless.

Authors:Shuichi Iwakiri, Jakob Miller, Florian Lang, Jakob Prettenthaler, Takashi Taniguchi, Kenji Watanabe, Sung Sik Lee, Pascal Becker, Detlef Günther, Thomas Ihn, Klaus Ensslin

Abstract: Graphene quantum dots are promising candidates for qubits due to weak spin-orbit and hyperfine interactions. The hyperfine interaction, controllable via isotopic purification, could be the key to further improving the coherence. Here, we use isotopically enriched graphite crystals of both $^{12}$C and $^{13}$C grown by high-pressure-high-temperature method to exfoliate graphene layers. We fabricated Hall bar devices and performed quantum transport measurements, revealing mobilities exceeding $10^{5}$$\textrm{cm}^{2}/Vs$ and a long mean free path of microns, which are as high as natural graphene. Shubnikov-de Haas oscillations, quantum Hall effect up to the filling factor of one, and Brown-Zak oscillations due to the alignment of hBN and graphene are observed thanks to the high mobility. These results constitute a material platform for physics and engineering of isotopically-enriched graphene qubits.

Authors:Mahmoud Zeer, Dongwook Go, Peter Schmitz, Tom G. Saunderson, Hao Wang, Jamal Ghabboun, Stefan Blügel, Wulf Wulfhekel, Yuriy Mokrousov

Abstract: We conduct a first-principles study of Hall effects in rare-earth dichalcogenides, focusing on monolayers of the H-phase EuX$_2$ and GdX$_2$, where X = S, Se, and Te. Our predictions reveal that all EuX$_2$ and GdX$_2$ systems exhibit high magnetic moments and wide bandgaps. We observe that while in case of EuX$_2$ the $p$ and $f$ states hybridize directly below the Fermi energy, the absence of $f$ and $d$ states of Gd at the Fermi energy results in $p$-like spin-polarized electronic structure of GdX$_2$, which mediates $p$-based magnetotransport. Notably, these systems display significant anomalous, spin, and orbital Hall conductivities. We find that in GdX$_2$ the strength of correlations controls the relative position of $p$, $d$ and $f$-states and their hybridization which has a crucial impact on $p$-state polarization and the anomalous Hall effect, but not the spin and orbital Hall effect. Moreover, we find that the application of strain can significantly modify the electronic structure of the monolayers, resulting in quantized charge, spin and orbital transport in GdTe$_2$ via a strain-mediated orbital inversion mechanism taking place at the Fermi energy. Our findings suggest that rare-earth dichalcogenides hold promise as a platform for topological spintronics and orbitronics.

Authors:Muzakkiy P. M. Akhir, Edi Suprayoga, Adam B. Cahaya

Abstract: The hybrid skyrmion, a type of magnetic skyrmion with intermediate helicity between Bloch and N\'eel skyrmion, has gained more attraction. It is tolerant toward the skyrmion Hall effect and a potential candidate for quantum bits. We investigated the stabilization and helicity control of the hybrid skyrmion in a two-dimensional magnetic system using an analytical model and micromagnetic simulation. We look at the interplaying factors of the bulk ($D_b$) and interfacial ($D_i$) Dzyaloshinskii-Moriya (DM) interactions along with the dipolar interaction. We show that the hybrid skyrmion can stabilize through the interplay between interfacial DM and either bulk DM or dipolar interaction. We can also control the helicity of the hybrid skyrmion by tuning the ratio of $D_i/D_b$ when there is no dipolar interaction, or simply by adjusting the $D_i$ when the $D_b$ is absent. Our results suggest that hybrid skyrmions can exist within $0 < |D_i| < 0.4$ mJ/m$^2$ for Co-based magnetic systems.

Authors:M. Zinkiewicz, M. Grzeszczyk, T. Kazimierczuk, M. Bartos, K. Nogajewski, W. Pacuski, K. Watanabe, T. Taniguchi, A. Wysmołek, P. Kossacki, M. Potemski, A. Babiński, M. R. Molas

Abstract: Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, $i.e.$ in-plane E$'$ and out-of-plane A$'_1$, but the appearance of 1$^{st}$, 2$^{nd}$, and higher-order phonon modes is recognised. The intensity profiles of the A$'_1$ modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe$_2$ ML, the A$'_1$ mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, $i.e.$ bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.

Authors:Serban Lepadatu

Abstract: It is shown micromagnetic and atomistic spin dynamics simulations can use multiple GPUs in order to reduce computation time, but also to allow for a larger simulation size than is possible on a single GPU. Whilst interactions which depend on neighbouring spins, such as exchange interactions, may be implemented efficiently by transferring data between GPUs using halo regions, or alternatively using direct memory accesses, implementing the long-range demagnetizing interaction is the main difficulty in achieving good performance scaling, where the data transfer rate between GPUs is a significant bottleneck. A multi-GPU convolution algorithm is developed here, which relies on single-GPU FFTs executed in parallel. It is shown that even for micromagnetic simulations where the demagnetizing interaction computation time dominates, good performance scaling may be achieved, with speedup factors up to 1.8, 2.5, and 3.1, for 2, 3, and 4 GPUs respectively. The code developed here can be used for any number of GPUs in parallel, with performance scaling strongly dependent on inter-GPU data transfer rate and connection topology. This is further improved in micromagnetic simulations which include a spin transport solver, obtaining speedup factors up to 1.96, 2.8, and 3.7, for 2, 3, and 4 GPUs respectively. The best case scenario is obtained for atomistic spin dynamics simulations, where the demagnetizing interaction is implemented with spin-averaged cells. Using a single workstation with 4 GPUs, it is shown atomistic spin dynamics simulations with up to 1 billion spins, and atomistic Monte Carlo simulations with up to 2 billion spins are possible, with a near-ideal performance scaling.

Authors:Vera Uzunova, Lukas Körber, Agapi Kavvadia, Gwendolyn Quasebarth, Helmut Schultheiss, Attila Kákay, Boris Ivanov

Abstract: Soft magnetic dots in the form of thin rings have unique topological properties. They can be in a vortex state with no vortex core. Here, we study the magnon modes of such systems both analytically and numerically. In an external magnetic field, magnetic rings are characterized by easy-cone magnetization and shows a giant splitting of doublets for modes with the opposite value of the azimuthal mode quantum number. The effect of the splitting can be refereed as a magnon analog of the topology-induced Aharonov-Bohm effect. For this we develop an analytical theory to describe the non-monotonic dependence of the mode frequencies on the azimuthal mode number, influenced by the balance between the local exchange and non-local dipole interactions.

Authors:Erik Zimmermann, Justus Teller, Michael Schleenvoigt, Gerrit Behner, Peter Schüffelgen, Hans Lüth, Detlev Grützmacher, Thomas Schäpers

Abstract: We present the current-induced switching of the internal magnetization direction in a magnetic topological insulator/topological insulator heterostructure in the quantum anomalous Hall regime. The switching process is based on the bias current dependence of the coercive field, which is attributed to the effect of the spin-orbit torque provided by the unpolarized bias current. Increasing the bias current leads to a decrease in the magnetic order in the sample. When the applied current is subsequently reduced, the magnetic moments align with an externally applied magnetic field, resulting in repolarization in the opposite direction. This includes a reversal of the spin polarisation and hence a reversal of the chiral edge mode. Possible applications in spintronic devices are discussed.

Authors:Victor H. González, Artem Litvinenko, Roman Khymyn, Johan Åkerman

Abstract: A spinwave Ising machine (SWIM) is a newly proposed type of time-multiplexed hardware solver for combinatorial optimization that employs feedback coupling and phase sensitive amplification to map an Ising Hamiltonian into phase-binarized propagating spin-wave RF pulses in an Yttrium-Iron-Garnet (YIG) film. In this work, we increase the mathematical complexity of the SWIM by adding a global Zeeman term to a 4-spin MAX-CUT Hamiltonian using a continuous external electrical signal with the same frequency as the spin pulses and phase locked with with one of the two possible states. We are able to induce ferromagnetic ordering in both directions of the spin states despite antiferromagnetic pairwise coupling. Embedding a planar antiferromagnetic spin system in a magnetic field has been proven to increase the complexity of the graph associated to its Hamiltonian and thus this straightforward implementation helps explore higher degrees of complexity in this evolving solver.

Authors:A. Erpenbeck, E. Gull, G. Cohen

Abstract: Nonequilibrium quantum transport is of central importance in nanotechnology. Its description requires the understanding of strong electronic correlations, which couple atomic-scale phenomena to the nanoscale. So far, research in correlated transport focused predominantly on few-channel transport, precluding the investigation of cross-scale effects. Recent theoretical advances enable the solution of models that capture the interplay between quantum correlations and confinement beyond a few channels. This problem is the focus of this study. We consider an atomic impurity embedded in a metallic nanosheet spanning two leads, showing that transport is significantly altered by tuning only the phase of a single, local hopping parameter. Furthermore -- depending on this phase -- correlations reshape the electronic flow throughout the sheet, either funneling it through the impurity or scattering it away from a much larger region. This demonstrates the potential for quantum correlations to bridge length scales in the design of nanoelectronic devices and sensors.

Authors:Partha Das, Samit Kumar Hazra, Tarak Nath Dey

Abstract: We theoretically investigate rapid adiabatic passage(RAP)-based super-resolution imaging in a two-level quantum dot system interacting with two structured beams. To understand the physical mechanism behind the formation of super-resolution for the experiment of Kaldewey {\it et. al.,}[Nature Photonics 10.1038/s41566-017-0079-y (2018)], we first use Liouville's density matrix where photon-mediated radiative and non-radiative decays are incorporated. A suitably chosen spatiotemporal envelope of the structured beams enables the formation of a super-resolution image. We also find that the feature size of the image depends on the intensity of the Laguerre Gaussian beam(LG). However, the created image resolution undergoes distortion due to the existence of a low-intensity circular ring. The unwanted circular ring arises from the dominance of the LG beam tail over the super-Gaussian(SG) beam tail, initiating the residual population transfer from the ground state to the excited state. This limitation can be overcome by using the Bessel-modulated truncated structured LG and SG beams. We next study the dynamics of the semiconductor quantum dot system at finite temperatures wherein the phonon interaction becomes imperative. We employ the polaron-transformed master equation to explore the system at higher temperatures. Our numerical results confirm that the sharpness of the image remains intact at low temperatures with weak phonon coupling. Hence, the proposed scheme may open up applications in nano-scale imaging with quantum dots.

Authors:Mingsheng Tian, Ivan Velkovsky, Tao Chen, Fengxiao Sun, Qiongyi He, Bryce Gadway

Abstract: Weyl points are three-dimensional linear points between bands that exhibit unique stability to perturbations and are accompanied by topologically non-trivial surface states. However, the discovery and control of Weyl points in nature poses significant challenges. While recent advances have allowed for engineering Weyl points in photonic crystals and metamaterials, the topological transition between Weyl semimetals with distinct types of Weyl points remains yet to be reported. Here, exploiting the flexible measurement-feedback control of synthetic mechanical systems, we experimentally simulate Weyl semimetals and observe for the first time the transition between states with type-I and type-II Weyl points. We directly observe the change in the band structures accompanying the transition and identify the Fermi arc surface states connecting the Weyl points. Further, making use of the non-reciprocal feedback control, we demonstrate that the introduction of non-Hermiticity significantly impacts the topological transition point, as well as the edge localization of the Fermi arc surface states. Our findings offer valuable insights into the design and realization of Weyl points in mechanical systems, providing a promising avenue for exploring novel topological phenomena in non-Hermitian physics.

Authors:Deepankur Thureja

Abstract: Confining particles to distances below their de Broglie wavelength discretizes their motional state. This fundamental effect is observed in many physical systems, ranging from electrons confined in atoms or quantum dots to ultracold atoms trapped in optical tweezers. In solid-state photonics, a long-standing goal has been to achieve fully tunable quantum confinement of optically active electron-hole pairs known as excitons. To confine excitons, existing approaches mainly rely on material modulation, which suffers from poor control over the energy and position of trapping potentials. This has severely impeded the engineering of large-scale quantum photonic systems. In this doctoral thesis, we demonstrate electrically controlled quantum confinement of neutral excitons in two-dimensional semiconductors. By combining gate-defined in-plane electric fields with inherent interactions between excitons and free charges in a lateral p-i-n junction, we achieve tunable exciton confinement lengths reaching values below 10 nm. Quantization of excitonic motion manifests in the measured optical response as a ladder of discrete voltage-dependent states below the continuum. Moreover, we observe that our confining potentials lead to a strong modification of the relative wave function of excitons. We further highlight the versatility of our approach by extending our confinement scheme to create quantum-dot-like zero-dimensional structures with a fully tunable confinement length. Our technique provides an experimental route towards achieving polariton blockade and creating scalable arrays of identical single-photon sources, which has wide-ranging implications for realizing strongly correlated photonic phases and on-chip optical quantum information processors.

Authors:Jiaan Qi, Zhi-Hai Liu, H. Q. Xu

Abstract: We study the implications of spin-orbit interaction (SOI) for two-qubit gates (TQGs) in semiconductor spin qubit platforms. The exchange interaction governing qubit pairs is anisotropic under SOI, posing a problem for conventional TQGs derived under the Heisenberg exchange. After developing a concise form of the effective two-qubit Hamiltonian under SOI, we use it to derive properties of rotating-frame evolution. Two main observations are made. First, in contrary to past belief, we find that an appropriate amount of SOI can significantly enhance the controlled-phase gate fidelity compared to the no-SOI case. Second, SOI enables novel two-qubit dynamics, that are conventionally inaccessible through DC evolution, such as the reflection gate and the controlled-not gate.

Authors:Hui Zeng, Wenhui Duan, Huaqing Huang

Abstract: We generalize the nested Wilson loop formalism, which has been playing an important role in the study of topological quadrupole insulators, to two-dimensional (2D) and 3D nonsymmorphic materials with higher-order topology. In particular, certain 3D Dirac semimetals exhibit 1D higher-order Fermi arc (HOFA) states localizing on hinges where two surfaces meet and connecting the projection of the bulk Dirac points. We discover that the generalized nested Berry phase (gNBP) derived from this formalism is the bulk topological indicator determining the existence/absence of HOFAs, revealing a direct bulk-hinge correspondence in 3D Dirac semimetals. Finally, we study the Dirac semimetals NaCuSe and KMgBi based on first-principles calculations and explicitly show that the change in gNBP adjacent to the Dirac point corresponds to the termination of HOFAs at the projection of Dirac points on the hinge. Our findings not only improve the understanding of the bulk-hinge correspondence in topological Dirac semimetals but also provide a general formalism for studying the higher-order topology in nonsymmorphic systems.

Authors:O. V. Kibis, M. V. Boev, D. S. Eliseev, V. M. Kovalev

Abstract: Within the Floquet theory of periodically driven quantum systems, we demonstrate that a circularly polarized off-resonant electromagnetic field can destroy the electron states bound by three-dimensional attractive potentials. As a consequence, the optically induced delocalization of bound electrons appears. The effect arises from the changing of topological structure of a potential landscape under a circularly polarized off-resonant electromagnetic field which turns simply connected potentials into doubly connected ones. Possible manifestations of the effect are discussed for conduction electrons in condensed-matter structures.

Authors:Yuya Ominato, Ai Yamakage, Mamoru Matsuo

Abstract: We study a dynamical spin response of surface Majorana fermions in a topological superconductor-magnet hybrid under microwave irradiation. We find a method to toggle between dissipative and non-dissipative Majorana Ising spin dynamics by adjusting the external magnetic field angle and the microwave frequency. This reflects the topological nature of the Majorana fermions, enhancing the Gilbert damping of the magnet, thereby, providing a detection method for the Majorana Ising spins. Our findings illuminate a magnetic probe for Majorana fermions, paving the path to innovative spin devices.

Authors:Kai-Zhi Bai, Bo Fu, Zhenyu Zhang, Shun-Qing Shen

Abstract: The quantum anomalous Hall effect (QAHE) is a topological state of matter with a quantized Hall resistance. It has been observed in some two-dimensional insulating materials such as magnetic topological insulator films and twisted bilayer graphene. These materials are insulating in the bulk, but possess chiral edge states carrying the edge current around the systems. Here we discover a metallic QAHE in a topological insulator film with magnetic sandwich heterostructure, in which the Hall conductance is quantized to $e^{2}/h$, but the longitudinal conductance remains finite. This effect is attributed to the existence of a pair of massless Dirac cones of surface fermions, with each contributing half of the Hall conductance due to quantum anomaly. It is not characterized by a Chern number and not associated to any chiral edge states. Our study offers novel insights into topological transport phenomena and topological metallic states of matter.

Authors:Lev V. Ginzburg, Yuze Wu, Marc P. Röösli, Pedro Rosso Gomez, Rebekka Garreis, Chuyao Tong, Veronika Stará, Carolin Gold, Khachatur Nazaryan, Serhii Kryhin, Hiske Overweg, Christian Reichl, Matthias Berl, Takashi Taniguchi, Kenji Watanabe, Werner Wegscheider, Thomas Ihn, Klaus Ensslin

Abstract: We measure electron transport through point contacts in an electron gas in AlGaAs/GaAs heterostructures and graphene for a range of temperatures, magnetic fields and electron densities. We find a magnetoconductance peak around B = 0. With increasing temperature, the width of the peak increases monotonically, while its amplitude first increases and then decreases. For GaAs point contacts the peak is particularly sharp at relatively low temperatures $T\approx$1.5 K: the curve rounds on a scale of few tens of $\mu$T hinting at length scales of several millimeters for the corresponding scattering processes. We propose a model based on the transition between different transport regimes with increasing temperature: from ballistic transport to few electron-electron scatterings to hydrodynamic superballistic flow to hydrodynamic Poiseuille-like flow. The model is in qualitative and, in many cases, quantitative agreement with the experimental observations.

Authors:Fan Xu, Zheng Sun, Tongtong Jia, Chang Liu, Cheng Xu, Chushan Li, Yu Gu, Kenji Watanabe, Takashi Taniguchi, Bingbing Tong, Jinfeng Jia, Zhiwen Shi, Shengwei Jiang, Yang Zhang, Xiaoxue Liu, Tingxin Li

Abstract: The interplay between strong correlations and topology can lead to the emergence of intriguing quantum states of matter. One well-known example is the factional quantum Hall effect, where exotic electron fluids with fractional charge excitations form in partially filled landau levels. The emergence of topological moir\'e flat bands provides exciting opportunities to realize the lattice analogs of both the integer and fractional quantum Hall states without the need for an external magnetic field. These states are known as the integer and fractional quantum anomalous Hall (IQAH and FQAH) states. Here, we present direct transport evidence of the existence of both IQAH and FQAH states in twisted bilayer MoTe2 (AA stacked). At zero magnetic field, we observe well-quantized Hall resistance of h/e2 around moir\'e filling factor {\nu} = -1 (corresponding to one hole per moir\'e unit cell), and nearly-quantized Hall resistance of 3h/2e2 around {\nu} = -2/3, respectively. Concomitantly, the longitudinal resistance exhibits distinct minima around {\nu} = -1 and -2/3. The application of an electric field induces topological quantum phase transition from the IQAH state to a charge transfer insulator at {\nu} = -1, and from the FQAH state to a generalized Wigner crystal state, further transitioning to a metallic state at {\nu} = -2/3. Our study paves the way for the investigation of fractional charge excitations and anyon statistics at zero magnetic field based on semiconductor moir\'e materials.

Authors:Yasushi Shinohara, Haruki Sanada, Katsuya Oguri

Abstract: The tunneling process, a prototypical phenomenon of nonperturbative dynamics, is a natural consequence of photocarrier generation in materials irradiated by a strong laser. Common treatments for Zener tunneling are based on a one-body problem with a field-free electronic structure. In a literature (Ikemachi et al., Phys. Rev. A 98, 023415 (2018)), a characteristic of gap shrinking or excitation can occur due to the electron-hole interaction for slow and strong time-varying electric fields. We have developed a theoretical framework called the quasi-Hartree-Fock (qHF) method to enable a more flexible imitation of the electronic structures and electron-hole attraction strength of materials compared to the original Hartree-Fock method. In the qHF framework, band gap, reduced effective mass, and electron-hole interaction strength can be independently selected to reproduce common crystals. In this study, we investigate the effect of electron-hole attraction on Zener tunneling subjected to a DC electric field for four different systems using the qHF method. Our findings demonstrate that the electron-hole attraction promotes the tunneling rates in all four material systems assumed as examples. Specifically, systems that have a strong electron-hole interaction show a few factor enhancements for tunneling rates under DC fields, while systems with a weak interaction show higher enhancements of a few tens of percent.

Authors:Takeru Utsugi, Takuma Kuno, Noriyuki Lee, Ryuta Tsuchiya, Toshiyuki Mine, Digh Hisamoto, Shinichi Saito, Hiroyuki Mizuno

Abstract: The ability to transport single electrons on a quantum dot array dramatically increases the freedom in designing quantum computation schemes that can be implemented on solid-state devices. So far, however, routing schemes to precisely control the transport paths of single electrons have yet to be established. Here, we propose a silicon single-electron router that transports pumped electrons along the desired route on the branches of a T-shaped quantum dot array by inputting a synchronous phase-controlled signal to multiple gates. Notably, we show that it is possible to achieve a routing accuracy above 99% by implementing assist gates in front of the branching paths. The results suggest new possibilities for fast and accurate transport of single electrons on quantum dot arrays.

Authors:Krzysztof Sawicki, Dmitriy Dovzhenko, Yuan Wang, Helgi Sigurðsson, Pavlos G. Lagoudakis

Abstract: We study the magneto-photoluminescence of an optically trapped exciton-polariton condensate in a planar semiconductor microcavity with multiple In0.08Ga0.92As quantum wells. Extremely high condensate coherence time and continuous control over the polariton confinement are amongst the advantages provided by optical trapping. This allows us to resolve magnetically induced {\mu}eV fine energy shifts in the condensate, and identify unusual dynamical regions in its parameter space. We observe polariton Zeeman splitting and, in small traps with tight confinement, demonstrate its full parametric screening when the condensate density exceeds a critical value, reminiscent of the spin- Meissner effect. For larger optical traps, we observe a complete inversion in the Zeeman splitting as a function of power, underlining the importance of condensate confinement and interactions with its background reservoir excitons.

Authors:Jiang Yao, Linhu Li

Abstract: Higher-order topological insulators (HOTIs) are a novel type of topological phases which supports $d$-dimensional topological boundary states in $D$-dimensional systems with $D-d>1$. In this work, we theoretically predict that interlayer couplings in AB-stacked bilayer transition-metal dichalcogenides (TMDs) lead to the emergence of extrinsic second-order topological phases, where corner states are induced by the band inversion of zigzag edge bands. We find that the systems feature a quantized multiband Berry phase defined for a zigzag nanoribbon geometry, unveiling the nontrivial topological properties of its two zigzag edges. With detailed investigation into the bilayer TMDs under different geometries, we find two types of boundary-obstructed corner states arising from different corner terminations of either the same type of or heterogeneous zigzag edges. The topological nature of these corner states and their degeneracy is further analyzed with both the crystalline symmetries of different geometries, and a topological phase transition of the Berry phase induced by a layer-dependent onsite energy.

Authors:Baksa Kolok, András Pályi

Abstract: A quantum system constrained to a degenerate energy eigenspace can undergo a nontrival time evolution upon adiabatic driving, described by a non-Abelian Berry phase. This type of dynamics may provide logical gates in quantum computing that are robust against timing errors. A strong candidate to realize such holonomic quantum gates is an electron or hole spin qubit trapped in a spin-orbit-coupled semiconductor, whose twofold Kramers degeneracy is protected by time-reversal symmetry. Here, we propose and quantitatively analyze protocols to measure the non-Abelian Berry phase by pumping a spin qubit through a loop of quantum dots. One of these protocols allows to characterize the local internal Zeeman field directions in the dots of the loop. We expect a near-term realisation of these protocols, as all key elements have been already demonstrated in spin-qubit experiments. These experiments would be important to assess the potential of holonomic quantum gates for spin-based quantum information processing.

Authors:Koushik R. Das, Sudipta Dutta

Abstract: Electronic conductance through a single molecule is sensitive towards its structural orientation between two electrodes, owing to the distribution of molecular orbitals and their coupling to the electrode levels, that are governed by quantum confinement effects. Here, we vary the contact geometry of the porphine molecule by attaching two Au tip electrodes that resemble the mechanical break junction, via thiol anchoring groups. We investigate the current-voltage characteristics of all the contact geometries using non-equilibrium Green's function formalism along with density functional theory and tight-binding framework. We observe varying current responses with changing contact sites, originating from varied wave-function delocalization and quantum interference effect. Our calculations show asymmetric current-voltage characteristics under forward and reverse biases due to structural asymmetry of the tip electrodes in either sides of the molecule. We establish this phenomenon as a universal feature for any molecular electronic device, irrespective of the inherent structural symmetry of a molecule. This will provide fundamental insights of electronic transport through single molecule in real experimental setup. Furthermore, our observations of varying current response can further motivate the fabrication of sensor devices with porphine based biomolecules that control important physiological activities, in view of their applications in advanced diagnostics.

Authors:S. Mikkola, V. Shahnazaryan, I. Chestnov, I. Iorsh

Abstract: The existence of spontaneous magnetization that fingerprints a ground-state ferromagnetic order was recently observed in two-dimensional (2D) van der Waals materials. Despite progress in the fabrication and manipulation of the atom-thick magnets, investigation of nanoscale magnetization properties is still challenging due to the concomitant technical issues. We propose a promising approach for a direct visualization of the domain walls formed in 2D magnetic materials. By interfacing 2D magnet with a transition metal dichalcogenide (TMD) monolayer, the strong proximity effects enable pinning the TMD excitons on the domain wall. The emergent localization stems from the proximity-induced exchange mixing between spin-dark and spin-bright TMD excitons due to the local in-plane magnetization characteristic of the domain wall in the magnetic monolayer.

Authors:Alexandre Assouline, Taige Wang, Haoxin Zhou, Liam A. Cohen, Fangyuan Yang, Ruining Zhang, Takashi Taniguchi, Kenji Watanabe, Roger S. K. Mong, Michael P. Zaletel, Andrea F. Young

Abstract: Bernal bilayer graphene hosts even denominator fractional quantum Hall states thought to be described by a Pfaffian wave function with nonabelian quasiparticle excitations. Here we report the quantitative determination of fractional quantum Hall energy gaps in bilayer graphene using both thermally activated transport and by direct measurement of the chemical potential. We find a transport activation gap of 5.1K at B = 12T for a half-filled N=1 Landau level, consistent with density matrix renormalization group calculations for the Pfaffian state. However, the measured thermodynamic gap of 11.6K is smaller than theoretical expectations for the clean limit by approximately a factor of two. We analyze the chemical potential data near fractional filling within a simplified model of a Wigner crystal of fractional quasiparticles with long-wavelength disorder, explaining this discrepancy. Our results quantitatively establish bilayer graphene as a robust platform for probing the non-Abelian anyons expected to arise as the elementary excitations of the even-denominator state.

Authors:Huan-Wen Wang, Bo Fu, Shun-Qing Shen

Abstract: The pursuit of understanding parity anomaly in condensed matter systems has led to significant advancements in both theoretical and experimental research in recent years. In this study, we explore the parity anomaly of massless Dirac fermions in a semimagnetic topological insulator (TI) thin film subjected to a finite magnetic field. Our findings reveal an anomalous half-quantized Hall conductance arising from the occupied electronic states far below the Fermi level, which is directly associated with the parity anomaly. This observation demonstrates a crossover from one-half quantized Hall conductance in a metallic phase at zero field to one or zero quantized Hall conductance in the insulating phase at a strong field in the presence of disorders, serving as a key indicator for confirming parity anomaly. Our work provides valuable insights into the intricate relationship between band topology in condensed matter systems and quantum anomaly in quantum field theory.

Authors:Anuranan Das, Adil Khan, Ankan Mukherjee, Bhaskaran Muralidharan

Abstract: Recent breakthroughs in the transport spectroscopy of 2-D material quantum-dot platforms have engendered a fervent interest in spin-valley qubits. In this context, Pauli blockades in double quantum dot structures form an important basis for multi-qubit initialization and manipulation. Focusing on double quantum dot structures, and the experimental results, we first build theoretical models to capture the intricate interplay between externally fed gate voltages and the physical properties of the 2-D system in such an architecture, allowing us to effectively simulate Pauli blockades. Employing the master equations for transport and considering extrinsic factors such as electron-photon interactions, we thoroughly investigate all potential occurrences of Pauli blockades. Notably, our research reveals two remarkable phenomena: (i) the existence of multiple resonances within a bias triangle, and (ii) the occurrence of multiple Pauli blockades. Leveraging our model to train a machine learning algorithm, we successfully develop an automated method for real-time detection of multiple Pauli blockade regimes. Through numerical predictions and validations against test data, we identify where and how many Pauli blockades are likely to occur. We propose that our model can effectively detect the generic class of Pauli blockades in practical experimental setups and hence serves as the foundation for future experiments on qubits that utilize 2-D material platforms.

Authors:Xin Cong, Parisa Ali Mohammadi, Mingyang Zheng, Kenji Watanabe, Takashi Taniguchi, Daniel Rhodes, Xiao-Xiao Zhang

Abstract: The interactions between charges and excitons involve complex many-body interactions at high densities. The exciton-polaron model has been adopted to understand the Fermi sea screening of charged excitons in monolayer transition metal dichalcogenides. The results provide good agreement with absorption measurements, which are dominated by dilute bright exciton responses. Here we investigate the Fermi sea dressing of spin-forbidden dark excitons in monolayer WSe2. With a Zeeman field, the valley-polarized dark excitons show distinct p-doping dependence in photoluminescence when the carriers reach a critical density. This density can be interpreted as the onset of strongly modified Fermi sea interactions and shifts with increasing exciton density. Through valley-selective excitation and dynamics measurements, we also infer an intervalley coupling between the dark trions and exciton-polarons mediated by the many-body interactions. Our results reveal the evolution of Fermi sea screening with increasing exciton density and the impacts of polaron-polaron interactions, which lay the foundation for understanding electronic correlations and many-body interactions in 2D systems.

Authors:Daniel Miravet, Abdulmenaf Altıntaş, Alina Wania Rodrigues, Maciej Bieniek, Marek Korkusinski, Paweł Hawrylak

Abstract: We develop here a theory of the electronic properties of a finite number of valence holes in gated WSe$_2$ quantum dots, considering the influence of spin, valley, electronic orbitals, and many-body interactions. The single-particle wave functions are constructed by combining the spin-up and down states of the highest valence bulk bands employing a multi-million atom ab-initio based tight-binding model solved in the wave-vector space, allowing to study up to 100 nm radius quantum dots atomistically. The effects of the many-body interactions are determined using the configuration interaction (CI) technique, applied up to $N = 6$ holes occupying up to 6 electronic shells with 42 orbitals. Our results show that N=2 holes are in valley and spin anti-ferromagnetic ground state, independent of the interaction strength and the quantum dot size. However, we predict that higher number of holes can undergo a transition to spontaneously broken symmetry valley and spin polarized ferromagnetic phases, highlighting the interplay between the many-body effects and the quantum dot lateral size and confining potential depth.

Authors:Navketan Batra, Zezhu Wei, Smitha Vishveshwara, D. E. Feldman

Abstract: Anyonic Fabry-P\'erot and Mach-Zehnder interferometers have been proposed theoretically and implemented experimentally as tools to probe electric charges and statistics of anyons. The experimentally observed visibility of Aharonov-Bohm oscillations is maximal at a high transmission through an interferometer but simple theoretical expressions for the electric currents and noises are only available at low visibility. We consider an alternative version of a Mach-Zehnder interferometer, in which anyons tunnel between co-propagating chiral channels on the edges of quantum Hall liquids at the filling factors $n/(2n+1)$. We find simple exact solutions for any transmission. The solutions allow a straight-forward interpretation in terms of fractional charges and statistics.

Authors:Tomonari Mizoguchi, Yoshihito Kuno, Yasuhiro Hatsugai

Abstract: Spatial non-uniformity in tight-binding models serves as a source of rich phenomena. In this paper, we study a diamond-chain tight-binding model with a spatially-modulated magnetic flux at each plaquette. In the numerical studies with various combinations of the minimum and maximum flux values, we find the characteristic dynamics of a particle, namely, a particle slows down when approaching the plaquette with $\pi$-flux. This originates from the fact that the sharply localized eigenstates exist around the $\pi$-flux plaquette. These localized modes can be understood from a squared model of the original one. This characteristic blocked dynamics will be observed in photonic waveguides or cold atoms.

Authors:Tenta Tani, Takuto Kawakami, Mikito Koshino

Abstract: We calculate the perpendicular electrical conductivity in twisted three-dimensional graphite (rotationally-stacked graphite pieces) by using the effective continuum model and the recursive Green's function method. In the low twist angle regime $(\theta \lesssim 2^\circ)$, the conductivity shows a non-monotonous dependence with a peak and dip structure as a function of the twist angle. By analyzing the momentum-resolved conductance and the local density of states, this behavior is attributed to the Fano resonance between continuum states of bulk graphite and interface-localized states, which is a remnant of the flat band in the magic-angle twisted bilayer graphene. We also apply the formulation to the high-angle regime near the commensurate angle $\theta \approx 21.8^\circ$, and reproduce the conductance peak observed in the experiment.

Authors:Partha Goswami, Udai Prakash Tyagi

Abstract: In this exceedingly short review article, we have provided some information on acoustic topological insulator for pedagogical purpose. Since, intrinsically acoustic systems do not have Kramers doublets due to spin-zero status, artificially acoustic spin-half states could be engineered as reported in refs. 5-26 maintaining time reversal symmetry. The high point of this article is an explanation of emergent Dirac physics in acoustic topological insulators.

Authors:Danielle Holmes CQC2T, School of Electrical Engineering and Telecommunications, UNSW Sydney, Australia, Benjamin Wilhelm CQC2T, School of Electrical Engineering and Telecommunications, UNSW Sydney, Australia, Alexander M. Jakob CQC2T, School of Physics, The University of Melbourne, Australia, Xi Yu CQC2T, School of Electrical Engineering and Telecommunications, UNSW Sydney, Australia, Fay E. Hudson CQC2T, School of Electrical Engineering and Telecommunications, UNSW Sydney, Australia Diraq, Sydney, Australia, Kohei M. Itoh School of Fundamental Science and Technology, Keio University, Japan, Andrew S. Dzurak CQC2T, School of Electrical Engineering and Telecommunications, UNSW Sydney, Australia Diraq, Sydney, Australia, David N. Jamieson CQC2T, School of Physics, The University of Melbourne, Australia, Andrea Morello CQC2T, School of Electrical Engineering and Telecommunications, UNSW Sydney, Australia

Abstract: Donor spins in silicon-28 ($^{28}$Si) are among the most performant qubits in the solid state, offering record coherence times and gate fidelities above 99%. Donor spin qubits can be fabricated using the semiconductor-industry compatible method of deterministic ion implantation. Here we show that the precision of this fabrication method can be boosted by implanting molecule ions instead of single atoms. The bystander ions, co-implanted with the dopant of interest, carry additional kinetic energy and thus increase the detection confidence of deterministic donor implantation employing single ion detectors to signal the induced electron-hole pairs. This allows the placement uncertainty of donor qubits to be minimised without compromising on detection confidence. We investigate the suitability of phosphorus difluoride (PF$_2^+$) molecule ions to produce high quality P donor qubits. Since $^{19}$F nuclei have a spin of $I = 1/2$, it is imperative to ensure that they do not hyperfine couple to P donor electrons as they would cause decoherence by adding magnetic noise. Using secondary ion mass spectrometry, we confirm that F diffuses away from the active region of qubit devices while the P donors remain close to their original location during a donor activation anneal. PF$_2$-implanted qubit devices were then fabricated and electron spin resonance (ESR) measurements were performed on the P donor electron. A pure dephasing time of $T_2^* = 20.5 \pm 0.5$ $\mu$s and a coherence time of $T_2^{Hahn} = 424 \pm 5$ $\mu$s were extracted for the P donor electron-values comparable to those found in previous P-implanted qubit devices. Closer investigation of the P donor ESR spectrum revealed that no $^{19}$F nuclear spins were found in the vicinity of the P donor. Molecule ions therefore show great promise for producing high-precision deterministically-implanted arrays of long-lived donor spin qubits.

Authors:Sergey Trishin, Christian Lotze, Nils Krane, Katharina J. Franke

Abstract: Two-dimensional (2D) transition-metal dichalcogenides (TMDC) are considered highly promising platforms for next-generation optoelectronic devices. Owing to its atomically thin structure, device performance is strongly impacted by a minute amount of defects. Although defects are usually considered to be disturbing, defect engineering has become an important strategy to control and design new properties of 2D materials. Here, we produce single S vacancies in a monolayer of MoS$_2$ on Au(111). Using a combination of scanning tunneling and atomic force microscopy, we show that these defects are negatively charged and give rise to a Kondo resonance, revealing the presence of an unpaired electron spin exchange coupled to the metal substrate. The strength of the exchange coupling depends on the density of states at the Fermi level, which is modulated by the moir\'e structure of the MoS$_2$ lattice and the Au(111) substrate. In the absence of direct hybridization of MoS$_2$ with the metal substrate, the S vacancy remains charge-neutral. Our results suggest that defect engineering may be used to induce and tune magnetic properties of otherwise non-magnetic materials.

Authors:Biswajit Pabi, Štepán Marek, Adwitiya Pal, Puja Kumari, Soumya Jyoti Ray, Arunabha Thakur, Richard Korytár, Atindra Nath Pal

Abstract: Achieving highly transmitting molecular junctions through resonant transport at low bias is key to the next-generation low-power molecular devices. Although, resonant transport in molecular junctions was observed by connecting a molecule between the metal electrodes via chemical anchors by applying a high source-drain bias (> 1V), the conductance was limited to < 0.1 G$_0$, G$_0$ being the quantum of conductance. Here, we report electronic transport measurements by directly connecting a Ferrocene molecule between Au electrodes at the ambient condition in a mechanically controllable break junction setup (MCBJ), revealing a conductance peak at ~ 0.2 G$_0$ in the conductance histogram. A similar experiment was repeated for Ferrocene terminated with amine (-NH2) and cyano (-CN) anchors, where conductance histograms exhibit an extended low conductance feature including the sharp high conductance peak, similar to pristine ferrocene. Statistical analysis of the data along with density functional theory-based transport calculation suggests the possible molecular conformation with a strong hybridization between the Au electrodes and Fe atom of Ferrocene molecule is responsible for a near-perfect transmission in the vicinity of the Fermi energy, leading to the resonant transport at a small applied bias (< 0.5V). Moreover, calculations including Van der Waals/dispersion corrections reveal a covalent like organometallic bonding between Au and the central Fe atom of Ferrocene, having bond energies of ~ 660 meV. Overall, our study not only demonstrates the realization of an air-stable highly transmitting molecular junction, but also provides an important insight about the nature of chemical bonding at the metal/organo-metallic interface.

Authors:Zhuoming Zhang, Sushrut Ghonge, Yang Ding, Shubin Zhang, Mona Berciu, Richard D. Schaller, Boldizsár Jankó, Masaru Kuno

Abstract: Lead-halide perovskite nanocrystals have attracted intense scrutiny due to their unusual interactions with light. They exhibit near-unity photoluminescence quantum yields, cooperative superfluorescence in colloidal superlattices, and possibly refrigerate optically when excited below gap. For the latter, multiple-phonon-assisted energy up-conversion to the band edge is followed by emission of higher energy, band gap photons. Unexpectedly strong electron-phonon interactions, however, are needed to explain near-unity up-conversion efficiencies wherein multiple-phonon absorption rates become competitive with multiple-phonon-emission mediated non-radiative relaxation. Here, we resolve this seeming contradiction to rationalize near-unity anti-Stokes photoluminescence efficiencies ($\eta_{ASPL}$) in CsPbBr3 nanocrystals as the consequence of resonant multiple-phonon absorption by polarons. The theory self-consistently explains paradoxically large efficiencies for intrinsically-disfavored, multiple-phonon-assisted anti-Stokes photoluminescence in nanocrystals. It also explains observed non-Arrhenius $\eta_{ASPL}$ temperature and energy detuning dependencies. Beyond this, the developed microscopic mechanism has immediate and important implications for applications of anti-Stokes photoluminescence towards condensed phase optical refrigeration, radiation-balanced lasers, and the development of ultra-stable optical cavities.

Authors:Donovan Buterakos, Sankar Das Sarma

Abstract: It has been demonstrated that small plaquettes of quantum dot spin qubits are capable of simulating condensed matter phenomena which arise from the Hubbard model, such as the collective Coulomb blockade and Nagaoka ferromagnetism. Motivated by recent materials developments, we investigate a bilayer arrangement of quantum dots with four dots in each layer which exhibits a complex ground state behavior. We find using a generalized Hubbard model with long-range Coulomb interactions, several distinct magnetic phases occur as the Coulomb interaction strength is varied, with possible ground states that are ferromagnetic, antiferromagnetic, or having both one antiferromagnetic and one ferromagnetic layer. We map out the full phase diagram of the system as it depends on the inter- and intra-layer Coulomb interaction strengths, and find that for a single layer, a similar but simpler effect occurs. We also predict interesting contrasts among electron, hole, and electron-hole bilayer systems arising from complex correlation physics. Observing the predicted magnetic configuration in already-existing few-dot semiconductor bilayer structures could prove to be an important assessment of current experimental quantum dot devices, particularly in the context of spin-qubit-based analog quantum simulations.

Authors:Domonkos Laszlo Farkas, Gyorgy Csaba

Abstract: We demonstrate that parametrically excited eigenmodes in nearby nanomagnets can be coupled to each other. Both positive (in-phase) and negative (anti-phase) couplings can be realized by a combination of appropriately chosen geometry and excitation field frequency. The oscillations are sufficiently stable against thermal fluctuations. The phase relation between field-coupled nanomagnets shows a hysteretic behavior with the phase relation being locked over a wide frequency range. We envision that this computational study lays the groundwork to use field-coupled nanomagnets as parametrons as building blocks of logic devices, neuromorphic systems or Ising machines.

Authors:Rosa López, Pascal Simon, Minchul Lee

Abstract: We investigate the quantum transport of the heat and the charge through a quantum dot (QD) coupled to fermionic contacts under the influence of time modulation of temperatures. We derive within the nonequilibrium Keldysh Green's function (NEGF) formalism, exact formulas for the charge, heat currents and the dissipation power by employing the concept of gravitational field firstly introduced by Luttinger in the sixties. The gravitational field, entering into the system Hamiltonian and being coupled to the energies stored in the contacts, plays a similar role as the electrostatic potential that is coupled to the charge density. We extend the original idea of Luttinger to correctly handle the dynamical transport driven under a time-modulated (ac) temperature, by coupling the gravitational field to not only the contact energy but also to half of the energy stored in the tunneling barriers connecting the QD to the contacts. The validity of our formalism is supported in that it satisfies the Onsager reciprocity relations and also reproduces the dynamics obtained from the scattering theory for noninteracting cases. Our formalism provides a systematical procedure to obtain general expressions for the charge and heat currents in the linear response regime solely in terms of the retarded and advanced components of the interacting QD NEGFs by a help of the charge conservation and some sum rules. In order to demonstrate the utility of our formalism, we apply it to the noninteracting and interacting QD junctions and interpret the charge and heat transports in terms of the equivalent quantum RC circuits. Via a systematic consideration of the effect of the Coulomb interaction under the ac drive of temperature, our formalism reveals that the interaction can modify the response for charging and energy relaxation with a significant different temperature dependence.

Authors:Xukun Feng, Chit Siong Lau, Shi-Jun Liang, Ching Hua Lee, Shengyuan A. Yang, Yee Sin Ang

Abstract: Two-dimensional (2D) ferrovalley semiconductor (FVSC) with spontaneous valley polarization offers an exciting material platform for probing Berry phase physics. How FVSC can be incorporated in valleytronic device applications, however, remain an open question. Here we generalize the concept of metal/semiconductor (MS) contact into the realm of valleytronics. We propose a half-valley Ohmic contact based on FVSC/graphene heterostructure where the two valleys of FVSC separately forms Ohmic and Schottky contacts with those of graphene, thus allowing current to be valley-selectively injected through the `Ohmic' valley while being blocked in the `Schottky' valley. We develop a theory of contact-limited valley-contrasting current injection and demonstrate that such transport mechanism can produce gate-tunable valley-polarized injection current. Using RuCl$_2$/graphene heterostructure as an example, we illustrate a device concept of valleytronic barristor where high valley polarization efficiency and sizable current on/off ratio, can be achieved under experimentally feasible electrostatic gating conditions. These findings uncover contact-limited valley-contrasting current injection as an efficient mechanism for valley polarization manipulation, and reveals the potential of valleytronic MS contact as a functional building block of valleytronic device technology.

Authors:Moritz M. Hirschmann, Johannes Mitscherling

Abstract: Recent density-functional theory (DFT) calculations on copper-doped lead apatite $\text{Pb}_9\text{Cu}(\text{PO}_4)_6\text{O}$ indicated various interesting band structure properties in the close vicinity to the Fermi surface including symmetry-enforced band crossings, narrow bands, and van-Hove singularities. These studies assume a regular arrangement of the dopant, such that the space group (SG) 176 (P6$\text{}_3$/m) is reduced to SG 143 (P3). We construct tight-binding models for this space group with two and four bands. A first analysis of these models show excellent agreement with the key features of the DFT results. We show that the symmetry enforced band crossings at $\Gamma$ and $A$ are double Weyl points, implying Chern bands for $k_z\neq 0,\pi$. We map out the distribution of Berry curvature and quantum metric and discuss their relation to the orbital character. For a specific set of parameters we find a singular flat band.

Authors:Raphaela De Oliveira, Alisson R. Cadore, Raul O. Freitas, Ingrid D. Barcelos

Abstract: Phyllosilicates emerge as a promising class of large bandgap lamellar insulators. Their applications have been explored from fabrication of graphene-based devices to 2D heterostructures based on transition metal dicalcogenides with enhanced optical and polaritonics properties. In this review, we provide an overview on the use of IR s-SNOM for studying nano-optics and local chemistry of a variety of 2D natural phyllosilicates. Finally, we bring a brief update on applications that combine natural lamellar minerals into multifunctional nanophotonic devices driven by electrical control.

Authors:Mohamed M. Elsayed, Sang Wook Kim, Juan M. Vanegas, Valeri N. Kotov

Abstract: Introducing quasiparticle anisotropy in graphene via uniaxial strain has a profound effect on the polarization charge density induced by external impurities, both Coulomb and short-range. In particular the charge distribution induced by a Coulomb impurity exhibits a power law tail modulated by a strain-dependent admixture of angular harmonics. The appearance of distributed charge is in sharp contrast to the response in pristine/isotropic graphene, where for subcritical impurities the polarization charge is fully localized at the impurity position. It is also interesting to note that our results are obtained strictly at zero chemical potential, and the behavior is fundamentally distinct from the typical Friedel oscillations observed at finite chemical potential. For weak to moderate strain, the $d$-wave symmetry is dominant. The presence of Dirac cone tilt, relevant to some 2D materials beyond graphene, can also substantially affect the induced charge distribution. Finally we consider impurities with short range potentials, and study the effect of strain on the charge response. Our results were obtained in the continuum via perturbation theory valid for weak (subcritical) potentials, and supported by numerical lattice simulations based on density functional theory.

Authors:Eitan Dvorquez, Benjamín Pavez, Qiang Sun, Felipe Pinto, Andrew D. Greentree, Brant C. Gibson, Jerónimo R. Maze

Abstract: We focus on the transmission and reflection coefficients of light in systems involving of topological insulators (TI). Due to the electro-magnetic coupling in TIs, new mixing coefficients emerge leading to new components of the electromagnetic fields of propagating waves. We have discovered a simple heterostructure that provides a 100-fold enhancement of the mixing coefficients for TI materials. Such effect increases with the TI's wave impedance. We also predict a transverse deviation of the Poynting vector due to these mixed coefficients contributing to the radiative electromagnetic field of an electric dipole. Given an optimal configuration of the dipole-TI system, this deviation could amount to $0.18\%$ of the Poynting vector due to emission near not topological materials, making this effect detectable.

Authors:Yukio Kaneko, Tatsuhiko N. Ikeda

Abstract: We study quantum Floquet (periodically-driven) systems having continuous dynamical symmetry (CDS) consisting of a time translation and a unitary transformation on the Hilbert space. Unlike the discrete ones, the CDS strongly constrains the possible Hamiltonians $H(t)$ and allows us to obtain all the Floquet states by solving a finite-dimensional eigenvalue problem. Besides, Noether's theorem leads to a time-dependent conservation charge, whose expectation value is time-independent throughout evolution. We exemplify these consequences of CDS in the seminal Rabi model, an effective model of a nitrogen-vacancy center in diamonds without strain terms, and Heisenberg spin models in rotating fields. Our results provide a systematic way of solving for Floquet states and explain how they avoid hybridization in quasienergy diagrams.

Authors:Yedi Shen, Zeyu Li, Qian Niu, Zhenhua Qiao

Abstract: Topological insulators have been extended to higher-order versions that possess topological hinge or corner states in lower dimensions. However, their robustness against disorder is still unclear. Here, we theoretically investigate the phase transitions of three-dimensional (3D) chiral second-order topological insulator (SOTI) in the presence of disorders. Our results show that, by increasing disorder strength, the nonzero densities of states of side surface and bulk emerge at critical disorder strengths of $W_{S}$ and $W_{B}$, respectively. The spectral function indicates that the bulk gap is only closed at one of the $R_{4z}\mathcal{T}$-invariant points, i.e., $\Gamma_{3}$. The closing of side surface gap or bulk gap is ascribed to the significant decrease of the elastic mean free time of quasi-particles. Because of the localization of side surface states, we find that the 3D chiral SOTI is robust at an averaged quantized conductance of $2e^{2}/h$ with disorder strength up to $W_{B}$. When the disorder strength is beyond $W_{B}$, the 3D chiral SOTI is then successively driven into two phases, i.e., diffusive metallic phase and Anderson insulating phase. Furthermore, an averaged conductance plateau of $e^{2}/h$ emerges in the diffusive metallic phase.

Authors:Guanxiong Qu, Masamitsu Hayashi, Masao Ogata, Junji Fujimoto

Abstract: We study the intrinsic spin Hall effect of a Dirac Hamiltonian system with ferromagnetic exchange coupling, a minimal model combining relativistic spin-orbit interaction and ferromagnetism. The energy bands of the Dirac Hamiltonian are split after introducing a Stoner-type ferromagnetic ordering which breaks the spherical symmetry of pristine Dirac model. The totally antisymmetric spin Hall conductivity (SHC) tensor becomes axially anisotropic along the direction of external electric field. Interestingly, the anisotropy does not vanish in the asymptotic limit of zero magnetization. We show that the ferromagnetic ordering breaks the spin degeneracy of the eigenfunctions and modifies the selection rules of the interband transitions for the intrinsic spin Hall effect. The difference in the selection rule between the pristine and the ferromagnetic Dirac phases causes the anisotropy of the SHC, resulting in a discontinuity of the SHC as the magnetization, directed orthogonal to the electric field, is reduced to zero in the ferromagnetic Dirac phase and enters the pristine Dirac phase.

Authors:Floor van Riggelen-Doelman, Chien-An Wang, Sander L. de Snoo, William I. L. Lawrie, Nico W. Hendrickx, Maximilian Rimbach-Russ, Amir Sammak, Giordano Scappucci, Corentin Déprez, Menno Veldhorst

Abstract: Quantum links can interconnect qubit registers and are therefore essential in networked quantum computing. Semiconductor quantum dot qubits have seen significant progress in the high-fidelity operation of small qubit registers but establishing a compelling quantum link remains a challenge. Here, we show that a spin qubit can be shuttled through multiple quantum dots while preserving its quantum information. Remarkably, we achieve these results using hole spin qubits in germanium, despite the presence of strong spin-orbit interaction. We accomplish the shuttling of spin basis states over effective lengths beyond 300 $\mu$m and demonstrate the coherent shuttling of superposition states over effective lengths corresponding to 9 $\mu$m, which we can extend to 49 $\mu$m by incorporating dynamical decoupling. These findings indicate qubit shuttling as an effective approach to route qubits within registers and to establish quantum links between registers.

Authors:Bo Fu, Yanru Chen, Weiwei Chen, Wei Zhu, Ping Cui, Qunxiang Li, Zhenyu Zhang, Qinwei Shi

Abstract: Despite extensive existing studies, a complete understanding of the role of disorder in affecting the physical properties of two-dimensional Dirac fermionic systems remains a standing challenge, largely due to obstacles encountered in treating multiple scattering events for such inherently strong scattering systems. Using graphene as an example and a nonperturbative numerical technique, here we reveal that the low energy quasiparticle properties are considerably modified by multiple scattering processes even in the presence of weak scalar potentials. We extract unified power-law energy dependences of the self-energy with fractional exponents from the weak scattering limit to the strong scattering limit from our numerical analysis, leading to sharp reductions of the quasiparticle residues near the Dirac point, eventually vanishing at the Dirac point. The central findings stay valid when the Anderson-type impurities are replaced by correlated Gaussian- or Yukawa-type disorder with varying correlation lengths. The improved understanding gained here also enables us to provide better interpretations of the experimental observations surrounding the temperature and carrier density dependences of the conductivity in ultra-high mobility graphene samples. The approach demonstrated here is expected to find broad applicability in understanding the role of various other types of impurities in two-dimensional Dirac systems.

Authors:Valentin John, Francesco Borsoi, Zoltán György, Chien-An Wang, Gábor Széchenyi, Floor van Riggelen, William I. L. Lawrie, Nico W. Hendrickx, Amir Sammak, Giordano Scappucci, András Pályi, Menno Veldhorst

Abstract: Electrically-driven spin resonance is a powerful technique for controlling semiconductor spin qubits. However, it faces challenges in qubit addressability and off-resonance driving in larger systems. We demonstrate coherent bichromatic Rabi control of quantum dot hole spin qubits, offering a spatially-selective approach for large qubit arrays. By applying simultaneous microwave bursts to different gate electrodes, we observe multichromatic resonance lines and resonance anticrossings that are caused by the ac Stark shift. Our theoretical framework aligns with experimental data, highlighting interdot motion as the dominant mechanism for bichromatic driving.

Authors:G. M. Minkov, O. E. Rut, A. A. Sherstobitov, S. A. Dvoretski, N. N. Mikhailov, V. Ya. Aleshkin

Abstract: The magnetic field, temperature dependence and the Hall effect have been measured in order to determine the energy spectrum of the valence band in HgTe quantum wells with the width (20-200)nm. The comparison of hole densities determined from the period Shubnikov-de Haas oscillations and the Hall effect shows that states at the top of valence band are double degenerate in teh entry quantum wells width the width range. The cyclotron mass determined from temperature dependence of SdH oscillations increases monotonically from (0.2-0.3) mass of the free electron, with increasing hole density from 2e11 to 6e11 cm^-2. The determined dependence has been compared to theoretical one calculate within the four band kp model. The experimental dependence was found to be strongly inconsistent with this predictions. It has been shown that the inclusion of additional factors (electric field, strain) does not remove the contradiction between experiment and theory. Consequently it is doubtful that the mentioned kp calculations adequately describe the valence band for any width of quantum well.

Authors:Bi Hong Tiang, Yee Sin Ang, L. K. Ang

Abstract: We develop a theoretical model to calculate the quantum efficiency (QE) of photoelectron emission from materials with Rashba spin-orbit coupling (RSOC) effect. In the low temperature limit, an analytical scaling between QE and the RSOC strength is obtained as QE $\propto (\hbar\omega-W)^2+2E_R(\hbar \omega-W) -E_R^2/3$, where $\hbar\omega$, $W$ and $E_R$ are the incident photon energy, work function and the RSOC parameter respectively. Intriguingly, the RSOC effect substantially improves the QE for strong RSOC materials. For example, the QE of Bi$_2$Se$_3$ and Bi/Si(111) increases, by 149\% and 122\%, respectively due to the presence of strong RSOC. By fitting to the photoelectron emission characteristics, the analytical scaling law can be employed to extract the RSOC strength, thus offering a useful tool to characterize the RSOC effect in materials. Importantly, when the traditional Fowler-Dubridge model is used, the extracted results may substantially deviate from the actual values by $\sim90\%$, thus highlighting the importance of employing our model to analyse the photoelectron emission especially for materials with strong RSOC. These findings provide a theoretical foundation for the design of photoemitters using Rashba spintronic materials.

Authors:X. R. Wang, X. Gong, K. Y. Jing

Abstract: Superradiance is a phenomenon of multiple facets that occurs in classical and quantum physics under extreme conditions. Here we present its manifestation in spin waves under an easily realized condition. We show that an interface between a current-free (normal) ferromagnetic (FM) region and a current-flow (pumped) FM region can be a spin wave super-mirror whose reflection coefficient is larger than 1. The super-reflection is the consequence of current-induced spectrum inversion where phase and group velocities of spin waves are in the opposite directions. An incident spin wave activates a backward propagating refractive wave inside pumped FM region. The refractive spin wave re-enters the normal FM region to constructively interfere with the reflective wave. It appears that the pumped FM region coherently emits reflective waves, leading to a super-reflection. The process resembles superradiance of a spinning black hole through the Hawking radiation process, or Dicke superradiance of cavity photons inside population inverted media.

Authors:Szczepan Głodzik, Nicholas Sedlmayr

Abstract: Two dimensional topological superconductors with chiral edge modes are predicted to posses a quantized thermal Hall effect, exactly half that for chiral topological insulators, which is proportional to the Chern number. However not much work has been done in identifying this in the standard models in the literature. Here we introduce a model based on a proximity induced superconducting Bismuth bilayer, to directly calculate the thermal Hall conductance based on the lattice model. This model serves as a demonstration of the state of the art possible in such a calculation, as well as introducing an interesting paradigmatic topological superconductor with a rich phase diagram. We demonstrate the quantized thermal Hall plateaus in several different topological phases, and compare this to numerical calculations of the Chern number, as well as analytical calculations of the Chern number's parity invariant. We demonstrate that it is possible to get a reasonable topological phase diagram from the quantized thermal Hall calculations.

Authors:Thomas W. J. Metzger, Kirill A. Grishunin, Chris Reinhoffer, Roman M. Dubrovin, Atiqa Arshad, Igor Ilyakov, Thales V. A. G. de Oliveira, Alexey Ponomaryov, Jan-Christoph Deinert, Sergey Kovalev, Roman V. Pisarev, Mikhail I. Katsnelson, Boris A. Ivanov, Paul H. M. van Loosdrecht, Alexey V. Kimel, Evgeny A. Mashkovich

Abstract: Understanding spin-lattice interactions in antiferromagnets is one of the most fundamental issues at the core of the recently emerging and booming fields of antiferromagnetic spintronics and magnonics. Recently, coherent nonlinear spin-lattice coupling was discovered in an antiferromagnet which opened the possibility to control the nonlinear coupling strength and thus showing a novel pathway to coherently control magnon-phonon dynamics. Here, utilizing intense narrow band terahertz (THz) pulses and tunable magnetic fields up to 7 T, we experimentally realize the conditions of the Fermi magnon-phonon resonance in antiferromagnetic $CoF_{2}$. These conditions imply that both the spin and the lattice anharmonicities harvest energy transfer between the subsystems, if the magnon eigenfrequency $f_{m}$ is twice lower than the frequency of the phonon $2f_{m}=f_{ph}$. Performing THz pump-infrared probe spectroscopy in conjunction with simulations, we explore the coupled magnon-phonon dynamics in the vicinity of the Fermi-resonance and reveal the corresponding fingerprints of an impulsive THz-induced response. This study focuses on the role of nonlinearity in spin-lattice interactions, providing insights into the control of coherent magnon-phonon energy exchange.

Authors:Zi-Ting Sun, Jin-Xin Hu, Ying-Ming Xie, K. T. Law

Abstract: Recently, several experiments reported that the magnetic field interference pattern of the quantum hall edge states mediated Josephson junctions can exhibit Fraunhofer oscillations with a periodicity of either $h/e$ or $h/2e$. However, a unified understanding of such a phenomenon is still absent. In this work, we show that the competition between local Andreev reflections and crossed Andreev reflections results in the crossover between $h/e$ and $h/2e$ quantum oscillations in chiral edge-channel Josephson junctions. Our theory explains why recent experiments observed either $h/e$ or $h/2e$ oscillations in different samples. Furthermore, we predict a thermal-driven $h/e$ to $h/2e$ Fraunhofer oscillations crossover.

Authors:Molly P. Andersen, Evgeny Mikheev, Ilan T. Rosen, Lixuan Tai, Peng Zhang, Kang L. Wang, Marc A. Kastner, David Goldhaber-Gordon

Abstract: Quantum coherence of electrons can produce striking behaviors in mesoscopic conductors, including weak localization and the Aharonov-Bohm effect. Although magnetic order can also strongly affect transport, the combination of coherence and magnetic order has been largely unexplored. Here, we examine quantum coherence-driven universal conductance fluctuations in the antiferromagnetic, canted antiferromagnetic, and ferromagnetic phases of a thin film of the topological material MnBi$_2$Te$_4$. In each magnetic phase we extract a charge carrier phase coherence length of about 100 nm. The conductance magnetofingerprint is repeatable when sweeping applied magnetic field within one magnetic phase, but changes when the applied magnetic field crosses the antiferromagnetic/canted antiferromagnetic magnetic phase boundary. Surprisingly, in the antiferromagnetic and canted antiferromagnetic phase, but not in the ferromagnetic phase, the magnetofingerprint depends on the direction of the field sweep. To explain these observations, we suggest that conductance fluctuation measurements are sensitive to the motion and nucleation of magnetic domain walls in MnBi$_2$Te$_4$.

Authors:Andrés Ruderman, M. B. Oviedo, S. A. Paz, E. P. M. Leiva

Abstract: We present a new approach to studying nanoparticle collisions using Density Functional based Tight Binding (DFTB). A novel DFTB parameterisation has been developed to study the collision process of Sn and Si nanoparticles (NPs) using Molecular Dynamics (MD). While bulk structures were used as training sets, we show that our model is able to accurately reproduce the cohesive energy of the nanoparticles using Density Functional Theory (DFT) as a reference. A surprising variety of phenomena are revealed for the Si/Sn nanoparticle collisions, depending on the size and velocity of the collision: from core-shell structure formation to bounce-off phenomena.

Authors:K. Navamani

Abstract: In this letter, we present the unified paradigm on entropy-ruled Einstein diffusion-mobility relation ({\mu}/D ratio) for all dimensional systems (1D, 2D and 3D) of molecules and materials. The different dimension-associated fractional value of the variation in differential entropy with respect to the chemical potential ({\Delta}h/{\Delta}{\eta}) gives the quantum-classical transition version of {\mu}/D relation. This is a new alternative version for quantum devices, instead of Einstein original relation of {\mu}/D = q/kT; where q, k and T are the electric charge, Boltzmann constant and temperature, respectively. It is found that the fractional value of {\Delta}h/{\Delta}{\eta} for {\mu}/D ratio for different dimensional systems or devices is a direct consequences with the average energy-Fermi energy relation, which can varies with the typical dimensions, whether the system belongs to 1D or 2D or 3D. This unified entropy-ruled transport formalism works well for both the quantum and classical systems with equilibrium as well as non-equilibrium conditions. Based on the dimensional dependent entropy-ruled {\mu}/D factor, the Navamani-Shockley diode equation is transformed.

Authors:A. Yu. Aladyshkin, K. Schouteden

Abstract: Field-emission resonances (FERs) for two-dimensional Pb(111) islands grown on \mbox{Si(111)7$\times$7} surfaces were studied by low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) in a broad range of tunneling conditions with both active and disabled feedback loop. These FERs exist at quantized sample-to-tip distances $Z^{\,}_n$ above the sample surface, where $n$ is the serial number of the FER state. By recording the trajectory of the STM tip during ramping of the bias voltage $U$ (while keeping the tunneling current $I$ fixed), we obtain the set of the $Z^{\,}_n$ values corresponding to local maxima in the derived $dZ/dU(U)$ spectra. This way, the continuous evolution of $Z^{\,}_n$ as a function of $U$ for all FERs was investigated by STS experiments with active feedback loop for different $I$. Complementing these measurements by current-distance spectroscopy at a fixed $U$, we could construct a 4-dimensional $I-U-Z-dZ/dU$ diagram, that allows us to investigate the geometric localization of the FERs above the surface. We demonstrate that (i) the difference $\delta Z^{\,}_n=Z^{\,}_{n+1}-Z^{\,}_n$ between neighboring FER lines in the $Z-U$ diagram is independent of $n$ for higher resonances, (ii) the $\delta Z^{\,}_{n}$ value decreases as $U$ increases; (iii) the quantized FER states lead to the \emph{periodic} variations of $\ln I$ as a function of $Z$ with periodicity $\delta Z$; (iv) the periodic variations in the $\ln I - Z$ spectra allows to estimate the absolute height of the tip above the sample surface. Our findings contribute to a deeper understanding on how the FER states affect various types of tunneling spectroscopy experiments and how they lead to a non-exponential decay of the tunneling current as a function of $Z$ at high bias voltages in the regime of quantized electron emission.

Authors:Wen-Tian Lu, Zhe Yuan, Xiaohong Xu

Abstract: A systematic investigation of spin injection behavior in Au/FM (FM = Fe and Ni) multilayers is performed using the superdiffusive spin transport theory. By exciting the nonmagnetic layer, the laser-induced hot electrons may transfer spin angular momentum into the adjacent ferromagnetic (FM) metals resulting in ultrafast demagnetization or enhancement. We find that these experimental phenomena sensitively depend on the particular interface reflectivity of hot electrons and may reconcile the different observations in experiment. Stimulated by the ultrafast spin currents carried by the hot electrons, we propose the multilayer structures to generate highly spin polarized currents for development of future ultrafast spintronics devices. The spin polarization of the electric currents carried by the hot electrons can be significantly enhanced by the joint effects of bulk and interfacial spin filtering. Meanwhile the intensity of the generated spin current can be optimized by varying the number of repeated stacking units and the thickness of each metallic layer.

Authors:Bruno Bertin-Johannet, Alexandre Popoff, Flavio Ronetti, Jérôme Rech, Thibaut Jonckheere, Laurent Raymond, Benoît Grémaud, Thierry Martin

Abstract: We consider a two-dimensional electron system in the Laughlin sequence of the fractional quantum Hall regime to investigate the effect of strong correlations on the mutual interaction between two Levitons, single-electron excitations generated by trains of quantized Lorentzian pulses. We focus on two-Leviton states injected in a single period with a time separation $\Delta t$. In the presence of a quantum point contact operating in the weak-backscattering regime, we compute the backscattered charge by means of the Keldysh technique. In the limit of an infinite period and zero temperature, we show that the backscattered charge for a two-Leviton state is not equal to twice the backscattered charge for a single Leviton. We present an interpretation for this result in terms of the wave-packet formalism for Levitons, thus proposing that an effective interaction between the two Levitons is induced by the strongly-correlated background. Finally, we perform numerical calculations in the periodic case by using the Floquet formalism for photo-assisted transport. By varying the system parameters such as pulse width, filling factor and temperature we show that the value of the backscattered charge for two-Leviton states is strongly dependent on the pulse separation, thus opening scenarios where the effective interaction between Levitons can be controllably tuned.

Authors:Michael Onyszczak, Ayelet J. Uzan, Yue Tang, Pengjie Wang, Yanyu Jia, Guo Yu, Tiancheng Song, Ratnadwip Singha, Jason F. Khoury, Leslie M. Schoop, Sanfeng Wu

Abstract: Optical spectroscopy of quantum materials at ultralow temperatures is rarely explored, yet it may provide critical characterizations of quantum phases not possible using other approaches. We describe the development of a novel experimental platform that enables optical spectroscopic studies, together with standard electronic transport, of materials at millikelvin temperatures inside a dilution refrigerator. The instrument is capable of measuring both bulk crystals and micron-sized two-dimensional van der Waals materials and devices. We demonstrate the performance by implementing photocurrent-based Fourier transform infrared spectroscopy on a monolayer WTe$_2$ device and a multilayer 1T-TaS$_2$ crystal, with a spectral range available from near-infrared to terahertz range and in magnetic fields up to 5 T. In the far-infrared regime, we achieve spectroscopic measurements at a base temperature as low as ~ 43 mK and a sample electron temperature of ~ 450 mK. Possible experiments and potential future upgrades of this versatile instrumental platform are envisioned.

Authors:Wan-Qing Zhu, Wen-Yu Shan

Abstract: Control and detection of antiferromagnetic topological materials are challenging since the total magnetization vanishes. Here we investigate the magneto-optical Kerr and Faraday effects in bilayer antiferromagnetic insulator MnBi$_2$Te$_4$. We find that by breaking the combined mirror symmetries with either perpendicular electric field or external magnetic moment, Kerr and Faraday effects occur. Under perpendicular electric field, antiferromagnetic topological insulators (AFMTI) show sharp peaks at the interband transition threshold, whereas trivial insulators show small adjacent positive and negative peaks. Gate voltage and Fermi energy can be tuned to reveal the differences between AFMTI and trivial insulators. We find that AFMTI with large antiferromagnetic order can be proposed as a pure magneto-optical rotator due to sizable Kerr (Faraday) angles and vanishing ellipticity. Under external magnetic moment, AFMTI and trivial insulators are significantly different in the magnitude of Kerr and Faraday angles and ellipticity. For the qualitative behaviors, AFMTI shows distinct features of Kerr and Faraday angles when the spin configurations of the system change. These phenomena provide new possibilities to optically detect and manipulate the layered topological antiferromagnets.

Authors:Erik Zimmermann, Michael Schleenvoigt, Alina Rupp, Gerrit Behner, Jan Karthein, Justus Teller, Peter Schüffelgen, Hans Lüth, Detlev Grützmacher, Thomas Schäpers

Abstract: We present a symmetrization routine that optimizes and eases the analysis of data featuring the anomalous Hall effect. This technique can be transferred to any hysteresis with (point-)symmetric behaviour. The implementation of the method is demonstrated exemplarily using intermixed longitudinal and transversal data obtained from a chromium-doped ternary topological insulator revealing a clear hysteresis. Furthermore, by introducing a mathematical description of the anomalous Hall hysteresis based on the error function precise values of the height and coercive field are determined.

Authors:Kyoung-Min Kim, Gyungchoon Go, Moon Jip Park, Se Kwon Kim

Abstract: The investigation of twist engineering in easy-axis magnetic systems has revealed the remarkable potential for generating topological spin textures, such as magnetic skyrmions. Here, by implementing twist engineering in easy-plane magnets, we introduce a novel approach to achieve fractional topological spin textures such as merons. Through atomistic spin simulations on twisted bilayer magnets, we demonstrate the formation of a stable double meron pair in two magnetic layers, which we refer to as the "Meron Quartet" (MQ). Unlike merons in a single pair, which is unstable against pair annihilation, the merons within the MQ exhibit exceptional stability against pair annihilation due to the protective localization mechanism induced by the twist that prevents the collision of the meron cores. Furthermore, we showcase that the stability of the MQ can be enhanced by adjusting the twist angle, resulting in increased resistance to external perturbations such as external magnetic fields. Our findings highlight the twisted magnet as a promising platform for investigating the intriguing properties of merons, enabling their realization as stable magnetic quasiparticles in van der Waals magnets.

Authors:Meng Lian, Yin-Jie Chen, Yue Geng, Yun-Tian Chen, Jing-Tao Lü

Abstract: In isolated nonlinear optical waveguide arrays with bounded energy spectrum, simultaneous conservation of energy and power of the optical modes enables study of coupled thermal and particle transport in the negative temperature regime. Here, based on exact numerical simulation and rationale from Landauer formalism, we predict generic violation of the Wiedemann-Franz law in such systems. This is rooted in the spectral decoupling of thermal and power current of optical modes, and their different temperature dependence. Our work extends the study of coupled thermal and particle transport into unprecedented regimes, not reachable in natural condensed matter and atomic gas systems.

Authors:Yongxu Fu, Yi Zhang

Abstract: Surpassing the individual buildings of the non-Hermitian skin effect (NHSE) and the scale-free localization (SFL) observed lately, we systematically exploit the exponential decay behavior of bulk eigenstates via the transfer matrix approach in non-Hermitian systems. We concentrate on the one-dimensional (1D) finite-size non-Hermitian systems with 2*2 transfer matrices in the participation of boundary impurity. We analytically unveil that the unidirectional pure scale-free (UPSF) effect emerges with the singular transfer matrices, while the hybrid skin-scale-free (HSSF) effect emerges with the nonsingular transfer matrices even though impose open boundary condition (OBC). The UPSF effect exceeds the scope of the SFL in previous works, while the HSSF effect is a charming interplay between the finite-size NHSE and SFL. Our results reveal that the NHSE under OBC prevails in the blend with the SFL as the system tends to the thermodynamic limit. Our approach paves an avenue to rigorously explore the finite-size display of the NHSE and SFL in both Hermitian and non-Hermitian systems with generic boundary conditions.

Authors:L. Peri, G. A. Oakes, L. Cochrane, C. J. B. Ford, M. F. Gonzalez-Zalba

Abstract: Semiconductor quantum dots operated dynamically are the basis of many quantum technologies such as quantum sensors and computers. Hence, modelling their electrical properties at microwave frequencies becomes essential to simulate their performance in larger electronic circuits. Here, we develop a self-consistent quantum master equation formalism to obtain the admittance of a quantum dot tunnel-coupled to a charge reservoir under the effect of a coherent photon bath. We find a general expression for the admittance that captures the well-known semiclassical (thermal) limit, along with the transition to lifetime and power broadening regimes due to the increased coupling to the reservoir and amplitude of the photonic drive, respectively. Furthermore, we describe two new photon-mediated regimes Floquet broadening, determined by the dressing of the QD states, and broadening determined by photon loss in the system. Our results provide a method to simulate the high-frequency behaviour of QDs in a wide range of limits, describe past experiments, and propose novel explorations of QD-photon interactions.

Authors:Kazuki Sone, Motohiko Ezawa, Yuto Ashida, Nobuyuki Yoshioka, Takahiro Sagawa

Abstract: As first demonstrated by the characterization of the quantum Hall effect by the Chern number, topology provides a guiding principle to realize robust properties of condensed matter systems immune to the existence of disorder. The bulk-boundary correspondence guarantees the emergence of gapless boundary modes in a topological system whose bulk exhibits nonzero topological invariants. Although some recent studies have suggested a possible extension of the notion of topology to nonlinear systems such as photonics and electrical circuits, the nonlinear counterpart of topological invariant has not yet been understood. Here, we propose the nonlinear extension of the Chern number based on the nonlinear eigenvalue problems in two-dimensional systems and reveal the bulk-boundary correspondence beyond the weakly nonlinear regime. Specifically, we find the nonlinearity-induced topological phase transitions, where the existence of topological edge modes depends on the amplitude of oscillatory modes. We propose and analyze a minimal model of a nonlinear Chern insulator whose exact bulk solutions are analytically obtained and indicate the amplitude dependence of the nonlinear Chern number, for which we confirm the nonlinear counterpart of the bulk-boundary correspondence in the continuum limit. Thus, our result reveals the existence of genuinely nonlinear topological phases that are adiabatically disconnected from the linear regime, showing the promise for expanding the scope of topological classification of matter towards the nonlinear regime.

Authors:Nicholas J. Harmon, Emma Z. Kurth, Dana Coleman, Lana Flanigan

Abstract: The spin-orbit interaction is frequently the mechanism by which spin and charge are coupled for spintronic applications. The discovery of spin, a century ago, relied on spin-charge coupling by a magnetic field gradient; this mechanism has received scant attention as a means for generating spin and charge currents in semiconductors. Through the derivation of a set of coupled spin-charge drift-diffusion equations, our work shows that magnetic field gradients can be used to generate charge currents from non-equilibrium spin polarization, in solid state systems. We predict, in GaAs, an ``Stern-Gerlach" voltage comparable to what is measured by the inverse spin Hall effect. Non-intuitively, we find the spin diffusion length is reduced by the magnetic gradient. This is understood by invoking the idea of co-current and counter-current exchange which is a concept frequently invoked in fields as disparate as animal physiology and thermal engineering.

Authors:Alba R. Piacenti Clarendon Laboratory, Department of Physics, University of Oxford, OX1 3PU Oxford, United Kingdom, Casey Adam Clarendon Laboratory, Department of Physics, University of Oxford, OX1 3PU Oxford, United Kingdom, Nicholas Hawkins Department of Engineering Science, University of Oxford, OX1 3PJ Oxford, United Kingdom, Ryan Wagner School of Mechanical Engineering, Purdue University, West Lafayette, Indiana, 47907, United States, Jacob Seifert Clarendon Laboratory, Department of Physics, University of Oxford, OX1 3PU Oxford, United Kingdom, Yukinori Taniguchi Asylum Research, Oxford Instruments KK, Tokyo 103-0006, Japan, Roger Proksch Asylum Research-An Oxford Instruments Company, Santa Barbara, California 93117, United States, Sonia Contera Clarendon Laboratory, Department of Physics, University of Oxford, OX1 3PU Oxford, United Kingdom

Abstract: Polymeric materials are widely used in industries ranging from automotive to biomedical. Their mechanical properties play a crucial role in their application and function and arise from the nanoscale structures and interactions of their constitutive polymer molecules. Polymeric materials behave viscoelastically, i.e. their mechanical responses depend on the time scale of the measurements; quantifying these time-dependent rheological properties at the nanoscale is relevant to develop, for example, accurate models and simulations of those materials, which are needed for advanced industrial applications. In this paper, an atomic force microscopy (AFM) method based on the photothermal actuation of an AFM cantilever is developed to quantify the nanoscale loss tangent, storage modulus, and loss modulus of polymeric materials. The method is then validated on a styrene-butadiene rubber (SBR), demonstrating the method's ability to quantify nanoscale viscoelasticity over a continuous frequency range up to five orders of magnitude (0.2 Hz to 20,200 Hz). Furthermore, this method is combined with AFM viscoelastic mapping obtained with amplitude-modulation frequency-modulation (AM-FM) AFM, enabling the extension of viscoelastic quantification over an even broader frequency range, and demonstrating that the novel technique synergizes with preexisting AFM techniques for quantitative measurement of viscoelastic properties. The method presented here introduces a way to characterize the viscoelasticity of polymeric materials, and soft matter in general at the nanoscale, for any application.

Authors:Tharindu Fernando, Ting Cao

Abstract: We introduce a novel gauge-invariant, quantized interband index in two-dimensional (2D) multiband systems. It provides a bulk topological classification of a submanifold of parameter space (e.g., an electron valley in a Brillouin zone), and therefore overcomes difficulties in characterizing topology of submanifolds. We confirm its topological nature by numerically demonstrating a one-to-one correspondence to the valley Chern number in $k\cdot p$ models (e.g., gapped Dirac fermion model), and the first Chern number in lattice models (e.g., Haldane model). Furthermore, we derive a band-resolved topological charge and demonstrate that it can be used to investigate the nature of edge states due to band inversion in valley systems like multilayer graphene.

Authors:Carys Chase-Mayoral, L. Q. English, Yeongjun Kim, Sanghoon Lee, Noah Lape, Alexei Andreanov, P. G. Kevrekidis, Sergej Flach

Abstract: We generate compact localized states in an electrical diamond lattice, comprised of only capacitors and inductors, via local driving near its flatband frequency. We compare experimental results to numerical simulations and find very good agreement. We also examine the stub lattice, which features a flatband of a different class where neighboring compact localized states share lattice sites. We find that local driving, while exciting the lattice at that flatband frequency, is unable to isolate a single compact localized state due to their non-orthogonality. Finally, we introduce lattice nonlinearity and showcase the realization of nonlinear compact localized states in the diamond lattice. Our findings pave the way of applying flatband physics to complex electric circuit dynamics.

Authors:N. Fang, Y. R. Chang, S. Fujii, D. Yamashita, M. Maruyama, Y. Gao, C. F. Fong, D. Kozawa, K. Otsuka, K. Nagashio, S. Okada, Y. K. Kato

Abstract: The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in mixed-dimensional heterostructures consisting of two-dimensional tungsten diselenide and one-dimensional carbon nanotubes. Bright emission peaks originating from the interface are identified, spanning a broad energy range within the telecommunication wavelengths. The effect of band alignment is investigated by systematically varying the nanotube bandgap, and we assign the new peaks to interface excitons as they only appear in type-II heterostructures. Room-temperature localization of low-energy interface excitons is indicated by extended lifetimes as well as small excitation saturation powers, and photon correlation measurements confirm single-photon emission. With mixed-dimensional van der Waals heterostructures where band alignment can be engineered, new opportunities for quantum photonics are envisioned.

Authors:Toshihiro Taen, Andhika Kiswandhi, Toshihito Osada

Abstract: Materials with the mesoscopic scales have provided an excellent platform for quantum-mechanical studies. Among them, the periodic oscillations of the electrical resistivity against the direct and the inverse of the magnetic fields, such as the Aharonov-Bohm effect and the Shubnikov-de Haas effect, manifest the interference of the wavefunction relevant to the electron motion perpendicular to the magnetic field. In contrast, the electron motion along the magnetic field also leads to the magnetic-field periodicity, which is the so-called Sondheimer effect. However, the Sondheimer effect has been understood only in the framework of the semiclassical picture, and thereby its interpretation at the quasiquantum limit was not clear. Here, we show that thin-film graphite exhibits clear sinusoidal oscillations with a period of about 1-3 T over a wide range of the magnetic fields (from around 10 T to 30 T), where conventional quantum oscillations are absent. In addition, the sample with a designed step in the middle for eliminating the stacking disorder effect verifies that the period of the oscillations is inversely proportional to the thickness, which supports the emergence of the Sondheimer oscillations in the quasiquantum limit. These findings suggest that the Sondheimer oscillations can be reinterpreted as inter-Landau-level resonances even at the field range where the semiclassical picture fails. Our results expand the quantum oscillation family, and pave the way for the exploration of the out-of-plane wavefunction motion.

Authors:C. M. Wang, Z. Z. Du, Hai-Zhou Lu, X. C. Xie

Abstract: Recently, the planar Hall effect has attracted tremendous interest. In particular, an in-plane magnetization can induce an anomalous planar Hall effect with a $2\pi/3$ period for hexagon-warped energy bands. This effect is similar to the anomalous Hall effect resulting from an out-of-plane magnetization. However, this anomalous planar Hall effect is absent in the planar Hall experiments. Here, we explain its absence, by performing a calculation that includes not only the Berry curvature mechanism as those in the previous theories, but also the disorder contributions. The conventional $\pi$-period planar Hall effect will occur if the mirror reflection symmetry is broken, which buries the anomalous one. We show that an in-plane strain can enhance the anomalous Hall conductivity and changes the period from $2\pi/3$ to $2\pi$. We propose a scheme to extract the hidden anomalous planar Hall conductivity from the experimental data. Our work will be helpful in detecting the anomalous planar Hall effect and could be generalized to understand mechanisms of the planar Hall effects in a wide range of materials.

Authors:Christian Tzschaschel, Jian-Xiang Qiu, Xue-Jian Gao, Hou-Chen Li, Chunyu Guo, Hung-Yu Yang, Cheng-Ping Zhang, Ying-Ming Xie, Yu-Fei Liu, Anyuan Gao, Damien Bérubé, Thao Dinh, Sheng-Chin Ho, Yuqiang Fang, Fuqiang Huang, Johanna Nordlander, Qiong Ma, Fazel Tafti, Philip J. W. Moll, Kam Tuen Law, Su-Yang Xu

Abstract: Weyl semimetals have emerged as a promising quantum material system to discover novel electrical and optical phenomena, due to their combination of nontrivial quantum geometry and strong symmetry breaking. One crucial class of such novel transport phenomena is the diode effect, which is of great interest for both fundamental physics and modern technologies. In the electrical regime, giant electrical diode effect (the nonreciprocal transport) has been observed in Weyl systems. In the optical regime, novel optical diode effects have been theoretically considered but never probed experimentally. Here, we report the observation of the nonlinear optical diode effect (NODE) in the magnetic Weyl semimetal CeAlSi, where the magnetic state of CeAlSi introduces a pronounced directionality in the nonlinear optical second-harmonic generation (SHG). By physically reversing the beam path, we show that the measured SHG intensity can change by at least a factor of six between forward and backward propagation over a wide bandwidth exceeding 250 meV. Supported by density-functional theory calculations, we establish the linearly dispersive bands emerging from Weyl nodes as the origin of the extreme bandwidth. Intriguingly, the NODE directionality is directly controlled by the direction of magnetization. By utilizing the electronically conductive semimetallic nature of CeAlSi, we demonstrate current-induced magnetization switching and thus electrical control of the NODE in a mesoscopic spintronic device structure with current densities as small as 5 kA/cm$^2$. Our results advance ongoing research to identify novel nonlinear optical/transport phenomena in magnetic topological materials. The NODE also provides a way to measure the phase of nonlinear optical susceptibilities and further opens new pathways for the unidirectional manipulation of light such as electrically controlled optical isolators.

Authors:Ali Ghorashi, Sachin Vaidya, Mikael Rechtsman, Wladimir Benalcazar, Marin Soljačić, Thomas Christensen

Abstract: The topological characteristics of photonic crystals have been the subject of intense research in recent years. Despite this, the basic question of whether photonic band topology is rare or abundant -- i.e., its relative prevalence -- remains unaddressed. Here, we determine the prevalence of stable, fragile, and higher-order photonic topology in the 11 two-dimensional crystallographic symmetry settings that admit diagnosis of one or more of these phenomena by symmetry analysis. Our determination is performed on the basis of a data set of 550000 randomly sampled, two-tone photonic crystals, spanning 11 symmetry settings and 5 dielectric contrasts, and examined in both transverse electric (TE) and magnetic (TM) polarizations. We report the abundance of nontrivial photonic topology in the presence of time-reversal symmetry and find that stable, fragile, and higher-order topology are generally abundant. Below the first band gap, which is of primary experimental interest, we find that stable topology is more prevalent in the TE polarization than the TM; is only weakly, but monotonically, dependent on dielectric contrast; and that fragile topology is near-absent. In the absence of time-reversal symmetry, nontrivial Chern phases are also abundant in photonic crystals with 2-, 4-, and 6-fold rotational symmetries but comparatively rare in settings with only 3-fold symmetry. Our results elucidate the interplay of symmetry, dielectric contrast, electromagnetic polarization, and time-reversal breaking in engendering topological photonic phases and may inform general design principles for their experimental realization.

Authors:Tomoki Ozawa, Tomoya Hayata

Abstract: We explore gauge-independent properties of two-dimensional non-Hermitian lattice systems with an imaginary magnetic field. We find that the energy spectrum under the open boundary conditions is an example of such gauge-independent properties. We discuss how to obtain the asymptotic continuum energy spectrum upon increasing length of one side using the framework of the non-Bloch band theory. We also find an analog of the Aharonov-Bohm effect; the net change of the norm of the wavefunction upon adiabatically forming a closed path is determined by the imaginary magnetic flux enclosed by the path.

Authors:Luis M. Canonico, Jose H. García, Stephan Roche

Abstract: We unveil a hitherto concealed spin-orbit torque mechanism driven by orbital degrees of freedom in centrosymmetric two-dimensional transition metal dichalcogenides (focusing on PtSe${}_2$ ). Using first-principles simulations, tight-binding models and large-scale quantum transport calculations, we show that such a mechanism fundamentally stems from a spatial localization of orbital textures at opposite sides of the material, which imprints their symmetries onto spin-orbit coupling effects, further producing efficient and tunable spin-orbit torque. Our study suggests that orbital-spin entanglement at play in centrosymmetric materials can be harnessed as a resource for outperforming conventional spin-orbit torques generated by the Rashba-type effects.

Authors:Daniel Timmer, Moritz Gittinger, Thomas Quenzel, Sven Stephan, Yu Zhang, Marvin F. Schumacher, Arne Lützen, Martin Silies, Sergei Tretiak, Jin-Hui Zhong, Antonietta De Sio, Christoph Lienau

Abstract: The strong coherent coupling of quantum emitters to vacuum fluctuations of the light field offers opportunities for manipulating the optical and transport properties of nanomaterials, with potential applications ranging from ultrasensitive all-optical switching to creating polariton condensates. Often, ubiquitous decoherence processes at ambient conditions limit these couplings to such short time scales that the quantum dynamics of the interacting system remains elusive. Prominent examples are strongly coupled exciton-plasmon systems, which, so far, have mostly been investigated by linear optical spectroscopy. Here, we use ultrafast two-dimensional electronic spectroscopy to probe the quantum dynamics of J-aggregate excitons collectively coupled to the spatially structured plasmonic fields of a gold nanoslit array. We observe rich coherent Rabi oscillation dynamics reflecting a plasmon-driven coherent exciton population transfer over mesoscopic distances at room temperature. This opens up new opportunities to manipulate the coherent transport of matter excitations by coupling to vacuum fields.

Authors:Victor Elhomsy, Luca Planat, David J. Niegemann, Bruna Cardoso-Paz, Ali Badreldin, Bernhard Klemt, Vivien Thiney, Renan Lethiecq, Eric Eyraud, Matthieu C. Dartiailh, Benoit Bertrand, Heimanu Niebojewski, Christopher Bäuerle, Maud Vinet, Tristan Meunier, Nicolas Roch, Matias Urdampilleta

Abstract: Spins in semiconductor quantum dots hold great promise as building blocks of quantum processors. Trapping them in SiMOS transistor-like devices eases future industrial scale fabrication. Among the potentially scalable readout solutions, gate-based dispersive radiofrequency reflectometry only requires the already existing transistor gates to readout a quantum dot state, relieving the need for additional elements. In this effort towards scalability, traveling-wave superconducting parametric amplifiers significantly enhance the readout signal-to-noise ratio (SNR) by reducing the noise below typical cryogenic low-noise amplifiers, while offering a broad amplification band, essential to multiplex the readout of multiple resonators. In this work, we demonstrate a 3GHz gate-based reflectometry readout of electron charge states trapped in quantum dots formed in SiMOS multi-gate devices, with SNR enhanced thanks to a Josephson traveling-wave parametric amplifier (JTWPA). The broad, tunable 2GHz amplification bandwidth combined with more than 10dB ON/OFF SNR improvement of the JTWPA enables frequency and time division multiplexed readout of interdot transitions, and noise performance near the quantum limit. In addition, owing to a design without superconducting loops and with a metallic ground plane, the JTWPA is flux insensitive and shows stable performances up to a magnetic field of 1.2T at the quantum dot device, compatible with standard SiMOS spin qubit experiments.

Authors:Nisarg Chadha, Ali G. Moghaddam, Jeroen van den Brink, Cosma Fulga

Abstract: The dislocation skin effect exhibits the capacity of topological defects to trap an extensive number of modes in two-dimensional non-Hermitian systems. Similar to the corresponding skin effects caused by system boundaries, this phenomenon also originates from nontrivial topology. However, finding the relationship between the dislocation skin effect and nonzero topological invariants, especially in disordered systems, can be obscure and challenging. Here, we introduce a real-space topological invariant based on the spectral localizer to characterize the skin effect on two-dimensional lattices. We demonstrate that this invariant consistently predicts the occurrence and location of both boundary and dislocation skin effects, offering a unified approach applicable to both ordered and disordered systems. Our work demonstrates a general approach that can be utilized to diagnose the topological nature of various types of skin effects, particularly in the absence of translational symmetry when momentum-space descriptions are inapplicable.

Authors:Simone Traverso, Maura Sassetti, Niccolò Traverso Ziani

Abstract: Zigzag nanoribbons hosting the Haldane Chern insulator model are considered. In this context, an unreported reentrant topological phase, characterized by the emergence of quasi zero dimensional in-gap states, is discussed. The bound states, which reside in the gap opened by the hybridization of the counter-propagating edge modes of the Haldane phase, are localized at the ends of the strip and are found to be robust against on-site disorder. These findings are supported by the behavior of the Zak phase over the parameter space, which exhibits jumps of $\pi$ in correspondence to the phase transitions between the trivial and the non-trivial phases. Setups with non-uniform parameters also show topological bound states via the Jackiw-Rebbi mechanism. All the properties reported are shown to be extremely sensitive to the strip width.

Authors:Renato Buzio, Andrea Gerbi, Cristina Bernini, Luca Repetto, Andrea Silva, Andrea Vanossi

Abstract: Solution-processed few-layers graphene flakes, dispensed to rotating and sliding contacts via liquid dispersions, are gaining increasing attention as friction modifiers to achieve low friction and wear at technologically-relevant interfaces. Vanishing friction states, i.e. superlubricity, have been documented for nearly-ideal nanoscale contacts lubricated by individual graphene flakes; there is however no clear understanding if superlubricity might persist for larger and morphologically-disordered contacts, as those typically obtained by graphene wet transfer from a liquid dispersion. In this study we address the friction performance of solution-processed graphene flakes by means of colloidal probe Atomic Force Microscopy. We use an additive-free aqueous dispersion to coat micrometric silica beads, which are then sled under ambient conditions against prototypical material substrates, namely graphite and the transition metal dichalcogenides (TMDs) MoS2 and WS2. High resolution microscopy proves that the random assembly of the wet-transferred flakes over the silica probes results into an inhomogeneous coating, formed by graphene patches that control contact mechanics through tens-of-nanometers tall protrusions. Atomic-scale friction force spectroscopy reveals that dissipation proceeds via stick-slip instabilities. Load-controlled transitions from dissipative stick-slip to superlubric continuous sliding may occur for the graphene-graphite homojunctions, whereas single- and multiple-slips dissipative dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical simulations demonstrate that the thermally-activated single-asperity Prandtl-Tomlinson model comprehensively describes friction experiments involving different graphene-coated colloidal probes, material substrates and sliding regimes.

Authors:Julian Legendre, Karyn Le Hur

Abstract: We present a method to probe the topological properties of a circuit quantum electrodynamics (cQED) array described through a Haldane model on the honeycomb lattice. We develop the theory of microwave light propagating in a local probe or a microscope (a one-dimensional transmission line) capacitively coupled to the topological cQED lattice model. Interestingly, we show that even if the microwave light has no transverse polarization, the measured reflection coefficient, resolved in frequency through the resonance, allows us to reveal the geometrical properties and topological phase transition associated to the model. This spectroscopy tool developed for cQED lattice models reveals the same topological information as circularly polarized light, locally within the Brillouin zone of the honeycomb lattice. Furthermore, our findings hold significance for topological magnon systems and are a priori applicable to all Chern insulators, presenting an intriguing opportunity for their adaptation to other systems with different particle statistics.

Authors:Yu-Feng Mao, Yu-Liang Tao, Jiong-Hao Wang, Qi-Bo Zeng, Yong Xu

Abstract: Quasicrystals allow for symmetries that are impossible in crystalline materials, such as eight-fold rotational symmetry, enabling the existence of novel higher-order topological insulators in two dimensions without crystalline counterparts. However, it remains an open question whether three-dimensional higher-order topological insulators and Weyl-like semimetals without crystalline counterparts can exist. Here, we demonstrate the existence of a second-order topological insulator by constructing and exploring a three-dimensional model Hamiltonian in a stack of Ammann-Beenker tiling quasicrystalline lattices. The topological phase has eight chiral hinge modes that lead to quantized longitudinal conductances of $4 e^2/h$. We show that the topological phase is characterized by the winding number of the quadrupole moment. We further establish the existence of a second-order topological insulator with time-reversal symmetry, characterized by a $\mathbb{Z}_2$ topological invariant. Finally, we propose a model that exhibits a higher-order Weyl-like semimetal phase, demonstrating both hinge and surface Fermi arcs. Our findings highlight that quasicrystals in three dimensions can give rise to higher-order topological insulators and semimetal phases that are unattainable in crystals.

Authors:Fredrik Brange, Riya Baruah, Christian Flindt

Abstract: Recent experiments have observed Cooper pair splitting in quantum dots coupled to superconductors, and efficient schemes for controlling and timing the splitting process are now called for. Here, we propose and analyze an adiabatic Cooper pair splitter that can produce a regular flow of spin-entangled electrons in response to a time-dependent and periodic gate voltage. The splitting process is controlled by moving back and forth along an avoided crossing between the empty state and the singlet state of two quantum dots that are coupled to a superconductor, followed by the emission of the split Cooper pairs into two normal-state drains. The scheme does not rely on fine-tuned resonance conditions and is therefore robust against experimental imperfections in the driving signal. We identify a range of driving frequencies, where the output currents are quantized and proportional to the driving frequency combined with suppressed low-frequency noise. We also discuss the main sources of cycle-missing events and evaluate the statistics of electrons emitted within a period of the drive as well as the distribution of waiting times between them. Realistic parameter estimates indicate that the Cooper pair splitter can be operated in the gigahertz regime.

Authors:Sofia Zinzani, Francesca Baletto

Abstract: It is far well accepted that the morphology of nanoparticles and nanoalloys is of paramount importance to understand their properties. Furthemore, the morphology depends on the growth mechanism with coalescence generally accepted as one the most common mechanisms both in liquid and in the gas phase. Coalescence refers when two existing seeds collide and aggregate into a larger object. It is expected that the resulting aggregate shows a compact, often spherical structure, although strongly out of the equilibrium, referring to its global minimum. While the coalescence of liquid droplet is widely studied, the first stages of the coalescence between nanoseeds has attracted less interest, although important as multiple aggregation can take place. Here we simulate the coalescence of Au and Pd seeds by the Molecular Dynamics method, comparing the initial stage of the coalescence in vacuum and when there is an interacting surrounding around them. We show that the surface chemical composition of the resulting aggregate depend on the environment as well as the overall morphology.

Authors:Tusaradri Mohapatra, Colin Benjamin

Abstract: $\Delta_T$ noise generated due to temperature gradient in the absence of charge current has recently attracted a lot of interest. In this paper, for the first time, we derive spin-polarized charge $\Delta_T$ noise and spin $\Delta_T$ noise along with its shot noise-like and thermal noise-like contributions. Introducing a spin flipper at the interface of a bilayer metal junction with a temperature gradient, we examine the impact of spin-flip scattering. We do a detailed analysis of charge and spin $\Delta_T$ noise in four distinct setups for two distinct temperature regimes: the first case of one hot \& the other cold reservoir and the second case of reservoirs with comparable temperatures, and also two distinct bias voltage regimes: the first case of zero bias voltage and second case of finite bias voltage. In all these regimes, we ensure that the net charge current transported is zero always. We find negative charge $\Delta_T$ noise for reservoirs at comparable temperatures while for the one hot \& another cold reservoir case, charge $\Delta_T$ noise is positive. We also see that spin $\Delta_T$ noise and spin $\Delta_T$ thermal noise-like contributions are negative for one hot and the other cold reservoir case. Recent work on the general bound for spin $\Delta_T$ shot noise with a spin-dependent bias suggests it is always positive. In this paper, we see spin $\Delta_T$ shot noise-like contribution to be negative in contrast to positive charge $\Delta_T$ shot noise contribution, although in the absence of any spin-dependent bias. Spin-flip scattering exhibits the intriguing effect of a change in sign in both charge and spin $\Delta_T$ noise, which can help probe spin-polarized transport.

Authors:Yoichi Ando

Abstract: Topological insulators are characterized by insulating bulk and conducting surface, the latter is a necessity consequence of the nontrivial topology of the wavefunctions forming the valence band. This chapter gives a historical overview of the discovery of topological insulators and a concise description of the $Z_2$ topology which defines them. The concept of topological insulators have been extended to various other topologies, giving rise to the recognition of further topological states of matter such as topological crystalline insulators and higher-order topological insulators. Representative materials of topological insulators, their synthesis techniques, and the ways for the experimental confirmation of the topological nature are introduced. Among the interesting phenomena derived from topological insulators, topological superconductivity, Majorana zero modes, and quantum anomalous Hall effect are briefly discussed.

Authors:I. V. Iorsh, D. D. Sedov, S. A. Kolodny, R. E. Sinitskiy, O. V. Kibis

Abstract: Within the Floquet theory of periodically driven quantum systems, we demonstrate that an off-resonant high-frequency electromagnetic field can induce the Lifshitz phase transition in periodical structures described by the one-dimensional repulsive Hubbard model with the nearest and next-nearest neighbor hopping. The transition changes the topology of electron energy spectrum at the Fermi level, transforming it from the two Fermi-points to the four Fermi-points, what facilitates the emergence of the superconducting fluctuations in the structure. Possible manifestations of the effect and conditions of its experimental observability are discussed.

Authors:V. V. Enaldiev, C. Moulsdale, A. K. Geim, V. I. Fal'ko

Abstract: Boundaries between structural twins of bilayer graphene (so-called AB/BA domain walls) are often discussed in terms of the formation of topologically protected valley-polarised chiral states. Here, we show that, depending on the width of the AB/BA boundary, the latter can also support non-chiral one-dimensional (1D) states that are confined to the domain wall at low energies and take the form of quasi-bound states at higher energies, where the 1D bands cross into the two-dimensional spectral continuum. We present the results of modeling of electronic properties of AB/BA domain walls with and without magnetic field as a function of their width and interlayer bias.

Authors:Qian Du, Su-Peng Kou

Abstract: In the paper, we study the non-Hermitian system under dissipation in which the energy band shows an imaginary line gap and energy eigenstates are bound to a specific region. To describe these phenomena, we propose the concept of "non-Hermitian tearing", in which the degree of tearing we defined reveals a continuous phase transition at the exceptional point. The non-Hermitian tearing manifests in two forms -- bulk state separation and boundary state decoupling. For a deeper understanding of non-Hermitian tearing, we give the effective 2*2 Hamiltonian in the k-space by reducing the N*N Hamiltonian in the real space. In addition, we also explore the non-Hermitian tearing in the one-dimensional Su-Schrieffer-Heeger model and the Qi-Wu-Zhang model. Our results provide a theoretical approach for studying non-Hermitian tearing in more complex systems.

Authors:José Balduque, Rafael Sánchez

Abstract: Three-terminal coherent conductors are able to perform as quantum thermocouples when the heat absorbed from one terminal is transformed into useful power in the other two. Allowing for a phase coherent coupling to the heat source we introduce a way to control and improve the thermoelectric response via quantum interference. A simple setup composed of a scanning probe between two resonant tunneling regions is proposed that achieves better performance than incoherent analogues by enhancing the generated power and efficiency, and reducing the output current noise.

Authors:Dániel Molnár, Tímea Nóra Török, Roland Kövecs, László Pósa, Péter Balázs, György Molnár, Nadia Jimenez Olalla, Juerg Leuthold, János Volk, Miklós Csontos, András Halbritter

Abstract: Analog tunable memristors are widely utilized as artificial synapses in various neural network applications. However, exploiting the dynamical aspects of their conductance change to implement active neurons is still in its infancy, awaiting the realization of efficient neural signal recognition functionalities. Here we experimentally demonstrate an artificial neural information processing unit that can detect a temporal pattern in a very noisy environment, fire a single output spike upon successful detection and reset itself in a fully unsupervised, autonomous manner. This circuit relies on the dynamical operation of only two memristive blocks: a non-volatile Ta$_2$O$_5$ device and a volatile VO$_2$ unit. A fading functionality with exponentially tunable memory time constant enables adaptive operation dynamics, which can be tailored for the targeted temporal pattern recognition task. In the trained circuit false input patterns only induce short-term variations. In contrast, the desired signal activates long-term memory operation of the non-volatile component, which triggers a firing output of the volatile block.

Authors:Humian Zhou, Chui-Zhen Chen, Qing-Feng Sun, X. C. Xie

Abstract: The quantum Hall effect (QHE), which was observed in 2D electron gas under an external magnetic field, stands out as one of the most remarkable transport phenomena in condensed matter. However, a long standing puzzle remains regarding the observation of the relativistic quantum Hall effect (RQHE). This effect, predicted for a single 2D Dirac cone immersed in a magnetic field, is distinguished by the intriguing feature of half-integer Hall conductivity (HIHC). In this work, we demonstrate that the condensed-matter realization of the RQHE and the direct measurement of the HIHC are feasible by investigating the underlying quantum transport mechanism. We reveal that the manifestation of HIHC is tied to the presence of dissipative half-integer quantized chiral channels circulating along the interface of the RQHE system and a Dirac metal. Importantly, we find that the Ohmic scaling of the longitudinal conductance of the system plays a key role in directly measuring the HIHC in experiments. Furthermore, we propose a feasible experimental scheme based on the 3D topological insulators to directly measure the HIHC. Our findings not only uncover the distinct transport mechanism of the HIHC for the RQHE, but also paves the way to the measurement of the HIHC in future experiments.

Authors:Alerksandr A. Kurilovich, Vladimir N. Mantsevich, Aleksey V. Chechkin, Vladimir V. Palyulin

Abstract: We show how two different mobile-immobile type models explain the observation of negative diffusion of excitons reported in experimental studies in quasi-two-dimensional semiconductor systems. The main reason for the effect is the initial trapping and a delayed release of free excitons in the area close to the original excitation spot. The density of trapped excitons is not registered experimentally. Hence, the signal from the free excitons alone includes the delayed release of not diffusing trapped particles. This is seen as the narrowing of the exciton density profile or decrease of mean-squared displacement which is then interpreted as a negative diffusion. The effect is enhanced with the increase of recombination intensity as well as the rate of the exciton-exciton binary interactions.

Authors:Piotr Stefański

Abstract: A tunneling junction between normal electrode and a topological superconducting wire, mediated by a quantum dot, is considered theoretically. We show that the presence of the dot in the junction can be advantageous to Majorana zero modes identification. Namely, we demonstrate that for the dot strongly coupled to the wire, the Majorana mode from the upper chiral sub-band "leaks" into the dot, providing supplementary information on Majorana mode formation. Thus, both the Zeeman-split dot sub-levels detect Majorana partners of a Kramers pair, formed at the wire end. The characteristic three-peak structures in both spin sectors of the spectral density of the dot, distinguish from the trivial scenario of one Andreev resonance at Fermi energy produced exclusively by the dot's spin sub-levels.

Authors:K. Hori, S. Souma, C. -W. Chuang, Y. Nakata, K. Nakayama, S. Gupta, T. P. T. Nguyen, K. Yamauchi, T. Takahashi, F. Matsukura, F. H. Chang, H. J. Lin, C. T. Chen, A. Chainani, T. Sato

Abstract: Understanding the mechanism of ferromagnetism in ferromagnetic topological insulators (TIs) is a key to realize exotic time-reversal-symmetry-broken quantum phases. However, electronic states relevant to the ferromagnetism are highly controversial. Here we report angle-resolved photoemission spectroscopy on (CrxSb1-x)2Te3 thin films, high-Curie-temperature (T_C) ferromagnetic TIs, spanning the non-doped (T_C=0 K) to highly-doped (T_C=192 K) region. We found that, upon Cr doping to Sb2Te3, the bulk valence-band valley exhibits filling-in behavior while retaining band inversion, leading to the formation of a nearly-flat band in high-T_C regime and evolution from a six-petal flower to a Star-of-David Fermi surface. Despite the weakening of spin-orbit coupling with Cr doping, the Dirac-cone state persists up to the highest-T_C sample, and shows a clear magnetic-gap opening below TC accompanied with an unexpected band shift, signifying its strong coupling with spontaneous ferromagnetism. The present result lays the foundation for understanding the interplay between band topology and ferromagnetism in TIs.

Authors:Sougata Biswas, Arunava Chakrabarti

Abstract: An infinitely large Koch fractal is shown to be capable of sustaining only extended, Bloch-like eigenstates, if certain parameters of the Hamiltonian describing the lattice are numerically correlated in a special way, and a magnetic flux of a special strength is trapped in every loop of the geometry. We describe the system within a tight binding formalism and prescribe the desired correlation between the numerical values of the nearest neighbor overlap integrals, along with a special value of the magnetic flux trapped in the triangular loops decorating the fractal. With such conditions, the lattice, despite the absence of translational order of any kind whatsoever, yields an absolutely continuous eigenvalue spectrum, and becomes completely transparent to an incoming electron with any energy within the allowed band. The results are analytically exact. An in-depth numerical study of the inverse participation ratio and the two-terminal transmission coefficient corroborates our findings. Our conclusions remain valid for a large set of lattice models, built with the same structural units, but beyond the specific geometry of a Koch fractal, unraveling a subtle universality in a variety of such low dimensional systems.

Authors:Suvodeep Paul, Devesh Negi, Saswata Talukdar, Saheb Karak, Shalini Badola, Bommareddy Poojitha, Manasi Mandal, Sourav Marik, R. P. Singh, Nashra Pistawala, Luminita Harnagea, Aksa Thomas, Ajay Soni, Subhro Bhattacharjee, Surajit Saha

Abstract: Tailoring the quantum many-body interactions in layered materials through appropriate heterostructure engineering can result in emergent properties that are absent in the constituent materials thus promising potential future applications. In this article, we have demonstrated controlling the otherwise robust magnetic properties of transition metal phosphorus trisulphides (Mn/Fe/NiPS3) in their heterostructures with Weyl semimetallic MoTe2 which can be attributed to the Dzyaloshinskii Moriya (DM) interactions at the interface of the two different layered materials. While the DM interaction is known to scale with the strength of the spin-orbit coupling (SOC), we also demonstrate here that the effect of DM interaction strongly varies with the spin orientation/dimensionality of the magnetic layer and the low-energy electronic density of state of the spin-orbit coupled layer. The observations are further supported by a series of experiments on heterostructures with a variety of substrates/underlayers hosting variable SOC and electronic density of states.

Authors:Wojciech Rudziński, Józef Barnaś, Anna Dyrdał

Abstract: Van der Waals magnetic materials are currently of great interest as materials for applications in future ultrathin nanoelectronics and nanospintronics. Due to weak coupling between individual monolayers, these materials can be easily obtained in the monolayer and bilayer forms. The latter are of specific interest as they may be considered as natural two-dimensional spin valves. In this paper, we study theoretically spin waves in bilayers of transition metal dichalcogenides. The considerations are carried within the general spin wave theory based on effective spin Hamiltonian and Hollstein-Primakoff-Bogolubov transformation. The spin Hamiltonian includes intra-layer as well as inter-layer nearest-neighbour exchange interactions, easy-plane anisotropy, and additionally a weak in-plane easy-axis anisotropy. The bilayer systems consist of two ferromagnetic (in-plane magnetization) monolayers that are coupled either ferromagnetically or antiferromagnetically. In the latter case, we analyse the spin wave spectra in all magnetic phases, i.e. in the antiferromagnetic, spin-flop, and ferromagnetic ones.

Authors:A. Söll Department of Inorganic Chemistry University of Chemistry and Technology Prague, E. Lopriore Institute of Electrical and Microengineering École Polytechnique Fédérale de Lausanne Institute of Materials Science and Engineering École Polytechnique Fédérale de Lausanne, A. K. Ottesen Institute of Electrical and Microengineering École Polytechnique Fédérale de Lausanne Institute of Materials Science and Engineering École Polytechnique Fédérale de Lausanne, J. Luxa Department of Inorganic Chemistry University of Chemistry and Technology Prague, G. Pasquale Institute of Electrical and Microengineering École Polytechnique Fédérale de Lausanne Institute of Materials Science and Engineering École Polytechnique Fédérale de Lausanne, J. Sturala Department of Inorganic Chemistry University of Chemistry and Technology Prague, F. Hájek Institute of Physics of the Czech Academy of Sciences v.v.i, V. Jarý Institute of Physics of the Czech Academy of Sciences v.v.i, D. Sedmidubský Department of Inorganic Chemistry University of Chemistry and Technology Prague, K. Mosina Department of Inorganic Chemistry University of Chemistry and Technology Prague, A. Kis Institute of Electrical and Microengineering École Polytechnique Fédérale de Lausanne Institute of Materials Science and Engineering École Polytechnique Fédérale de Lausanne, Z. Sofer Department of Inorganic Chemistry University of Chemistry and Technology Prague

Abstract: Van der Waals heterostructures of two-dimensional materials have opened up new frontiers in condensed matter physics, unlocking unexplored possibilities in electronic and photonic device applications. However, the investigation of wide-gap high-$\kappa$ layered dielectrics for devices based on van der Waals structures has been relatively limited. In this work, we demonstrate an easily reproducible synthesis method for the rare earth oxyhalide LaOBr, and we exfoliate it as a 2D layered material with a measured static dielectric constant of $\epsilon_{0, \perp} \simeq 9$ and a wide bandgap of 5.3 eV. Furthermore, our research demonstrates that LaOBr can be used as a high-$\kappa$ dielectric in van der Waals field-effect transistors with high performance and low interface defect concentrations. Additionally, it proves to be an attractive choice for electrical gating in excitonic devices based on 2D materials. Our work demonstrates the versatile realization and functionality of 2D systems with wide-gap and high-$\kappa$ van der Waals dielectric environments.

Authors:Svend-Age Biehs, Achim Kittel, Zhenghua An

Abstract: We analytically calculate the contribution to the local density of states due to thermal sources in a disk-like patch within the framework of fluctuational electrodynamics. We further introduce a wavevector cutoff method to approximate this contribution. We compare the results obtained with the source and cutoff method with the numerical exact LDOS above a metal disk attained by SCUFF-EM calculations. By this comparison we highlight the difference and resemblance of thermal and geometrical boundary conditions which are both relevant for near-field scanning microscope measurements. Finally, we give an outlook to general lateral temperature profiles and compare it with surface profiles.

Authors:Junseo Jung, Hyeongmuk Lim, Bohm-Jung Yang

Abstract: We study the relation between the quantum geometry of wave functions and the Landau level (LL) spectrum of two-band Hamiltonians with a quadratic band crossing point (QBCP) in two-dimensions. By investigating the influence of interband coupling parameters on the wave function geometry of general QBCPs, we demonstrate that the interband coupling parameters can be entirely determined by the projected elliptic image of the wave functions on the Bloch sphere, which can be characterized by three parameters, i.e., the major $d_1$ and minor $d_2$ diameters of the ellipse, and one angular parameter $\phi$ describing the orientation of the ellipse. These parameters govern the geometric properties of the system such as the Berry phase and modified LL spectra. Explicitly, by comparing the LL spectra of two quadratic band models with and without interband couplings, we show that the product of $d_1$ and $d_2$ determines the constant shift in LL energy while their ratio governs the initial LL energies near a QBCP. Also, by examining the influence of the rotation and time-reversal symmetries on the wave function geometry, we construct a minimal continuum model which exhibits various wave function geometries. We calculate the LL spectra of this model and discuss how interband couplings give LL structure for dispersive bands as well as nearly flat bands.

Authors:Gauthier Philippe, Mathieu Moalic, Jarosław W. Kłos

Abstract: We demonstrate that the spin wave Cherenkov effect can be used to design the unidirectional spin wave emitter with tunable frequency and switchable direction of emission. In our numerical studies, we propose to use a pair of traveling profiles of the magnetic field which generate the spin waves, for sufficiently large velocity of their motion. In the considered system, the spin waves of shorter (longer) wavelengths are induced at the front (back) of the moving profiles and interfere constructively or destructively, depending on the velocity of the profiles. Moreover, we showed that the spin waves can be confined between the pair of traveling profiles of the magnetic field. This work opens the perspectives for the experimental studies in hybrid magnonic-superconducting systems where the magnetic vortices in a superconductor can be used as moving sources of the magnetic field driving the spin waves in the ferromagnetic subsystem.

Authors:Suman Chatterjee, Medha Dandu, Pushkar Dasika, Rabindra Biswas, Sarthak Das, Kenji Watanabe, Takashi Taniguchi, Varun Raghunathan, Kausik Majumdar

Abstract: Excitonic states trapped in harmonic moir\'e wells of twisted heterobilayers is an intriguing testbed. However, the moir\'e potential is primarily governed by the twist angle, and its dynamic tuning remains a challenge. Here we demonstrate anharmonic tuning of moir\'e potential in a WS$_2$/WSe$_2$ heterobilayer through gate voltage and optical power. A gate voltage can result in a local in-plane perturbing field with odd parity around the high-symmetry points. This allows us to simultaneously observe the first (linear) and second (parabolic) order Stark shift for the ground state and first excited state, respectively, of the moir\'e trapped exciton - an effect opposite to conventional quantum-confined Stark shift. Depending on the degree of confinement, these excitons exhibit up to twenty-fold gate-tunability in the lifetime ($100$ to $5$ ns). Also, exciton localization dependent dipolar repulsion leads to an optical power-induced blueshift of $\sim$1 meV/$\mu$W - a five-fold enhancement over previous reports.

Authors:Jian-Sheng Wang, Mauro Antezza

Abstract: Based on a tight-binding model for the electron system, we investigate the transfer of energy, momentum, and angular momentum mediated by electromagnetic fields among buckminsterfullerene (C$_{60}$) and graphene nano-strips. Our nonequilibrium Green's function approach enables calculations away from local thermal equilibrium where the fluctuation-dissipation theorem breaks down. For example, the forces between C$_{60}$ and current-carrying nano-strips are predicted. It is found that the presence of current usually enhances the van der Waals attractive forces. For two current-carrying graphene strips rotated at some angle, the fluctuational force and torque are much stronger at the nanoscale compared to that of the static Biot-Savart law.

Authors:Antonin Coutant, Bruno Lombard

Abstract: When semi-infinite phononic crystals (PCs) are in contact, localized modes may exist at their boundary. The central question is generally to predict their existence and to determine their stability. With the rapid expansion of the field of topological insulators, powerful tools have been developed to address these questions. In particular, when applied to one-dimensional systems with mirror symmetry, the bulk-boundary correspondence claims that the existence of interface modes is given by a topological invariant computed from the bulk properties of the phononic crystal, which ensures strong stability properties. This one-dimensional bulk-boundary correspondence has been proven in various works. Recent attempts have exploited the notion of surface impedance, relying on analytical calculations of the transfer matrix. In the present work, the monotonic evolution of surface impedance with frequency is proven for all one-dimensional phononic crystals with mirror symmetry. This result allows us to establish a stronger version of the bulk-boundary correspondence that guarantees not only the existence but also the uniqueness of a topologically protected interface state. The method is numerically illustrated in the physically relevant case of PCs with imperfect interfaces, where analytical calculations would be out of reach.

Authors:Isaac Alcón, Aron Cummings, Stephan Roche

Abstract: During the last 15 years bottom-up on-surface synthesis has been demonstrated as an efficient way to synthesize carbon nanostructures with atomic precision, opening the door to unprecedented electronic control at the nanoscale. Nanoporous graphenes (NPGs) fabricated as two-dimensional arrays of graphene nanoribbons (GNRs) represent one of the key recent breakthroughs in the field. NPGs interestingly display in-plane transport anisotropy of charge carriers, and such anisotropy was shown to be tunable by modulating quantum interference. Herein, using large-scale quantum transport simulations, we show that electrical anisotropy in NPGs is not only resilient to disorder but can further be massively enhanced by its presence. This outcome paves the way to systematic engineering of quantum transport in NPGs as a novel concept for efficient quantum devices and architectures.

Authors:D. V. Slobodianiuk, O. V. Prokopenko

Abstract: We demonstrate numerically that a pure time-harmonic bias AC current of some particular amplitude $\tau_f$ and angular frequency $\omega_f$ can excite the chaotic magnetization dynamics in a Josephson-like antiferromagnetic (AFM) spin Hall oscillator (SHO) having a biaxial magnetic anisotropy of an AFM layer. The nature of such a stochastic generation regime in a Josephson-like AFM SHO could be explained by the random hopping of the SHO's work point between several quasi-stable states under the action of applied AC current. We revealed that depending on the $\omega_f/\tau_f$ ratio several stochastic generation regimes interspersed with regular generation regimes can be achieved in an AFM SHO that can be used in spintronic sources of random signals and various nano-scale devices utilizing random signals including the spintronic p-bit device considered in this paper. The obtained results are important for the development and optimization of spintronic devices able to generate and process (sub-)THz-frequency random signals promising for ultra-fast probabilistic computing, cryptography, secure communications, etc.

Authors:Saba Karimeddiny, Thow Min Jerald Cham, Orion Smedley, Daniel C. Ralph, Yunqiu Kelly Luo

Abstract: Sagnac interferometry can provide a significant improvement in signal-to-noise ratio compared to conventional magnetic imaging based on the magneto-optical Kerr effect (MOKE). We show that this improvement is sufficient to allow quantitative measurements of current-induced magnetic deflections due to spin-orbit torque even in thin-film magnetic samples with perpendicular magnetic anisotropy for which the Kerr rotation is second-order in the magnetic deflection. Sagnac interfermometry can also be applied beneficially for samples with in-plane anisotropy, for which the Kerr rotation is first order in the deflection angle. Optical measurements based on Sagnac interferometry can therefore provide a cross-check on electrical techniques for measuring spin-orbit torque. Different electrical techniques commonly give quantitatively inconsistent results, so that Sagnac interferometry can help to identify which techniques are affected by unidentified artifacts.

Authors:B. Benhamou-Bui, C. Consejo, S. S. Krishtopenko, M. Szoła, K. Maussang, S. Ruffenach, E. Chauveau, S. Benlemqwanssa, C. Bray, X. Baudry, P. Ballet, S. V. Morozov, V. I. Gavrilenko, N. N. Mikhailov, S. A. Dvoretskii, B. Jouault, J. Torres, F. Teppe

Abstract: Two-dimensional Dirac fermions in HgTe quantum wells close to the topological phase transition can generate significant cyclotron emission that is magnetic field tunable in the Terahertz (THz) frequency range. Due to their relativistic-like dynamics, their cyclotron mass is strongly dependent on their electron concentration in the quantum well, providing a second tunability lever and paving the way for a gate-tunable, permanent-magnet Landau laser. In this work, we demonstrate the proof-of-concept of such a back-gate tunable THz cyclotron emitter at fixed magnetic field. The emission frequency detected at 1.5 Tesla is centered on 2.2 THz and can already be electrically tuned over 250 GHz. With an optimized gate and a realistic permanent magnet of 1.0 Tesla, we estimate that the cyclotron emission could be continuously and rapidly tunable by the gate bias between 1 and 3 THz, that is to say on the less covered part of the THz gap.

Authors:Klaus Zollner, Eike Icking, Jaroslav Fabian

Abstract: Van der Waals (vdW) heterostructures consisting of Bernal bilayer graphene (BLG) and hexagonal boron nitride (hBN) are investigated. By performing first-principles calculations we capture the essential BLG band structure features for several stacking and encapsulation scenarios. A low-energy model Hamiltonian, comprising orbital and spin-orbit coupling (SOC) terms, is employed to reproduce the hBN-modified BLG dispersion, spin splittings, and spin expectation values. Most important, the hBN layers open an orbital gap in the BLG spectrum, which can range from zero to tens of meV, depending on the precise stacking arrangement of the individual atoms. Therefore, large local band gap variations may arise in experimentally relevant moir\'{e} structures. Moreover, the SOC parameters are small (few to tens of $\mu$eV), just as in bare BLG, but are markedly proximity modified by the hBN layers. Especially when BLG is encapsulated by monolayers of hBN, such that inversion symmetry is restored, the orbital gap and spin splittings of the bands vanish. In addition, we show that a transverse electric field mainly modifies the potential difference between the graphene layers, which perfectly correlates with the orbital gap for fields up to about 1~V/nm. Moreover, the layer-resolved Rashba couplings are tunable by $\sim 5~\mu$eV per V/nm. Finally, by investigating twisted BLG/hBN structures, with twist angles between 6$^{\circ}$ -- 20$^{\circ}$, we find that the global band gap increases linearly with the twist angle. The extrapolated $0^{\circ}$ band gap is about 23~meV and results roughly from the average of the stacking-dependent local band gaps. Our investigations give new insights into proximity spin physics of hBN/BLG heterostructures, which should be useful for interpreting experiments on extended as well as confined (quantum dot) systems.

Authors:Daniela Andrade Damasceno, Henrique Musseli Cezar, Teresa Duarte Lanna, Alexsandro Kirch, Caetano Rodrigues Miranda

Abstract: We investigate the mechanical and adsorption properties of single-walled carbon nanotubes (SWCNTs) filled with greenhouse gases through Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations using a recently developed parameterization for the cross-terms of the Lenard-Jones (LJ) potential. Carbon nanotubes interact strongly with CO$_2$ compared to CH$_4$, resulting in a CO$_2$-rich composition inside the nanotubes, with the proportion of CO$_2$ decreasing as the diameter of the nanotubes increases. Contrarily, the smallest nanotubes showed a more even balance between CO$_2$ and CH$_4$ due to gas solidification. The gas does not affect the mechanical response of the nanotubes under tension, but under compression, it presents a complex relationship with the loading direction, nanotube's diameters, chirality, and to a minor extent, the gas composition. Filled zigzag nanotubes showed to be more stable in the presence of fillers, giving the best mechanical performance compared to the filled armchairs. The study confirms carbon nanotubes as effective means of separating CO$_2$ from CH$_4$, presenting good mechanical stability.

Authors:Henrique Musseli Cezar, Teresa Duarte Lanna, Daniela Andrade Damasceno, Alexsandro Kirch, Caetano Rodrigues Miranda

Abstract: Carbon nanotubes and graphene are promising nanomaterials to improve the performance of current gas separation membrane technologies. From the molecular modeling perspective, an accurate description of the interfacial interactions is mandatory to understand the gas selectivity in these materials. Most of the molecular dynamics simulations studies considered available force fields with the standard Lorentz-Berthelot (LB) mixing rules to describe the interaction among carbon dioxide (CO$_2$), methane (CH$_4$) and carbon structures. We performed a systematic study in which we showed the LB underestimates the fluid/solid interaction energies compared to the density functional theory (DFT) calculation results. To improve the classical description, we propose a new parametrization for the cross-terms of the Lenard-Jones (LJ) potential by fitting DFT forces and energies. The obtained model enhanced fluid/carbon interface description showed excellent transferability between single-walled carbon nanotubes (SWCNTs) and graphene. To investigate the effect of the new parametrization on the gas structuring within the SWCNTs with varying diameters, we performed Grand Canonical Monte Carlo (GCMC) simulations. We observed considerable differences in the CO$_2$ and CH$_4$ density within SWCNTs compared to those obtained with the standard approach. Our study highlights the importance of going beyond the traditional Lorentz-Berthelot mixing rules in the studies involving solid/fluid interfaces of confined systems.

Authors:Ishita Pushkarna, Árpád Pásztor, Christoph Renner

Abstract: Synthetic materials and heterostructures obtained by the controlled stacking of exfoliated monolayers are emerging as attractive functional materials owing to their highly tunable properties. We present a detailed scanning tunneling microscopy and spectroscopy study of single layer MoS$_2$-on-gold heterostructures as a function of twist angle. We find that their electronic properties are determined by the hybridization of the constituent layers and are modulated at the moir\'e period. The hybridization depends on the layer alignment and the modulation amplitude vanishes with increasing twist angle. We explain our observations in terms of a hybridization between the nearest sulfur and gold atoms, which becomes spatially more homogeneous and weaker as the moir\'e periodicity decreases with increasing twist angle, unveiling the possibility of tunable hybridization of electronic states via twist angle engineering.

Authors:Suvendu Ghosh, Snehasish Nandy, A. Taraphder

Abstract: We investigate quantum transport through a rectangular potential barrier in Weyl semimetals (WSMs) and multi-Weyl semimetals (MSMs), within the framework of Landauer-B\"uttiker formalism. Our study uncovers the role of nodal topology imprinted in the electric current and the shot noise. We find that, in contrast to the finite odd-order conductance and noise power, the even-order contributions vanish at the nodes. Additionally, depending on the topological charge ($J$), the linear conductance ($G_1$) scales with the Fermi energy ($E_F$) as $G_1^{E_F>U}\propto E_F^{2/J}$. We demonstrate that the $E_F$-dependence of the second-order conductance and shot noise power could quite remarkably distinguish an MSM from a WSM depending on the band topology, and may induce several smoking gun experiments in nanostructures made out of WSMs and MSMs. Analyzing shot noise and Fano factor, we show that the transport across the rectangular barrier follows the sub-Poissonian statistics. Interestingly, we obtain universal values of Fano factor at the nodes unique to their topological charges. The universality for a fixed $J$, however, indicates that only a fixed number of open channels participate in the transport through evanescent waves at the nodes. The proposed results can serve as a potential diagnostic tool to identify different topological systems in experiments.

Authors:YuanDong Wang, Zhen-Gang Zhu, Gang Su

Abstract: We propose a spin photogalvanic effect of magnons with broken inversion symmetry. The dc spin photocurrent is generated via the Aharonov-Casher effect, which includes the Drude, Berry curvature dipole, shift, injection, and rectification components with distinct quantum geometric origin. Based on a symmetry classification, we uncover that there exist linearly polarized (LP) magnon spin photocurrent responses in the breathing kagome-lattice ferromagnet with Dzyaloshinskii-Moriya interaction, and the circularly polarized (CP) responses due to the symmetry breaking by applying a uniaxial strain. We address that the topological phase transitions can be characterized by the spin photocurrents. This study presents a deeper insight into the nonlinear responses of light-magnon interactions, and suggests a possible way to generate and control the magnon spin current in real materials.

Authors:Oscar Javier Gomez Sanchez, Guan-Hao Peng, Wei-Hua Li, Ching-Hung Shih, Chao-Hsin Chien, Shun-Jen Cheng

Abstract: A light beam can be spatially structured in the complex amplitude to possess orbital angular momentum (OAM), which introduces a new degree of freedom alongside the intrinsic spin angular momentum (SAM) associated with circular polarization. Moreover, super-imposing two twisted lights with distinct SAM and OAM produces a vector vortex beam (VVB) in non-separable states where not only complex amplitude but also polarization are spatially structured and entangled with each other. In addition to the non-separability, the SAM and OAM in a VVB are intrinsically coupled by the optical spin-orbit interaction and constitute the profound spin-orbit physics in photonics. In this work, we present a comprehensive theoretical investigation, implemented on the first-principles base, of the intriguing light-matter interaction between VVBs and WSe$_{2}$ monolayers (WSe$_{2}$-MLs), one of the best-known and promising two-dimensional (2D) materials in optoelectronics dictated by excitons, encompassing bright exciton (BX) as well as various dark excitons (DXs). One of the key findings of our study is the substantial enhancement of the photo-excitation of gray excitons (GXs), a type of spin-forbidden dark exciton, in a WSe$_2$-ML through the utilization of a twisted light that possesses a longitudinal field associated with the optical spin-orbit interaction. Our research demonstrates that a spin-orbit-coupled VVB surprisingly allows for the imprinting of the carried optical information onto gray excitons in 2D materials, which is robust against the decoherence mechanisms in materials. This observation suggests a promising method for deciphering the transferred angular momentum from structured lights to excitons.

Authors:Malo Duportal, Luca M. Berger, Stefan A. Maier, Andreas Tittl, Katharina Krischer

Abstract: Surface-enhanced spectroscopy techniques are the method-of-choice to characterize adsorbed intermediates occurring during electrochemical reactions, which are crucial in realizing a green sustainable future. Characterizing species with low coverages or short lifetimes have so far been limited by low signal enhancement. Recently, metasurface-driven surface-enhanced infrared absorption spectroscopy (SEIRAS) has been pioneered as a promising narrowband technology to study single vibrational modes of electrochemical interfaces during CO oxidation. However, many reactions involve several species or configurations of adsorption that need to be monitored simultaneously requiring reproducible and broadband sensing platforms to provide a clear understanding of the underlying electrochemical processes. Here, we experimentally realize multi-band metasurface-driven SEIRAS for the in-situ study of electrochemical CO2 reduction on a Pt surface. We develop an easily reproducible and spectrally-tunable platinum nano-slot metasurface. Two CO adsorption configurations at 2030 cm-1 and 1840 cm-1 are locally enhanced as a proof of concept that can be extended to more vibrational bands. Our platform provides a 41-fold enhancement in the detection of characteristic absorption signals compared to conventional broadband electrochemically roughened platinum films. A straightforward methodology is outlined starting by baselining our system in CO saturated environment and clearly detecting both configurations of adsorption, in particular the hitherto hardly detectable CO bridge configuration. Then, thanks to the signal enhancement provided by our platform, we find that the CO bridge configuration on platinum does not play a significant role during CO2 reduction in an alkaline environment. We anticipate that our technology will guide researchers in developing similar sensing platforms.

Authors:Justin Schirmann, Selma Franca, Felix Flicker, Adolfo G. Grushin

Abstract: The discovery of the Hat, an aperiodic monotile, has revealed novel mathematical aspects of aperiodic tilings. However, the physics of particles propagating in such a setting remains unexplored. In this work we study spectral and transport properties of a tight-binding model defined on the Hat. We find that (i) the spectral function displays striking similarities to that of graphene, including six-fold symmetry and Dirac-like features; (ii) unlike graphene, the monotile spectral function is chiral, differing for its two enantiomers; (iii) the spectrum has a macroscopic number of degenerate states at zero energy; (iv) when the magnetic flux per plaquette ($\phi$) is half of the flux quantum, zero-modes are found localized around the reflected `anti-hats'; and (v) its Hofstadter spectrum is periodic in $\phi$, unlike other quasicrystals. Our work serves as a basis to study wave and electron propagation in possible experimental realizations of the Hat, which we suggest.

Authors:Jacob R. Taylor, Jay D. Sau, Sankar Das Sarma

Abstract: We develop a practical machine learning approach to determine the disorder landscape of Majorana nanowires by using training of the conductance matrix and inverting the conductance data in order to obtain the disorder details in the system. The inversion carried out through machine learning using different disorder parametrizations turns out to be unique in the sense that any input tunnel conductance as a function of chemical potential and Zeeman energy can indeed be inverted to provide the correct disorder landscape. Our work opens up a qualitatively new direction of directly determining the topological invariant and the Majorana wave-function structure corresponding to a transport profile of a device using simulations that quantitatively match the specific conductance profile. In addition, this also opens up the possibility for optimizing Majorana systems by figuring out the (generally unknown) underlying disorder only through the conductance data. An accurate estimate of the applicable spin-orbit coupling in the system can also be obtained within the same scheme.

Authors:Ronika Sarkar, Ayan Banerjee, Awadhesh Narayan

Abstract: Non-Abelian phenomena and non-Hermitian systems have both been widely explored in recent years. As a bridge between the two, we introduce and develop non-Abelian gauge engineering for realizing multi-fold spectral topology. As an example of our proposal, we engineer non-Hermiticity in the paradigmatic Su-Schrieffer-Heeger (SSH) model by introducing a generalized non-Abelian gauge, leading to an emergent two-fold spectral topology that governs the decoupled behaviour of the corresponding non-Hermitian skin effect. As a consequence of the non-Abelian gauge choice, our model exhibits a rich phase diagram consisting of distinct topological phases, which we characterize by introducing the notion of paired winding numbers, which, in turn, predict the direction of skin localization under open boundaries. We demonstrate that the choice of gauge parameters enables control over the directionality of the skin effect, allowing for it to be unilateral or bilateral. Furthermore, we discover non-dispersive flat bands emerging within the inherent SSH model framework, arising from the non-Abelian gauge. We also introduce a simplified toy model to capture the underlying physics, thereby giving direct physical insights. Our findings pave way for the exploration of unconventional spectral topology through non-Abelian gauges.

Authors:Thomas D. Honeychurch, Daniel S. Kosov

Abstract: We study the dynamics of two capacitively coupled quantum dots, each coupled to a lead. A Floquet Green's function approach described the system's dynamics, with the electron-electron interactions handled with the fluctuation-exchange approximation. While electrons cannot move between the separate sections of the device, energy transfer occurs with the periodic driving of one of the leads. This process was found to be explained with four stages. The energy transfer was also found to be sensitive to the driving frequency of the leads, with an optimal frequency corresponding to the optimal completion of the four stages of the identified process.

Authors:K. O. Nikolaev, S. R. Lake, G. Schmidt, S. O. Demokritov, V. E. Demidov

Abstract: Spin-wave based transmission and processing of information is a promising emerging nano-technology that can help overcome limitations of traditional electronics based on the transfer of electrical charge. Among the most important challenges for this technology is the implementation of spin-wave devices that can operate without the need for an external bias magnetic field. Here we experimentally demonstrate that this can be achieved using sub-micrometer wide spin-wave waveguides fabricated from ultrathin films of low-loss magnetic insulator - Yttrium Iron Garnet (YIG). We show that these waveguides exhibit a highly stable single-domain static magnetic configuration at zero field and support long-range propagation of spin waves with gigahertz frequencies. The experimental results are supported by micromagnetic simulations, which additionally provide information for optimization of zero-field guiding structures. Our findings create the basis for the development of energy-efficient zero-field spin-wave devices and circuits.

Authors:W. AfzaL, Z. Yue, Z. Li, M. Fuhrer, X. Wang

Abstract: We report the observation of anomalous Hall effect in Mn3Sn polycrystalline thin films deposited on Pt coated Al2O3 substrate with a large anomalous Hall conductivity of 65({\Omega}cm)-1 at 3K. The Hall and magnetic measurements show a very small hysteresis owing to a weak ferromagnetic moment in this material. The longitudinal resistivity decreases sufficiently for the thin films as compared to the polycrystalline bulk sample used as the target for the film deposition. The anomalous Hall resistivity and conductivity decreases almost linearly with the increase in the temperature. A negative magnetoresistance is observed for all the measured temperatures with the negative decrease in the magnitude with the increase in temperature.

Authors:Lu Gao, Qiang Cheng, Qing-Feng Sun

Abstract: We study the Andreev reflections and the quantum transport in the proximitized graphene/superconductor junction. The proximitized graphene possesses the pseudospin staggered potential and the intrinsic spin-orbit coupling induced by substrate, which are responsible for the spin-valley dependent double Andreev reflections and the anomalous transport properties in the junction. The pure specular Andreev reflection can happen in the superconducting gap for the $K\uparrow$ and $K'\downarrow$ electrons while the pure retro-Andreev reflection happens for the $K\downarrow$ and $K'\uparrow$ electrons. The coexisting two types of Andreev reflections related to the fixed spin-valley indices strongly depend on the chemical potential of the proximitized graphene. The condition of the emergence of the specific type of Andreev reflection for the electrons with the fixed spin-valley index is clarified. The spin-valley dependent Andreev reflections bring about the peculiar conductance spectra of the junction, which can help determine the values of the pseudospin staggered potential and the intrinsic spin-orbit coupling induced in graphene. Hence, our research results not only provide an experimental method to detect the induced potential and coupling in graphene but also establish the foundation of the superconductor electronics based on the spin-valley indices.

Authors:Tarun Tummuru, Anffany Chen, Patrick M. Lenggenhager, Titus Neupert, Joseph Maciejko, Tomáš Bzdušek

Abstract: We extend the notion of topologically protected semi-metallic band crossings to hyperbolic lattices in negatively curved space. Due to their distinct translation group structure, such lattices support non-Abelian Bloch states which, unlike conventional Bloch states, acquire a matrix-valued Bloch factor under lattice translations. Combining diverse numerical and analytical approaches, we uncover a quartic scaling in the density of states at low energies, and illuminate a nodal manifold of codimension five in the reciprocal space. The nodal manifold is topologically protected by a non-zero second Chern number, reminiscent of the characterization of Weyl nodes by the first Chern number.

Authors:David T. S. Perkins, Aires Ferreira

Abstract: We propose how to create, control, and read-out real-space localized spin qubits in proximitized finite graphene nanoribbon (GNR) systems using purely electrical methods. Our proposed nano-qubits are formed of in-gap singlet-triplet states that emerge through the interplay of Coulomb and relativistic spin-dependent interactions in GNRs placed on a magnetic substrate. Application of an electric field perpendicular to the GNR heterostructure leads to a sudden change in the proximity couplings, i.e. a quantum quench, which enables us to deterministically rotate the nano-qubit to any arbitrary point on the Bloch sphere. We predict these spin qubits to undergo Rabi oscillations with optimal visibility and frequencies in excess of 10 GHz. Our findings open up a new avenue for the realization of graphene-based quantum computing with ultra-fast all-electrical methods.

Authors:N. Krane, E. Turco, A. Bernhardt, D. Jacob, G. Gandus, D. Passerone, M. Luisier, M. Juríček, R. Fasel, J. Fernández-Rossier, P. Ruffieux

Abstract: Phenalenyl is a radical nanographene with triangular shape that hosts an unpaired electron with spin S = 1/2. The open-shell nature of phenalenyl is expected to be retained in covalently bonded networks. Here, we study a first step in that direction and report the synthesis of the phenalenyl dimer by combining in-solution synthesis and on-surface activation and its characterization both on Au(111) and on a monolayer of NaCl on top of Au(111) by means of inelastic electron tunneling spectroscopy (IETS). IETS shows inelastic steps that, together with a thorough theoretical analysis, are identified as the singlet-triplet excitation arising from interphenalenyl exchange. Two prominent features of our data permit to shed light on the nature of spin interactions in this system. First, the excitation energies with and without the NaCl decoupling layer are 48 and 41 meV, respectively, indicating a significant renormalization of the spin excitation energies due to exchange with the Au(111) electrons. Second, a position-dependent bias-asymmetry of the height of the inelastic steps is accounted for by an interphenalenyl hybridization of the singly occupied phenalenyl orbitals that is only possible via third neighbor hopping. This hybridization is also essential to activate kinetic interphenalenyl exchange. Our results set the stage for future work on the bottom-up synthesis of spin S = 1/2 spin lattices with large exchange interaction.

Authors:Gabriele Pasquale, Edoardo Lopriore, Zhe Sun, Kristiāns Čerņevičs, Fedele Tagarelli, Kenji Watanabe, Takashi Taniguchi, Oleg V. Yazyev, Andras Kis

Abstract: Two-dimensional flat-band systems have recently attracted considerable interest due to the rich physics unveiled by emergent phenomena and correlated electronic states at van Hove singularities. However, the difficulties in electrically detecting the flat band position in field-effect structures are slowing down the investigation of their properties. In this work, we employ Indium Selenide (InSe) as a flat-band system due to a van Hove singularity at the valence band edge in a few-layer form of the material without the requirement of a twist angle. We investigate tunneling photocurrents in gated few-layer InSe structures and relate them to ambipolar transport and photoluminescence measurements. We observe an appearance of a sharp change in tunneling mechanisms due to the presence of the van Hove singularity at the flat band. We further corroborate our findings by studying tunneling currents as a reliable probe for the flat-band position up to room temperature. Our results create an alternative approach to studying flat-band systems in heterostructures of 2D materials.

Authors:Gaëlle Bigeard, Alessandro Cresti

Abstract: We investigate the effect of a magnetic field on the band structure of a bilayer graphene with a magic twist angle of 1.08{\deg}. The coupling of tight-binding model and Peierls phase allows the calculation of the energy bands of periodic two-dimensional systems. For an orthogonal magnetic field, the Landau levels turn out to be dispersive, especially for magnetic lengths comparable or larger than the twisted bilayer cell size. A high in-plane magnetic field modifies the low-energy bands and gap, which we demonstrate to be a direct consequence of the minimal coupling.

Authors:Henrique Musseli Cezar, Caetano Rodrigues Miranda

Abstract: Silica (SiO$_2$) nanotubes (NTs) are used in a wide range of applications that go from sensors to nanofluidics. Currently, these NTs can be grown with diameters as small as 3 nm, with walls 1.5 nm thick. Recent experimental advances combined with first-principles calculations suggest that silica NTs could be obtained from a single silica sheet. In this work, we explore the water adsorption in such ultrathin silica NTs using molecular simulation and first-principles calculations. Combining molecular dynamics and density functional theory calculations we obtain putative structures for NTs formed by 10, 12, and 15-membered SiO$_2$ rings. Water adsorption isotherms for these NTs are obtained using Grand Canonical Monte Carlo simulations. Computing the accessible cross-section area ($A_\text{free}$) for the NTs, we were able to understand how this property correlates with condensation pressures. We found that $A_\text{free}$ does not necessarily grow with the NT size and that the higher the confinement (smaller $A_\text{free}$), the larger the condensation pressure.

Authors:Tzu-Chao Hung, Yokari Godinez-Loyola, Manuel Steinbrecher, Brian Kiraly, Alexander A. Khajetoorians, Nikos L. Doltsinis, Cristian A. Strassert, Daniel Wegner

Abstract: Luminescence of open-shell 3d metal complexes is often quenched due to ultrafast intersystem crossing (ISC) and cooling into a dark metal-centered excited state. We demonstrate successful activation of fluorescence from individual nickel phthalocyanine (NiPc) molecules in the junction of a scanning tunneling microscope (STM) by resonant energy transfer from other metal phthalocyanines at low temperature. By combining STM, scanning tunneling spectroscopy, STM- induced luminescence, and photoluminescence experiments as well as time-dependent density functional theory, we provide evidence that there is an activation barrier for the ISC, which in most experimental conditions is overcome. We show that this is also the case in an electroluminescent tunnel junction where individual NiPc molecules adsorbed on an ultrathin NaCl decoupling film on a Ag(111) substrate are probed. However, when placing an MPc (M = Zn, Pd, Pt) molecule close to NiPc by means of STM atomic manipulation, resonant energy transfer can excite NiPc without overcoming the ISC activation barrier, leading to Q-band fluorescence. This work demonstrates that the thermally activated population of dark metal-centered states can be avoided by a designed local environment at low temperatures paired with a directed molecular excitation into vibrationally cold electronic states. Thus, we can envisage the use of luminophores based on more abundant transition metal complexes that do not rely on Pt or Ir.

Authors:Laetitia Farinacci, Gael Reecht, Felix von Oppen, Katharina J. Franke

Abstract: Kagome lattices constitute versatile platforms for studying paradigmatic correlated phases. While molecular self-assembly of kagome structures on metallic substrates is promising, it is challenging to realize pristine kagome properties because of hybridization with the bulk degrees of freedom and modified electron-electron interactions. We suggest that a superconducting substrate offers an ideal support for a magnetic kagome lattice. Exchange coupling induces kagome-derived bands at the interface, which are protected from the bulk by the superconducting energy gap. We realize a magnetic kagome lattice on a superconductor by depositing Fe-porphin-chloride molecules on Pb(111) and using temperature-activated de-chlorination and self-assembly. This allows us to control the formation of smaller kagome precursors and long-range ordered kagome islands. Using scanning tunneling microscopy and spectroscopy at 1.6 K, we identify Yu-Shiba-Rusinov states inside the superconducting energy gap and track their hybridization from the precursors to larger islands, where the kagome lattice induces extended YSR bands. These YSR-derived kagome bands are protected inside the superconducting energy gap, motivating further studies to resolve possible spin-liquid or Kondo-lattice-type behavior.

Authors:Esteban A. Rodríguez-Mena, José Carlos Abadillo-Uriel, Gaëtan Veste, Biel Martinez, Jing Li, Benoît Sklénard, Yann-Michel Niquet

Abstract: We investigate the existence of linear-in-momentum spin-orbit interactions in the valence band of Ge/GeSi heterostructures using an atomistic tight-binding method. We show that symmetry breaking at the Ge/GeSi interfaces gives rise to a linear Dresselhaus-type interaction for heavy-holes. This interaction results from the heavy-hole/light-hole mixings induced by the interfaces and can be captured by a suitable correction to the minimal Luttinger-Kohn, four bands $\vec{k}\cdot\vec{p}$ Hamiltonian. It is dependent on the steepness of the Ge/GeSi interfaces, and is suppressed if interdiffusion is strong enough. Besides the Dresselhaus interaction, the Ge/GeSi interfaces also make a contribution to the in-plane gyromagnetic $g$-factors of the holes. The tight-binding calculations also highlight the existence of a small linear Rashba interaction resulting from the couplings between the heavy-hole/light-hole manifold and the conduction band enabled by the low structural symmetry of Ge/GeSi heterostructures. These interactions can be leveraged to drive the hole spin. The linear Dresselhaus interaction may, in particular, dominate the physics of the devices for out-of-plane magnetic fields. When the magnetic field lies in-plane, it is, however, usually far less efficient than the $g$-tensor modulation mechanisms arising from the motion of the dot in non-separable, inhomogeneous electric fields and strains.

Authors:Carlos Alberto Martins Junior, Henrique Musseli Cezar, Daniela Andrade Damasceno, Caetano Rodrigues Miranda

Abstract: The separation of carbon dioxide CO$_2$ from nitrogen gas (N$_2$), the main component of flue gas, has become an emerging action to mitigate climate change. Feasible and efficient approaches to exploring the separation properties of materials are molecular dynamics (MD) and Monte Carlo (MC) simulations. In these approaches, a careful choice of force fields is required to avoid unrealistic predictions of thermodynamic properties. However, most studies use Lorentz-Berthelot combining rules (LB) to obtain the interaction between different species, an approximation that could not capture the essence of interfacial interactions. In this context, we verified how accurate LB is in describing the interaction of N$_2$ molecules and carbon nanostructures by comparing the interaction energies from LB with those from density functional theory (DFT) calculations. We selected carbon nanomaterials because they are considered promising materials to perform N$_2$/CO$_2$ separation. The results show that the LB underestimates the interaction energies and affects the prediction of fundamental properties of solid-fluid interfacial interactions. To overcome this limitation, we parametrized a Lennard-Jones potential using energies and forces from DFT, obtained through the van der Waals functional KBM. The proposed potential show good transferability and agreement to ab-initio calculations. Grand Canonical Monte Carlo simulations were performed to verify the effects of employing LB in predicting the amount of nitrogen gas adsorbed inside different CNTs. LB predicts a lower density inside them. Moreover, our results suggest that LB leads to a different characterization of the adsorption properties of carbon nanotubes, by changing significantly the adsorption isotherm.

Authors:Jinjiang Zhu, Yang Feng, Xiaodong Zhou, Yongchao Wang, Zichen Lian, Weiyan Lin, Qiushi He, Yishi Lin, Youfang Wang, Hongxu Yao, Hao Li, Yang Wu, Jing Wang, Jian Shen, Jinsong Zhang, Yayu Wang, Yihua Wang

Abstract: The chiral edge current is the boundary manifestation of the Chern number of a quantum anomalous Hall (QAH) insulator. Its direct observation is assumed to require well-quantized Hall conductance, and is so far lacking. The recently discovered van der Waals antiferromagnet MnBi$_2$Te$_4$ is theorized as a QAH in odd-layers but has shown Hall resistivity below the quantization value at zero magnetic field. Here, we perform scanning superconducting quantum interference device (sSQUID) microscopy on these seemingly failed QAH insulators to image their current distribution. When gated to the charge neutral point, our device exhibits edge current, which flows unidirectionally on the odd-layer boundary both with vacuum and with the even-layer. The chirality of such edge current reverses with the magnetization of the bulk. Surprisingly, we find the edge channels coexist with finite bulk conduction even though the bulk chemical potential is in the band gap, suggesting their robustness under significant edge-bulk scattering. Our result establishes the existence of chiral edge currents in a topological antiferromagnet and offers an alternative for identifying QAH states.

Authors:Martin Lang, Swapneel Amit Pathak, Samuel J. R. Holt, Marijan Beg, Hans Fangohr

Abstract: The Bloch point is a point singularity in the magnetisation configuration, where the magnetisation vanishes. It can exist as an equilibrium configuration and plays an important role in many magnetisation reversal processes. In the present work, we focus on manipulating Bloch points in a system that can host stable Bloch points - a two-layer FeGe nanostrip with opposite chirality of the two layers. We drive Bloch points using spin-transfer torques and find that Bloch points can move collectively without any Hall effect and report that Bloch points are repelled from the sample boundaries and each other. We study pinning of Bloch points at wedge-shaped constrictions (notches) in the nanostrip and demonstrate that arrays of Bloch points can be moved past a series of notches in a controlled manner by applying consecutive current pulses of different strength. Finally, we simulate a T-shaped geometry and demonstrate that a Bloch point can be moved along different paths by applying current between suitable strip ends.

Authors:Yi-Hsien Wu, Leon C. Camenzind, Akito Noiri, Kenta Takeda, Takashi Nakajima, Takashi Kobayashi, Chien-Yuan Chang, Amir Sammak, Giordano Scappucci, Hsi-Sheng Goan, Seigo Tarucha

Abstract: Because of their long coherence time and compatibility with industrial foundry processes, electron spin qubits are a promising platform for scalable quantum processors. A full-fledged quantum computer will need quantum error correction, which requires high-fidelity quantum gates. Analyzing and mitigating the gate errors are useful to improve the gate fidelity. Here, we demonstrate a simple yet reliable calibration procedure for a high-fidelity controlled-rotation gate in an exchange-always-on Silicon quantum processor allowing operation above the fault-tolerance threshold of quantum error correction. We find that the fidelity of our uncalibrated controlled-rotation gate is limited by coherent errors in the form of controlled-phases and present a method to measure and correct these phase errors. We then verify the improvement in our gate fidelities by randomized benchmark and gate-set tomography protocols. Finally, we use our phase correction protocol to implement a virtual, high-fidelity controlled-phase gate.

Authors:Sara Zimny, Magdalena Tarnacka, Monika Geppert-Rybczyńska, Kamil Kamiński

Abstract: In this paper, we investigated the annealing experiments of poly(mercaptopropylmethylsiloxane, PMMS) confined within two types of porous templates (anodic aluminium oxide, AAO, and silica) characterized by different pore diameter, d= 8-120nm, using different thermal protocols (varying significantly in cooling/heating rate) by means of Broadband Dielectric Spectroscopy (BDS) supported by the complementary Differential Scanning Calorimetry (DSC) and temperature-dependent contact angle, {\theta}, measurements. It was found that relaxation times obtained from routine temperature-dependent dielectric investigations deviate from the bulk behavior when approaching the glass transition temperature. Importantly, this confinement induced effect can be easily removed by the annealing experiments performed at some specific range of temperatures. The analysis of the dielectric data collected during isothermal experiments of confined samples that was beforehand cooled with different rates revealed that (i) constant rates of annealing gets longer with cooling and weakly depend on the rate of cooling, and (ii) activation energy of the equilibration process, E_a, varies with the reduction of the pore diameter and material the porous template is made of. In fact, there is significant reduction in E_a from ~62 to ~23 kJ/mol obtained for the annealing process carried out in AAO (d= 10 nm) and silica (d= 8 nm) membranes, respectively. Such significant change in E_a can be explained taking into account temperature-dependence of {\theta} of PMMS indicating a notable change in wettability between both surfaces upon cooling. As a consequence, one can expect that the mass exchange between interfacial and core molecules as well as adsorption-desorption processes occurring at the interface at lower temperatures must be affected.

Authors:Maximilian Fürst, Denis Kochan, Cosimo Gorini, Klaus Richter

Abstract: Studies of the formation of Landau levels based on the Schr\"odinger equation for electrons constrained to curved surfaces have a long history. These include as prime examples surfaces with constant positive and negative curvature, the sphere [Phys. Rev. Lett. 51, 605 (1983)] and the pseudosphere [Annals of Physics 173, 185 (1987)]. Now, topological insulators, hosting Dirac-type surface states, provide a unique platform to experimentally examine such quantum Hall physics in curved space. Hence, extending previous work we consider solutions of the Dirac equation for the pseudosphere for both, the case of an overall perpendicular magnetic field and a homogeneous coaxial, thereby locally varying, magnetic field. For both magnetic-field configurations, we provide analytical solutions for spectra and eigenstates. For the experimentally relevant case of a coaxial magnetic field we find that the Landau levels split and show a peculiar scaling $\propto B^{1/4}$, thereby characteristically differing from the usual linear $B$ and $B^{1/2}$ dependence of the planar Schr\"odinger and Dirac case, respectively. We compare our analytical findings to numerical results that we also extend to the case of the Minding surface.

Authors:Yiming Pan, Fabio Caruso

Abstract: Valley degrees of freedom in transition-metal dichalcogenides influence thoroughly electron-phonon coupling and its nonequilibrium dynamics. We conducted a first-principles study of the quantum kinetics of chiral phonons following valley-selective carrier excitation with circularly-polarized light. Our numerical investigations treat the ultrafast dynamics of electrons and phonons on equal footing within a parameter-free ab-initio framework. We report the emergence of valley-polarized phonon populations in monolayer MoS$_2$ that can be selectively excited at either the K or K' valleys depending on the light helicity. The resulting vibrational state is characterized by a distinctive chirality, which lifts time-reversal symmetry of the lattice on transient timescales. We show that chiral valley phonons can further lead to fingerprints of vibrational dichroism detectable by ultrafast diffuse scattering and persisting beyond 10 ps. The valley polarization of nonequilibrium phonon populations could be exploited as information carrier, thereby extending the paradigm of valleytronics to the domain of vibrational excitations.

Authors:Simone Fratini, Sergio Ciuchi, Vladimir Dobrosavljevic, Louk Rademaker

Abstract: We present a theory for band-tuned metal-insulator transitions based on the Kubo formalism. Such a transition exhibits scaling of the resistivity curves, in the regime where $T\tau >1$ or $\mu \tau>1$, where $\tau$ is the scattering time and $\mu$ the chemical potential. At the critical value of the chemical potential, the resistivity diverges as a power law, $R_c \sim 1/T$. Consequently, on the metallic side there is a regime with negative $dR/dT$, which is often misinterpreted as insulating. We show that scaling and this `fake insulator' regime is observed in a wide range of experimental systems. In particular, we show that Mooij correlations in high-temperature metals with negative $dR/dT$ can be quantitatively understood with our scaling theory in the presence of $T$-linear scattering.

Authors:Mireia Tolosa-Simeón, Michael M. Scherer, Stefan Floerchinger

Abstract: Moir\'e materials have recently been established experimentally as a highly-tunable condensed matter platform, facilitating the controlled manipulation of band structures and interactions. In several of these moir\'e materials, Dirac cones are present in the low-energy regime near the Fermi level. Thus, fermionic excitations emerging in these materials close to the Dirac cones have a linear dispersion relation near the Fermi surface as massless relativistic Dirac fermions. Here, we study low-energy fermionic excitations of moir\'e Dirac materials in the presence of a mass gap that may be generated by symmetry breaking. Introducing a dynamical Fermi velocity and/or time-dependent mass gap for the Dirac quasiparticles, we exhibit the emergence of an analog of cosmological fermion pair production in terms of observables such as the expected occupation number or two-point correlation functions. We find that it is necessary and sufficient for quasiparticle production that only the ratio between the mass gap and the Fermi velocity is time-dependent. In this way, we establish that moir\'e Dirac materials can serve as analog models for cosmological spacetime geometries, in particular, for Friedmann-Lema\^itre-Robertson-Walker expanding cosmologies. We briefly discuss possibilities for experimental realization.

Authors:Huan Wang, Zhaochen Liu, Yadong Jiang, Jing Wang

Abstract: We propose a theory of anisotropic band flattening in moir\'e systems at the $\Gamma$ valley. We find that in twisted anisotropic two-dimensional crystals with a rectangular unit cell of $C_{2z}$ or mirror symmetries, a larger effective mass anisotropy $\eta=m_y/m_x$ has a stronger tendency to be further enhanced compared to that of monolayer, which leads to correlated physics in one dimension effectively. We predict twisted bilayer black phosphorus (tBBP) has giant anisotropic flattened moir\'e bands ($\eta\sim10^4$) from ab initio calculations and continuum model, where the low energy physics is described by the weakly coupled array of one-dimensional wires. We further calculate the phase diagram based on sliding Luttinger liquid by including the screened Coulomb interactions in tBBP, and find a large parameter space may host the non-Fermi liquid phase. We thus establish tBBP as a promising and experimentally accessible platform for exploring correlated physics in low dimensions.

Authors:Alessandro Braggio, Matteo Carrega, Björn Sothmann, Rafael Sánchez

Abstract: We investigate nonequilibrium effects in the transport of interacting electrons in quantum conductors, proposing the nonlocal thermoelectric response as a direct indicator of the presence of interactions, nonthermal states and the effect of correlations. This is done by assuming a quantum Hall setup where two channels (connected to reservoirs at different temperatures) co-propagate for a finite distance, such that a thermoelectrical response is only expected if the electron-electron interaction mediates heat exchange between the channels. This way, the nonlocal Seebeck response measures the interaction strength. Considering zero-range interactions, we solve the charge and energy currents and noises of a non-equilibrium integrable interacting system, determining the universal interaction-dependent length scale of energy equilibration. Further, a setup with two controllable quantum point contacts allows thermoelectricity to monitor the interacting system thermalisation as well as the fundamental role of cross-correlations in the heat exchange at intermediate length scales.

Authors:Sangeet S. Kumar, Brendan C. Mulkerin, Meera M. Parish, Jesper Levinsen

Abstract: Strong interactions between charges and light-matter coupled quasiparticles offer an intriguing prospect with applications from optoelectronics to light-induced superconductivity. Here, we investigate how the interactions between electrons and exciton-polaritons in a two-dimensional semiconductor microcavity can be resonantly enhanced due to a strong coupling to a trion, i.e., an electron-exciton bound state. We develop a microscopic theory that uses a strongly screened interaction between charges to enable the summation of all possible diagrams in the polariton-electron scattering process. The position and magnitude of the resonance is found to vary depending on the values of the light-matter coupling and detuning, thus indicating a large degree of tunability. We furthermore derive an analytic approximation of the interaction strength based on universal lowenergy scattering theory. This is found to match extremely well with our full calculation, indicating that the trion resonance is near universal, depending more on the strength of the light-matter coupling relative to the trion binding energy rather than on the details of the electronic interactions. Thus, we expect the trion resonance in polariton-electron scattering to appear in a broad range of microcavity systems with few semiconductor layers, such as doped monolayer MoSe2 where such resonances have recently been observed experimentally [Sidler et al., Nature Physics 13, 255 (2017)].

Authors:Sourav Manna, Ankur Das

Abstract: Shot noise at a conductance plateau in a quantum point contact (QPC) can be explained by considering equilibrations at the quantum Hall edges. The indication from recent experiments is that the charge equilibration length is much shorter than the thermal equilibration length. We discuss how this discovery gives rise to different thermal equilibration regimes in the presence of full charge equilibration. In this work, we classify these distinct regimes via dc current-current correlations \emph{(electrical shot noise)} at distinct QPC conductance plateaus for the edges of integer, particle-like, and hole-like filling fractions in a two dimensional electron gas.

Authors:Zhao Wang

Abstract: This study employs molecular dynamics simulations to examine the physisorption behavior of hydrocarbon molecules on a covalent graphene-nanotube hybrid nanostructure. The results indicate that the adsorbed molecules undergo self-diffusion into the nanotubes without the need for external driving forces, primarily driven by significant variations in binding energy throughout different regions. Notably, these molecules remain securely trapped within the tubes even at room temperature, thanks to a ``gate'' effect observed at the neck region, despite the presence of a concentration gradient that would typically hinder such trapping. This mechanism of passive mass transport and retention holds implications for the storage and separation of gas molecules.

Authors:Stephan Wong, Terry A. Loring, Alexander Cerjan

Abstract: Nonlinear topological insulators have garnered substantial recent attention as they have both enabled the discovery of new physics due to interparticle interactions, and may have applications in photonic devices such as topological lasers and frequency combs. However, due to the local nature of nonlinearities, previous attempts to classify the topology of nonlinear systems have required significant approximations that must be tailored to individual systems. Here, we develop a general framework for classifying the topology of nonlinear materials in any discrete symmetry class and any physical dimension. Our approach is rooted in a numerical $K$-theoretic method called the spectral localizer, which leverages a real-space perspective of a system to define local topological markers and a local measure of topological protection. Our nonlinear spectral localizer framework yields a quantitative definition of topologically non-trivial nonlinear modes that are distinguished by the appearance of a topological interface surrounding the mode. Moreover, we show how the nonlinear spectral localizer can be used to understand a system's topological dynamics, i.e., the time-evolution of nonlinearly induced topological domains within a system. We anticipate that this framework will enable the discovery and development of novel topological systems across a broad range of nonlinear materials.

Authors:Divya Rawat, Aksa Thomas, Ajay Soni

Abstract: Ultra-thin 2D materials have shown complete paradigm shift of understanding of physical and electronic properties because of confinement effects, symmetry breaking and novel phenomena at nanoscale. Bulk 2H-TaS2 undergoes an incommensurate charge density wave (I-CDW) transition temperature, TI-CDW - 76 K, however, onset of CDW in atomically thin layers is not clear. We explored the evidence of CDW instability in exfoliated atomically thin 2H-TaS2 using low temperature Raman spectroscopy. We have emphasized on CDW associated modes, M1 - 125 cm-1, M2 -158 cm-1, and M3 -334 cm-1, with thickness - 3 nm (one unit-cell). The asymmetric (Fano) line shape of M2 suggests evidence of strong electron-phonon coupling, which mainly drives the CDW instability. Our observations provide key evidence that the CDW can persists even in one-unit cell with a TI-CDW well above - 200 K, which is higher than bulk 2H-TaS2.

Authors:Jiaxin Du, Mei Li, Xue Zhang, Bin Xi, Yongjun Liu, Chun-Gui Duan, Jie Lu

Abstract: Steady-flow dynamics of ferromagnetic 180$^{\circ}$ domain walls (180DWs) in long and narrow spin valves (LNSVs) with interfacial Dzyaloshinskii-Moriya interaction (IDMI) under spin currents with Slonczewski $g-$factor are examined. Depending on the magnetization orientation of polarizers (pinned layers of LNSVs), dynamics of 180DWs in free layers of LNSVs are subtly manipulated: (i) For parallel polarizers, stronger spin polarization leads to higher Walker limit thus ensures the longevity of faster steady flows. Meantime, IDMI induces both the stable-region flapping and its width enlargement. (ii) For perpendicular polarizers, a wandering of 180DWs between bi- and tri-stability persists with the criticality adjusted by the IDMI. (iii) For planar-transverse polarizers, IDMI makes the stable region of steady flows completely asymmetric and further imparts a high saturation wall velocity under large current density. Under the last two polarizers, the ultrahigh differential mobility of 180DWs survives. The combination of Slonczewski spin current and IDMI provides rich possibilities of fine controlling on 180DW dynamics, hence opens avenues for magnetic nanodevices with rich functionality and high robustness.

Authors:Anjana Uday, Gertjan Lippertz, Kristof Moors, Henry F. Legg, Andrea Bliesener, Lino M. C. Pereira, A. A. Taskin, Yoichi Ando

Abstract: Inducing Cooper pairing in a thin ferromagnetic topological insulator in the quantum anomalous Hall state (called quantum anomalous Hall insulator, QAHI) is a promising way to realize topological superconductivity with associated chiral Majorana edge states. However, finding evidence of superconducting proximity effect in a QAHI has proven to be a considerable challenge due to inherent experimental difficulties. Here we report the observation of crossed Andreev reflection (CAR) across a narrow superconducting Nb electrode contacting the chiral edge state of a QAHI, evinced by a negative nonlocal voltage measured downstream from the grounded Nb electrode. This is an unambiguous signature of induced superconducting pair correlation in the chiral edge state. Our theoretical analysis demonstrates that CAR processes of the chiral edge are not strongly dependent on the nature of the superconductivity that mediates them. Nevertheless, the characteristic length of the CAR process is found to be much longer than the superconducting correlation length in Nb, which suggests that the CAR is in fact mediated by superconductivity induced on the QAHI surface. The approach and results presented here provide a foundation for future studies of topological superconductivity and Majorana physics, as well as for the search for non-Abelian zero modes.

Authors:Arya Thenapparambil, Graciely Elias dos Santos, Chang-An Li, Mohamed Abdelghany, Wouter Beugeling, Hartmut Buhmann, Charles Gould, Song-Bo Zhang, Björn Trauzettel, Laurens W. Molenkamp

Abstract: Nonlinear planar magnetotransport is ubiquitous in topological HgTe structures, both in tensile (topological insulator) or compressively strained layers (Weyl semimetal phase). We show that the common reason for the nonlinear planar magnetotransport is the presence of tilted Dirac cones combined with the formation of charge puddles. The origin of the tilted Dirac cones is the mix of the Zeeman term due to the in-plane magnetic field and quadratic contributions to the dispersion relation. We develop a network model that mimics transport of tilted Dirac fermions in the landscape of charge puddles. The model captures the essential features of the experimental data. It should be relevant for nonlinear planar magnetotransport in a variety of topological and small band gap materials.

Authors:Adam Rycerz, Maciej Fidrysiak, Danuta Goc-Jagło

Abstract: Magnetic flux piercing a carbon nanotube induce periodic gap oscillations which represent the Aharonov-Bohm effect at nanoscale. Here we point out, by analyzing numerically the anisotropic Hubbard model on a honeycomb lattice, that similar oscillations should be observable when uniaxial strain is applied to a nanotube. In both cases, a vector potential (magnetic- or strain-induced) may affect the measurable quantities at zero field. The analysis, carried out within the Gutzwiller Approximation, shows that for small semiconducting nanotube with zigzag edges and realistic value of the Hubbard repulsion ($U/t_0=1.6$, with $t_0\approx{}2.5\,$eV being the equilibrium hopping integral) energy gap can be reduced by a factor of more than $100$ due to the strain.

Authors:Kittithat Krongchon, Tawfiqur Rakib, Shivesh Pathak, Elif Ertekin, Harley T. Johnson, Lucas K. Wagner

Abstract: An uncertainty in studying twisted bilayer graphene (TBG) is the minimum energy geometry, which strongly affects the electronic structure. The minimum energy geometry is determined by the potential energy surface, which is dominated by van der Waals (vdW) interactions. In this work, large-scale diffusion quantum Monte Carlo (QMC) simulations are performed to evaluate the energy of bilayer graphene at various interlayer distances for four stacking registries. An accurate registry-dependent potential is fit to the QMC data and is used to describe interlayer interactions in the geometry of near-magic-angle TBG. The band structure for the optimized geometry is evaluated using the accurate local-environment tight-binding model. We find that compared to QMC, DFT-based vdW interactions can result in errors in the corrugation magnitude by a factor of 2 or more near the magic angle. The error in corrugation then propagates to the flat bands in twisted bilayer graphene, where the error in corrugation can affect the bandwidth by about 30% and can change the nature and degeneracy of the flat bands.

Authors:Martin von Helversen, Lara Greten, Imad Limame, Chin-Wen Shih, Paul Schlaugat, Carlos Antón-Solanas, Christian Schneider, Bárbara Rosa1, Andreas Knorr, Stephan Reitzenstein

Abstract: In recent years, much research has been undertaken to investigate the suitability of two-dimensional materials to act as single-photon sources with high optical and quantum optical quality. Amongst them, transition-metal dichalcogenides, especially WSe$_{2}$, have been one of the subjects of intensive studies. Yet, their single-photon purity and photon indistinguishability, remain the most significant challenges to compete with mature semiconducting systems such as self-assembled InGaAs quantum dots. In this work, we explore the emission properties of quantum emitters in a WSe$_{2}$ monolayer which are induced by metallic nanoparticles. Under quasi-resonant pulsed excitation, we verify clean single-photon emission with a $g^{(2)}(0) = 0.036\pm0.004$. Furthermore, we determine its temperature dependent coherence time via Michelson interferometry, where a value of (13.5$\pm$1.0) ps is extracted for the zero-phonon line (ZPL) at 4 K, which reduces to (9$\pm$2) ps at 8 K. Associated time-resolved photoluminescence experiments reveal a decrease of the decay time from (2.4$\pm$0.1) ns to (0.42$\pm$0.05) ns. This change in decay time is explained by a model which considers a F\"orster-type resonant energy transfer process, which yields a strong temperature induced energy loss from the SPE to the nearby Ag nanoparticle.

Authors:Alina Joch, Götz S. Uhrig

Abstract: Optically driven electronic spins coupled in quantum dots to nuclear spins show a pre-pulse signal (revival amplitude) after having been trained by long periodic sequences of pulses. The size of this revival amplitude depends on the external magnetic field in a specific way due to the varying commensurability of the nuclear Larmor precession period with the time $T_\text{rep}$ between two consecutive pulses. In theoretical simulations, sharp dips occur at fields when an integer number of precessions fits in $T_\text{rep}$; this feature could be used to identify nuclear isotopes spectroscopically. But these sharp and characteristic dips have not (yet) been detected in experiment. We study whether the nuclear quadrupolar interaction is the reason for this discrepancy because it perturbs the nuclear precessions. But our simulations show that the absolute width of the dips and their relative depth are not changed by quadrupolar interactions. Only the absolute depth decreases. We conclude that quadrupolar interaction alone cannot be the reason for the absence of the characteristic dips in experiment.

Authors:Zhixiong Li, Xiansi Wang, Xuejuan Liu, Peng Yan

Abstract: Twisted magnons (TMs) have great potential applications in communication and computing owing to the orbital angular momentum (OAM) degree of freedom. Realizing the unidirectional propagation of TMs is the key to design functional magnonics devices. Here we theoretically study the propagation of TMs in one-dimensional magnetic nanodisk arrays. By performing micromagnetic simulations, we find that the one-dimensional nanodisk array exhibits a few bands due to the collective excitations of TMs. A simple model by considering the exchange interaction is proposed to explain the emerging multiband structure and theoretical results agree well with micromagnetic simulations. Interestingly, for a zigzag structure, the dispersion curves and propagation images of TMs show obvious nonreciprocity for specific azimuthal quantum number ($l$), which originates from a geometric effect depending on the phase difference of TMs and the relative angle between two adjacent nanodisks. Utilizing this feature, one can conveniently realize the unidirectional propagation of TMs with arbitrary nonzero $l$. Our work provides important theoretical references for controlling the propagation of TMs.

Authors:Edvin G. Idrisov, Eddwi H. Hasdeo, Byjesh N. Radhakrishnan, Thomas L. Schmidt

Abstract: We study a two-dimensional (2D) electron system with a linear spectrum in the presence of Rashba spin-orbit (RSO) coupling in the hydrodynamic regime. We derive a semiclassical Boltzmann equation with a collision integral due to Coulomb interactions in the basis of the eigenstates of the system with RSO coupling. Using the local equilibrium distribution functions, we obtain a generalized hydrodynamic Navier-Stokes equation for electronic systems with RSO coupling. In particular, we discuss the influence of the spin-orbit coupling on the viscosity and the enthalpy of the system and present some of its observable effects in hydrodynamic transport.

Authors:I. M. Flor, A. Donis Vela, C. W. J. Beenakker, G. Lemut

Abstract: The chiral edge modes of a topological superconductor can transport fermionic quasiparticles, with Abelian exchange statistics, but they can also transport non-Abelian anyons: Majorana zero-modes bound to a {\pi}-phase domain wall that propagates along the boundary. Such an edge vortex is injected by the application of an h/2e flux bias over a Josephson junction. Existing descriptions of the injection process rely on the instantaneous scattering approximation of the adiabatic regime, where the internal dynamics of the Josephson junction is ignored. Here we go beyond that approximation in a time-dependent many-body simulation of the injection process, followed by a braiding of the mobile edge vortex with an immobile Abrikosov vortex in the bulk of the superconductor. Our simulation sheds light on the properties of the Josephson junction needed for a successful implementation of a flying Majorana qubit.

Authors:Guoan Li, Guang Yang, Ting Lin, M. Rossi, G. Badawy, Zhiyuan Zhang, Xiaofan Shi, Jiayu Shi, Degui Qian, Fang Lu, Lin Gu, An-Qi Wang, Zhaozheng Lyu, Guangtong Liu, Fanming Qu, Ziwei Dou, Qinghua Zhang, E. P. A. M. Bakkers, M. P. Nowak, P. Wójcik, Li Lu, Jie Shen

Abstract: With the development of quantum technology, hybrid devices that combine superconductors (S) and semiconductors (Sm) have attracted great attention due to the possibility of engineering structures that benefit from the integration of the properties of both materials. However, until now, none of the experiments have reported good control of band alignment at the interface, which determines the strength of S-Sm coupling and the proximitized superconducting gap. Here, we fabricate hybrid devices in a generic way with argon milling to modify the interface while maintaining its high quality. First, after the milling the atomically connected S-Sm interfaces appear, resulting in a large induced gap, as well as the ballistic transport revealed by the multiple Andreev reflections and quantized above-gap conductance plateaus. Second, by comparing transport measurement with Schr\"odinger-Poisson (SP) calculations, we demonstrate that argon milling is capable of varying the band bending strength in the semiconducting wire as the electrons tend to accumulate on the etched surface for longer milling time. Finally, we perform nonlocal measurements on advanced devices to demonstrate the coexistence and tunability of crossed Andreev reflection (CAR) and elastic co-tunneling (ECT) -- key ingredients for building the prototype setup for realization of Kitaev chain and quantum entanglement probing. Such a versatile method, compatible with the standard fabrication process and accompanied by the well-controlled modification of the interface, will definitely boost the creation of more sophisticated hybrid devices for exploring physics in solid-state systems.

Authors:P. S. Alekseev, A. P. Dmitriev

Abstract: In high-quality conductors, the hydrodynamic regime of electron transport has been recently realized. In this work we theoretically investigate magnetotransport of a viscous electron fluid in samples with electron-impermeable obstacles. We use the two approaches to describe the fluid flow. The first one is based on the equations of hydrodynamics of a charged fluid, which assume that the kinetic equation takes into account the two harmonics of the electron distribution function. The second approach is based on the equations that are obtained by taking into account three harmonics of the distribution function (''quasi-hydrodynamics''). Within the hydrodynamic approach, we consider the cases of the rough and the smooth edges of the disks, on which the electron scattering is diffusive or specular, respectively. The longitudinal magnetoresistivity turns out to be strong and negative, the same for both rough and smooth discs edges to within small corrections. For rough discs, the Hall resistivity is equal to its standard value. For smooth discs the Hall resistance acquire a small correction to the standard value, proportional to the Hall viscosity. In the quasi-hydrodynamic approach, we considered the case of smooth discs and small magnetic fields. In the regime when the flow is substantially different from the hydrodynamic one, the longitudinal resistivity does not depend on the shear stress relaxation time (but depends on the relaxation time of the third angular harmonic), while the correction to the standard Hall resistivity does not depend on both relaxation times. We compare the results of the hydrodynamic calculation of the longitudinal resistance with the experimental data on magnetotransport in high-quality GaAs quantum wells with macroscopic defects. A good agreement of theory and experiment evidences in favor of the realization of the hydrodynamic transport regime in such systems.

Authors:M. Coraiola, D. Z. Haxell, D. Sabonis, M. Hinderling, S. C. ten Kate, E. Cheah, F. Krizek, R. Schott, W. Wegscheider, F. Nichele

Abstract: Harnessing spin and parity degrees of freedom is of fundamental importance for the realization of emergent quantum devices. Nanostructures embedded in superconductor--semiconductor hybrid materials offer novel and yet unexplored routes for addressing and manipulating fermionic modes. Here we spectroscopically probe the two-dimensional band structure of Andreev bound states in a phase-controlled hybrid three-terminal Josephson junction. Andreev bands reveal spin-degeneracy breaking, with level splitting in excess of 9 GHz, and zero-energy crossings associated to ground state fermion parity transitions, in agreement with theoretical predictions. Both effects occur without the need of external magnetic fields or sizable charging energies and are tuned locally by controlling superconducting phase differences. Our results highlight the potential of multiterminal hybrid devices for engineering quantum states.

Authors:M. Hinderling, S. C. ten Kate, D. Z. Haxell, M. Coraiola, S. Paredes, E. Cheah, F. Krizek, R. Schott, W. Wegscheider, D. Sabonis, F. Nichele

Abstract: Properties of superconducting devices depend sensitively on the parity (even or odd) of the quasiparticles they contain. Encoding quantum information in the parity degree of freedom is central in several emerging solid-state qubit architectures. Yet, accurate, non-destructive, and time-resolved parity measurement is a challenging and long-standing issue. Here we report on control and real-time parity measurement in a superconducting island embedded in a superconducting loop and realized in a hybrid two-dimensional heterostructure using a microwave resonator. Device and readout resonator are located on separate chips, connected via flip-chip bonding, and couple inductively through vacuum. The superconducting resonator detects the parity-dependent circuit inductance, allowing for fast and non-destructive parity readout. We resolved even and odd parity states with signal-to-noise ratio SNR $\approx3$ with an integration time of $20~\mu$s and detection fidelity exceeding 98%. Real-time parity measurement showed state lifetime extending into millisecond range. Our approach will lead to better understanding of coherence-limiting mechanisms in superconducting quantum hardware and provide novel readout schemes for hybrid qubits.

Authors:Aurore Finco, Vincent Jacques

Abstract: Despite the considerable interest for antiferromagnets which appeared with the perspective of using them for spintronics, their experimental study, including the imaging of antiferromagnetic textures, remains a challenge. To address this issue, quantum sensors, and in particular the nitrogen-vacancy (NV) defects in diamond have become a widespread technical solution. We review here the recent applications of single NV centers to study a large variety of antiferromagnetic materials, from quantitative imaging of antiferromagnetic domains and non-collinear states, to the detection of spin waves confined in antiferromagnetic textures and the non-perturbative measurement of spin transport properties. We conclude with recent developments improving further the magnetic sensitivity of scanning NV microscopy, opening the way to detailed investigations of the internal texture of antiferromagnetic objects.

Authors:Pablo Moles, Francisco Domínguez-Adame, Leonor Chico

Abstract: We carry out an extensive numerical study of low-temperature electronic transport in quantum point contacts based on twisted bilayer graphene. Assuming ballistic electron dynamics, quantized plateaus in the conductance are observed in defect-free samples when the twisting angle is large enough. However, plateaus are smeared out and hardly noticeable on decreasing the angle. Close to the magic angle, the conductance around the charge neutrality point drops significantly and the quantization steps visible at higher angles are no longer appreciable. Furthermore, we consider the effects of a random distribution of vacancies on the quantum point contact. Whereas the electron-hole symmetry is broken in pristine samples, we find that this symmetry is restored upon increasing the concentration of vacancies. We explain this effect by a reduction of the effective interlayer coupling due to the presence of the vacancies.

Authors:Satyaki Kar

Abstract: Su-Schrieffer-Heeger (SSH) model is one of the simplest models to show topological end/edge states and the existence of Majorana fermions. Here we consider a SSH like model both in one and two dimensions where a nearest neighbor hopping features spatially periodic modulations. In the 1D chain, we witness appearance of new in-gap end states apart from a pair of Majorana zero modes (MZM) when the hopping periodicity go beyond two lattice spacings. The pair of MZMs, that appear in the topological regime, characterize the end modes each existing in either end of the chain. These, however, crossover to both-end end modes for small hopping detuning strength in a finite chain. Contrarily in a 2D SSH model with symmetric hopping that we consider, both non-zero and zero energy topological states appear in a finite square lattice even with a simple staggered hopping, though the zero energy modes disappear in a ribbon configuration. Apart from edge modes, the 2D system also features corner modes as well as modes with satellite peaks distributed non-randomly within the lattice. In both the dimensions, an increase in the periodicity of hopping modulation causes the zero energy Majorana modes to become available for either sign of the detuning. But interestingly with different periodicity for hopping modulations in the two directions, the zero energy modes in a 2D model become rarer and does not appear for all strength and sign of the detuning.

Authors:Sakthi Priya Amirtharaj, Zhiyuan Xie, Josephine Si Yu See, Gabriele Rolleri, Wen Chen, Alexandre Bouhelier, Emanuel Lörtscher, Christophe Galland

Abstract: Electrically connected and plasmonically enhanced molecular junctions combine the optical functionalities of high field confinement and enhancement (cavity function), and of high radiative efficiency (antenna function) with the electrical functionalities of molecular transport and electrically driven light emission. They are supposed to play a leading role in emerging nanoscale optoelectronic devices; yet, this development is hindered by an insufficient control and understanding of atomic-scale phenomena that govern the optical and electrical behavior of plasmonic nanojunctions under ambient operating conditions. For instance, displacement of a single atom may drastically influence the junction's conductance and its optical near-field distribution. Here, we investigate tunneling-induced light emission from a self-assembled metal-molecule-metal junction embedded in a plasmonic cavity at room-temperature. We find that despite the presence of hundreds of molecules in the junction, electrical conductance and light emission are both highly sensitive to atomic-scale fluctuations - a phenomenology reminiscent of picocavities observed in Raman scattering and of luminescence blinking from photo-excited plasmonic junctions. We present a minimal electrical model that is able to capture all main experimental features. Contrasting with these microscopic fluctuations, the overall plasmonic and electronic functionalities of our devices feature an excellent long-term stability and reproducibility at room temperature and under electrical bias of several volts, allowing for measurements over several months. Our work contributes to the understanding of atomic fluctuations in molecular plasmonic junctions and to the development of more robust and scalable platforms for nanoscale optoelectronics.

Authors:Klaus E. Hermann

Abstract: The shape of crystalline nanoparticles (NP) can often be described by polyhedra with flat facet surfaces. Thus, structural studies of polyhedral bodies can help to describe geometric details of NPs. Here we consider compact polyhedra of cubic point symmetry Oh.as simple models. Their surfaces are described by facets with normal vectors along selected directions (a, b, c) together with their symmetry equivalents forming a direction family {abc}. For given {abc} this yields generic polyhedra with up to 48 facets where we focus on polyhedra with facets of {abc} = {100}, {110}, and {111}, suggested for metal NPs with cubic lattices. The resulting generic polyhedra, cubic, rhombohedral, and octahedral, can serve for the description of non-generic polyhedra as intersections of corresponding generic species. Their structural properties are shown to be fully determined by only three structure parameters, facet distances R100, R110, and R111 of three types of facets. This provides a phase diagram to completely classify the corresponding Oh symmetry polyhedra. Structural properties of all polyhedra, such as shape, size, and facet geometries, are discussed in analytical and numerical detail with visualization of characteristic examples. The results may be used for respective nanoparticle simulations but also as a repository assisting the interpretation of structures of real compact nanoparticles observed by experiment.

Authors:Pierre Vallet, Jérôme Cayssol

Abstract: In superconductors (SC), the Higgs amplitude mode is a coherent oscillation of the order parameter typically generated by THz laser irradiation. In this paper we propose to probe the Higgs mode using electronic transport in ballistic superconducting hybrid devices. We first confirm the existence of a non-zero amplitude mode in the clean case using the Keldysh-Eilenberger formalism. We then investigate two different geometries, respectively a normal-insulating-superconductor (NIS) tunnel junction and a NSN junction with transparent interfaces, the superconductor being irradiated in both situations. In the NIS case, the Higgs manifests itself in the second-order AC current response which is resonant at the Higgs frequency. In the NSN case, the DC differential conductance allows to probe the gaps generated by the Higgs mode in the Floquet spectrum.

Authors:Luise Siegl, Michaela Lammel, Akashdeep Kamra, Hans Huebl, Wolfgang Belzig, Sebastian T. B. Goennenwein

Abstract: A magnonic spin current crossing a ferromagnet-metal interface is accompanied by spin current shot noise arising from the discrete quanta of spin carried by magnons. In thin films, e.g., the spin of so-called squeezed magnons have been shown to deviate from the common value $\hbar$, with corresponding changes in the spin noise. In experiments, spin currents are typically converted to charge currents via the inverse spin Hall effect. We here analyze the magnitude of the spin current shot noise in the charge channel for a typical electrically detected spin pumping experiment, and find that the voltage noise originating from the spin current shot noise is much smaller than the inevitable Johnson-Nyquist noise. Furthermore, due to the local nature of the spin-charge conversion, the ratio between spin current shot noise and Johnson-Nyquist noise does not scale with sample geometry and sensitively depends on material-specific transport properties. Our analysis thus provides guidance for the experimental detection of squeezed magnons through spin pumping shot noise.

Authors:Jan M. van Ruitenbeek, Richard Korytár, Ferdinand Evers

Abstract: Chirality-induced spin selectivity has been reported in many experiments, but a generally accepted theoretical explanation has not yet been proposed. Here, we introduce a simple model system of a straight cylindrical free-electron wire, containing a helical string of atomic scattering centers, with spin-orbit interaction. The advantage of this simple model is that it allows deriving analytical expressions for the spin scattering rates, such that the origin of the effect can be easily followed. We find that spin-selective scattering can be viewed as resulting from constructive interference of partial waves scattered by the spin-orbit terms. We demonstrate that forward scattering rates are independent of spin, while back scattering is spin dependent over wide windows of energy. Although the model does not represent the full details of electron transmission through chiral molecules, it clearly reveals a mechanism that could operate in chiral systems.

Authors:Natalya A. Zimbovskaya

Abstract: We present a theoretical analysis of heat transport through a single-molecule junction with two possible transport channels for electrons where interactions between electrons on the molecule and phonons in the nuclear environment is strong and Marcus-type processes predominate in the electron transport. We show that within the steady state regime the competition between transport channels may result in negative differential heat conductance and cooling of the molecule environment. Also, we analyze the effect of a slowly driven molecule level (provided that another level is fixed) on the heat transport and power generated in the system.

Authors:David T. S. Perkins, Aires Ferreira

Abstract: Spintronics has become a broad and important research field that intersects with magnetism, nano-electronics, and materials science. Its overarching aim is to provide a fundamental understanding of spin-dependent phenomena in solid-state systems that can enable a new generation of spin-based logic devices. Over the past decade, graphene and related 2D van der Waals crystals have taken center stage in expanding the scope and potential of spintronic materials. Their distinctive electronic properties and atomically thin nature have opened new opportunities to probe and manipulate internal electronic degrees of freedom. Purely electrical control over conduction-electron spins can be attained in graphene-transition metal dichalcogenide heterostructures, due to proximity effects combined with graphene's high electronic mobility. Specifically, graphene experiences a proximity-induced spin-orbit coupling that enables efficient spin-charge interconversion processes; the two most well-known and at the forefront of current research are the spin Hall and inverse spin galvanic effects, wherein an electrical current yields a spin current and non-equilibrium spin polarization, respectively. This article provides an overview of the basic principles, theory, and experimental methods underpinning the nascent field of 2D material-based spintronics.

Authors:Yoshihiro Fujiwara, Motoya Shinozaki, Kazuma Matsumura, Kosuke Noro, Riku Tataka, Shoichi Sato, Takeshi Kumasaka, Tomohiro Otsuka

Abstract: Semiconductor quantum dots are useful for controlling and observing quantum states and can also be used as sensors for reading out quantum bits and exploring local electronic states in nanostructures. However, challenges remain for the sensor applications, such as the trade-off between sensitivity and dynamic range and the issue of instability due to external disturbances. In this study, we demonstrate proportional-integral-differential feedback control of the radio-frequency reflectometry in GaN nanodevices using a field-programmable gate array. This technique can maintain the operating point of the charge sensor with high sensitivity. The system also realizes a wide dynamic range and high sensor sensitivity through the monitoring of the feedback signal. This method has potential applications in exploring dynamics and instability of electronic and quantum states in nanostructures.

Authors:Hau Tian Teo, Subhaskar Mandal, Yang Long, Haoran Xue, Baile Zhang

Abstract: It has recently been shown that the non-Hermitian skin effect can be suppressed by magnetic fields. In this work, using a two-dimensional tight-binding lattice, we demonstrate that a pseudomagnetic field can also lead to the suppression of the non-Hermitian skin effect. With an increasing pseudomagnetic field, the skin modes are found to be pushed into the bulk, accompanied by the reduction of skin topological area and the restoration of Landau level energies. Our results provide a time-reversal invariant route to localization control and could be useful in various classical wave devices that are able to host the non-Hermitian skin effect but inert to magnetic fields.

Authors:Pieter M. Gunnink, Tim Ludwig, Rembert A. Duine

Abstract: Spin torque oscillators are conventionally described by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, at the onset of oscillations, the predictions of the conventional LLGS equation differ qualitatively from experimental results and thus appear to be incomplete. In this work we show that taking charge conservation into account leads to a previously-overlooked self-induced torque, which modifies the LLGS equation. We show that the self-induced torque originates from the pumping current that a precessing magnetization drives through a magnetic tunnel junction. To illustrate the importance of the self-induced torque, we consider an in-plane magnetized nanopillar, where it gives clear qualitative corrections to the conventional LLGS description.

Authors:Ze Zheng, Kainan Chang, Jin Luo Cheng

Abstract: Generating photogalvanic effects in centrosymmetric materials can provide new opportunities for developing passive photodetectors and energy harvesting devices. In this work, we investigate the photogalvanic effects in centrosymmetric two-dimensional materials, AA- and AB-stacked bilayer graphene, by applying an external gate voltage to break the symmetry. Using a tight-binding model to describe the electronic states, the injection coefficients for circular photogalvanic effects and shift conductivities for linear photogalvanic effects are calculated for both materials with light wavelengths ranging from THz to visible. We find that gate voltage induced photogalvanic effects can be very significant for AB-stacked bilayer graphene, with generating a maximal dc current in the order of mA for a 1 $\mu$m wide sample illuminated by a light intensity of 0.1 GW/cm$^2$, which is determined by the optical transition around the band gap and van Hove singularity points. Although such effects in AA-stacked bilayer graphene are about two orders of magnitude smaller than those in AB-stacked bilayer graphene, the spectrum is interestingly limited in a very narrow photon energy window, which is associated with the interlayer coupling strength. A detailed analysis of the light polarization dependence is also performed. The gate voltage and chemical potential can be used to effectively control the photogalvanic effects.

Authors:Mark O. Goerbig

Abstract: Volkov-Pankratov surface bands arise in smooth topological interfaces, i.e. interfaces between a topological and a trivial insulator, in addition to the chiral surface state imposed by the bulk-surface correspondence of topological materials. These two-dimensional bands become Landau-quantized if a magnetic field is applied perpendicular to the interface. I show that the energy scales, which are typically in the 10-100 meV range, can be controlled both by the perpendicular magnetic field and the interface width. The latter can still be varied with the help of a magnetic-field component in the interface. The Landau levels of the different Volkov-Pankratov bands are optically coupled, and their arrangement may allow one to obtain population inversion by resonant optical pumping. This could serve as the elementary brick of a multi-level laser based on Landau levels. Moreover, the photons are absorbed and emitted either parallel or perpendicular to the magnetic field, respectively in the Voigt and Faraday geometry, depending on the Volkov-Pankratov bands and Landau levels involved in the optical transitions.

Authors:Sourav Manna, Ankur Das, Moshe Goldstein

Abstract: Bulk-boundary correspondence allows one to probe the bulk topological order by studying the transport properties of the edge modes. However, edge modes in a fractional quantum Hall (FQH) state can undergo edge reconstruction; moreover, they can be in the coherent regime or exhibit varying degrees of charge and thermal equilibration, giving rise to a zoo of intriguing scenarios. Even more possibilities arise when a quantum point contact (QPC) is introduced and tuned into a conductance plateau. Distinguishing among the different models and equilibration regimes is an outstanding problem, which cannot be resolved by dc electrical conductance measurement. In this work we show that \emph{electrical shot noise} at a QPC conductance plateau can serve as such diagnostic. As a prototypical example we consider the $\nu=2/3$ FQH state, and show that different inequalities between the auto- and cross-correlation electrical shot noise hold for different edge models. In particular, our results offer several possible scenarios for the QPC conductance plateaus $e^2/3h$ (observed previously), $e^2/2h$ (recently observed), and $5e^2/9h$ (our prediction), as well as how to distinguish among them via shot noise.

Authors:Sourav Manna, Ankur Das, Moshe Goldstein

Abstract: The $\nu = 2/3$ filling is the simplest paradigmatic example of a fractional quantum Hall state, which contains counter-propagating edge modes. These modes can be either in the coherent or equilibrated to different extents, on top of the possible edge reconstruction. In the coherent regime, two distinct renormalization group fixed points have been previously proposed, namely Kane-Fischer-Polchinski and Wang-Meir-Gefen. In the equilibration regime, different degree of thermal equilibration can exist, while charge is fully equilibrated. Here, we show that these rich variety of models can give rise to three possible conductance plateaus at $e^2/2h$ (recently observed in experiments), $5e^2/9h$ (we predict), and $e^2/3h$ (observed earlier in experiments) in a quantum point contact geometry. We identify different mechanisms for \emph{electrical shot noise} generation at these plateaus, which provides an experimentally accessible venue for distinguishing among the distinct models.

Authors:Yuval Abulafia, Amit Goft, Nadav Orion, Eric Akkermans

Abstract: We present a general method to identify topological materials by studying the local electronic density $\delta \rho \left(\boldsymbol{r}\right)$. More specifically, certain types of defects or spatial textures such as vacancies, turn graphene into a topological material characterised by invariant Chern or winding numbers. We show that these numbers are directly accessible from a dislocation pattern of $\delta \rho \left(\boldsymbol{r}\right)$, resulting from an interference effect induced by topological defects. For non topological defects such as adatoms, this pattern is scrambled by Friedel oscillations absent in topological cases. A Kekule distortion is discussed and shown to be equivalent to a vacancy.

Authors:Leonard Deuschle, Jonathan Backman, Mathieu Luisier, Jiang Cao

Abstract: We present first-principles quantum transport simulations of single-walled carbon nanotubes based on the NEGF method and including carrier-carrier interactions within the self-consistent GW approximation. Motivated by the characteristic enhancement of interaction between charge carriers in one-dimensional systems, we show that the developed framework can predict Auger recombination, hot carrier relaxation, and impact ionization in this type of nanostructures. Using the computed scattering rates, we infer the inverse electron-hole pair lifetimes for different Auger processes in several device configurations.

Authors:H. Grebel, Y. Zhang

Abstract: In the past, when Au nanoparticles (AuNPs) were placed just above (but not included in) active carbon (A-C) electrodes, a 10-fold amplification of gravitational specific capacitance were demonstrated, despite the small mass-ratio between the AuNPs and A-C; a ratio of 1:5000. We use surface enhance Raman spectroscopy (SERS) in further studying this system. Supercapacitors are volumetric elements while SERS is a surface interrogating method, whose signal could be impacted by many factors, including local colloid's preferential points (hot spots). Here, we use the ratio between the D- and G-lines of the A-C electrode, ID/IG, as a marker to eliminate the surface inconsistencies. At some concentration levels of AuNPs, the A-C Raman signature shows a clear preference towards the 1600 cm-1 vibration mode (G-line). Following that point, the cell exhibits a large specific capacitance.

Authors:Yi-Ting Tu, Sankar Das Sarma

Abstract: The experimental work [J. Crossno et al., Science 351, 1058 (2016)], which reported the violation of the Wiedemann-Franz law in monolayer graphene characterized by a sharp peak of the Lorenz ratio at a finite temperature, has not been fully explained. Our previous work [Y.-T. Tu and S. Das Sarma, Phys. Rev. B 107, 085401 (2023)] provided a possible explanation through a Boltzmann-transport model with bipolar diffusion and an energy gap possibly induced by the substrate. In this paper, we extend our calculation to include a weak magnetic field perpendicular to the graphene layer, which is experimentally relevant, and may shed light on the possible violation or not of the Wiedemann-Franz law. We find that the magnetic field enhances the size of the peak of the Lorenz ratio but has little effect on its position, and that the transverse component of the Lorenz ratio can be either positive or negative depending on the parameter regime. In addition, we do the same calculation for bilayer graphene in the presence of a magnetic field and show the qualitative similarity with monolayer graphene. Our work should motivate magnetic-field-dependent experiments elucidating the nature of the charge carriers in graphene layers.

Authors:Shuji Ito, Moeta Tsukamoto, Kensuke Ogawa, Tokuyuki Teraji, Kento Sasaki, Kensuke Kobayashi

Abstract: Nitrogen-vacancy (NV) centers in diamonds are a powerful tool for accurate magnetic field measurements. The key is precisely estimating the field-dependent splitting width of the optically detected magnetic resonance (ODMR) spectra of the NV centers. In this study, we investigate the optical power dependence of the ODMR spectra using NV ensemble in nanodiamonds (NDs) and a single-crystal bulk diamond. We find that the splitting width exponentially decays and is saturated as the optical power increases. Comparison between NDs and a bulk sample shows that while the decay amplitude is sample-dependent, the optical power at which the decay saturates is almost sample-independent. We propose that this unexpected phenomenon is an intrinsic property of the NV center due to non-axisymmetry deformation or impurities. Our finding indicates that diamonds with less deformation are advantageous for accurate magnetic field measurements.

Authors:Uddipta Kar Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan Nano Science and Technology, Taiwan International Graduate Program, Academia Sinica and National Taiwan University, Taipei, Taiwan, Elisha Cho-Hao Lu Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan, Akhilesh Kr. Singh Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan, P. V. Sreenivasa Reddy Department of Physics, National Taiwan University, Taipei, Taiwan, Youngjoon Han Department of Physics, California Institute of Technology, Pasadena, California, USA, Xinwei Li Department of Physics, California Institute of Technology, Pasadena, California, USA, Cheng-Tung Cheng Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan, Song Yang Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu, Taiwan, Chun-Yen Lin Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu, Taiwan, I-Chun Cheng Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, Taiwan, Chia-Hung Hsu Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu, Taiwan, D. Hsieh Department of Physics, California Institute of Technology, Pasadena, California, USA, Wei-Cheng Lee Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, New York, USA, Guang-Yu Guo Department of Physics, National Taiwan University, Taipei, Taiwan Physics Division, National Center for Theoretical Sciences, Taipei, Taiwan, Wei-Li Lee Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan

Abstract: The identification of distinct charge transport features, deriving from nontrivial bulk band and surface states, has been a challenging subject in the field of topological systems. In topological Dirac and Weyl semimetals, nontrivial conical bands with Fermi-arc surfaces states give rise to negative longitudinal magnetoresistance due to chiral anomaly effect and unusual thickness dependent quantum oscillation from Weyl-orbit effect, which were demonstrated recently in experiments. In this work, we report the experimental observations of large nonlinear and nonreciprocal transport effects for both longitudinal and transverse channels in an untwinned Weyl metal of SrRuO$_3$ thin film grown on a SrTiO$_{3}$ substrate. From rigorous measurements with bias current applied along various directions with respect to the crystalline principal axes, the magnitude of nonlinear Hall signals from the transverse channel exhibits a simple sin$\alpha$ dependent at low temperatures, where $\alpha$ is the angle between bias current direction and orthorhombic [001]$_{\rm o}$, reaching a maximum when current is along orthorhombic [1-10]$_{\rm o}$. On the contrary, the magnitude of nonlinear and nonreciprocal signals in the longitudinal channel attains a maximum for bias current along [001]$_{\rm o}$, and it vanishes for bias current along [1-10]$_{\rm o}$. The observed $\alpha$-dependent nonlinear and nonreciprocal signals in longitudinal and transverse channels reveal a magnetic Weyl phase with an effective Berry curvature dipole along [1-10]$_{\rm o}$ from surface states, accompanied by 1D chiral edge modes along [001]$_{\rm o}$.

Authors:Jeong Woo Han, Pavlo Sai, Dmytro But, Ece Uykur, Stephan Winnerl, Gagan Kumar, Matthew L. Chin, Rachael L. Myers-Ward, Matthew T. Dejarld, Kevin M. Daniels, Thomas E. Murphy, Wojciech Knap, Martin Mittendorff

Abstract: Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong (~1{\deg}) ultrafast Faraday rotation (~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm-2.

Authors:Maryam Khosravian, Elena Bascones, Jose L. Lado

Abstract: Twisted van der Waals materials have risen as highly tunable platform for realizing unconventional superconductivity. Here we demonstrate how a topological superconducting state can be driven in a twisted graphene multilayer at a twist angle of approximately 1.6 degrees proximitized to other 2D materials. We show that an encapsulated twisted bilayer subject to induced Rashba spin-orbit coupling, s-wave superconductivity and exchange field generates a topological superconducting state enabled by the moire pattern. We demonstrate a variety of topological states with different Chern numbers highly tunable through doping, strain and bias voltage. Our proposal does not depend on a fine tuning of the twist angle, but solely on the emergence of moire minibands and is applicable for twist angles between 1.3 and 3 degrees. Our results establish the potential of twisted graphene bilayers to create artificial topological superconductivity without requiring ultraflat dispersions.

Authors:Assem Alassaf, János Koltai, László Oroszlány

Abstract: In this paper we establish a connection between the bulk topological structure and the magnetic properties of drumhead surface states of nodal loop semimetals. We identify the magnetic characteristics of the surface states and compute the system's magnon spectrum by treating electron-electron interactions on a mean-field level. We draw attention to a subtle connection between a Lifshitz-like transition of the surface states driven by mechanical distortions and the magnetic characteristics of the system. Our findings may be experimentally verified e.g. by spin polarized electron energy loss spectroscopy of nodal semimetal surfaces.

Authors:Rahnuma Rahman, Supriyo Bandyopadhyay

Abstract: Probabilistic computing with binary stochastic neurons (BSN) implemented with low- or zero-energy barrier nanoscale ferromagnets (LBMs) possessing in-plane magnetic anisotropy has emerged as an efficient paradigm for solving computationally hard problems. The fluctuating magnetization of an LBM at room temperature encodes a p-bit which is the building block of a BSN. Its only drawback is that the dynamics of common (transition metal) ferromagnets are relatively slow and hence the number of uncorrelated p-bits that can be generated per second - the so-called "flips per second" (fps) - is insufficient, leading to slow computational speed in autonomous co-processing with p-computers. Here, we show that a simple way to increase fps is to replace commonly used ferromagnets (e.g. Co, Fe, Ni), which have large saturation magnetization Ms, with a dilute magnetic semiconductor like GaMnAs with much smaller saturation magnetization. The smaller Ms reduces the energy barrier within the LBM and increases the fps significantly. It also offers other benefits such as increased packing density for increased parallelization and reduced device to device variation. This provides a way to realize the hardware acceleration and energy efficiency promise of p-computers.

Authors:Liyang Liao, Jorge Puebla, Kei Yamamot, Junyeon Kim, Sadamichi Meakawa, Yunyoung Hwang, You Ba, Yoshichika Otani

Abstract: Phonons and magnons are engineered by periodic potential landscapes in phononic and magnonic crystals, and their combined studies may enable valley phonon transport tunable by the magnetic field. Through nonreciprocal surface acoustic wave transmission, we demonstrate valley-selective phonon-magnon scattering in magnetoelastic superlattices. The lattice symmetry and the out-of-plane magnetization component control the sign of nonreciprocity. The phonons in the valleys play a crucial role in generating nonreciprocal transmission by inducing circularly polarized strains that couple with the magnons. The transmission spectra show a nonreciprocity peak near a transmission gap, matching the phononic band structure. Our results open the way for manipulating valley phonon transport through periodically varying magnon-phonon coupling.

Authors:Bristi Ghosh, Sushanta Dattagupta, Malay Bandyopadhyay

Abstract: We present here a general formulation for the interband dynamical optical conductivity in the nonlinear regime of graphene in the presence of a quantum bath comprising phonons and electrons. Our main focus is the relaxation behavior of the quantum solid of graphene perturbed by an oscillatory electric field. Considering the optical range of the frequency and a considerable amount of the amplitude of the field, one can observe a nonlinear response by formulating a quantum master equation of the density operator associated with the Hamiltonian encapsulated in the form of a spin-Boson model of dissipative quantum statistical mechanics. Mapping the valence and conduction states as the eigenstates of the Pauli spin operators and utilizing the rotating wave approximation to omit off-resonant terms, one can solve the rate equation for the mean population of the conduction and valence states and the mixing matrix elements between them. Our results reveal the nonlinear steady-state regime's population inversion and interband coherence. It is characterized by a single dimensionless parameter that is directly proportional to the incident field strength and inversely proportional to the optical frequency. Our method is also capable of calculating the nonlinear interband optical conductivity of doped and gapped graphene at finite temperatures. The effects of different bath spectra for phonons and electrons are examined in detail. Although our general formulation can address a variety of nonequilibrium response of the two-band system, it also facilitates a connection with phenomenological modeling of nonlinear optical conductivity.

Authors:David Scheer, Fabian Hassler

Abstract: A single Josephson junction in the phase-slip regime exhibits Bloch oscillations in the voltage when biased with a DC current $I_\text{DC}$. The frequency of the oscillation is given by $\pi I_\text{DC}/e$, with $e$ the elementary charge, linking the current to the frequency via fundamental constants of nature. If an additional AC drive is applied, the Bloch oscillations may synchronize with the external drive. This leads to the emergence of dual Shapiro steps at fixed current in the $IV$ characteristics of the device. For applications as a current standard, frequencies of the order of 10\,GHz are required. These are challenging to implement experimentally without detrimental effects due to stray capacitances. Here, we propose to employ an additional Josephson junction with a DC voltage bias as an on chip AC source due to the AC Josephson effect. We study the back action of the Bloch oscillations on the Josephson oscillations and identify a parameter regime in which it is minimized. Furthermore, we find that the back action can even be utilized to further enhance the driving signal which can lead to increased widths of the resulting dual Shapiro steps. Finally, we show dual Shapiro steps for a set of realistic experimental parameters at finite temperatures.

Authors:Jesús D. Cifuentes, Philip Y. Mai, Frédéric Schlattner, H. Ekmel Ercan, MengKe Feng, Christopher C. Escott, Andrew S. Dzurak, Andre Saraiva

Abstract: The boom of semiconductor quantum computing platforms created a demand for computer-aided design and fabrication of quantum devices. Path integral Monte Carlo (PIMC) can have an important role in this effort because it intrinsically integrates strong quantum correlations that often appear in these multi-electron systems. In this paper we present a PIMC algorithm that estimates exchange interactions of three-dimensional electrically defined quantum dots. We apply this model to silicon metal-oxide-semiconductor (MOS) devices and we benchmark our method against well-tested full configuration interaction (FCI) simulations. As an application, we study the impact of a single charge trap on two exchanging dots, opening the possibility of using this code to test the tolerance to disorder of CMOS devices. This algorithm provides an accurate description of this system, setting up an initial step to integrate PIMC algorithms into development of semiconductor quantum computers.

Authors:Yong Xu, Fan Zhang, Albert Fert, Henri-Yves Jaffres, Yongshan Liu, Renyou Xu, Yuhao Jiang, Houyi Cheng, Weisheng Zhao

Abstract: Orbitronics is based on the use of orbit currents as information carriers. Up to now, orbit currents were created from the conversion of charge or spin currents, and inversely, they could be converted back to charge or spin currents. Here we demonstrate that orbit currents can also be generated by femtosecond light pulses on Ni. In multilayers associating Ni with oxides and nonmagnetic metals such as Cu, we detect the orbit currents by their conversion into charge currents and the resulting terahertz emission. We show that the orbit currents extraordinarily predominate the light-induced spin currents in Ni-based systems, whereas only spin currents can be detected with CoFeB-based systems. In addition, the analysis of the time delays of the terahertz pulses leads to relevant information on the velocity and propagation of orbit carriers. Our finding of light-induced orbit currents and our observation of their conversion into charge currents opens new avenues in orbitronics, including the development of orbitronic terahertz devices.

Authors:Zijing Zhao, Junzhe Kang, Ashwin Tunga, Hojoon Ryu, Ankit Shukla, Shaloo Rakheja, Wenjuan Zhu

Abstract: As a promising alternative to the Von Neumann architecture, in-memory computing holds the promise of delivering high computing capacity while consuming low power. Content addressable memory (CAM) can implement pattern matching and distance measurement in memory with massive parallelism, making them highly desirable for data-intensive applications. In this paper, we propose and demonstrate a novel 1-transistor-per-bit CAM based on the ferroelectric reconfigurable transistor. By exploiting the switchable polarity of the ferroelectric reconfigurable transistor, XOR/XNOR-like matching operation in CAM can be realized in a single transistor. By eliminating the need for the complementary circuit, these non-volatile CAMs based on reconfigurable transistors can offer a significant improvement in area and energy efficiency compared to conventional CAMs. NAND- and NOR-arrays of CAMs are also demonstrated, which enable multi-bit matching in a single reading operation. In addition, the NOR array of CAM cells effectively measures the Hamming distance between the input query and stored entries. Furthermore, utilizing the switchable polarity of these ferroelectric Schottky barrier transistors, we demonstrate reconfigurable logic gates with NAND/NOR dual functions, whose input-output mapping can be transformed in real-time without changing the layout. These reconfigurable circuits will serve as important building blocks for high-density data-stream processors and reconfigurable Application-Specific Integrated Circuits (r-ASICs). The CAMs and transformable logic gates based on ferroelectric reconfigurable transistors will have broad applications in data-intensive applications from image processing to machine learning and artificial intelligence.

Authors:Navketan Batra, D. E. Feldman

Abstract: Fractional charges of anyons can be extracted from shot noise in two ways. One can use either the auto-correlation noise of the current in one drain or the cross-correlation noise between two drains on the two sides of the device. The former approach typically overestimates the charge. This may happen due to upstream edge modes. We propose a mechanism for the excess auto-correlation noise without upstream modes. It applies to systems with multiple co-propagating edge modes and assumes that the noise is measured at a low but non-zero frequency.

Authors:A. S. Dotdaev, Ya. I. Rodionov, K. I. Kugel, B. A. Aronzon

Abstract: We study the effects of anisotropy on the magnetoresistance of Weyl semimetals (WSMs) in the ultraquantum regime. We utilize the fact that many Weyl semimetals are approximately axially anisotropic. We find that anisotropy manifests itself in the strong dependence of the magnetoresistance on the polar and azimuthal angles determining the orientation of the anisotropy axis with respect to the applied magnetic field and electric current. We also predict that the ratio of magnetoresistances in the geometries, where the magnetic field and anisotropy axes are aligned and where they are orthogonal, scales as $(v_\bot/v_\parallel)^2$ where $v_\bot$ and $v_\parallel$ are the corresponding Fermi velocities.

Authors:D. A. Bahamon, G. Gómez-Santos, D. K. Efetov, T. Stauber

Abstract: We propose a minimal transport experiment in the linear regime that can probe the chirality of twisted moir\'e structures. First, we point out that usual two-terminal conductance measurements cannot access the chirality of a system. Only with a third contact and in the presence of an in-plane magnetic field, a chiral system displays non-reciprocal transport even if all contacts are symmetric. We thus propose to use the third lead as a voltage probe and show that opposite enantiomers give rise to different voltage drops on the third lead. The third lead can also be used as a current probe in the case of layer-discriminating contacts that can detect different handedness even in the absence of a magnetic field. Our exact symmetry considerations are supported by numerical calculations that confirm our conclusions and also demonstrate that there is a change of chirality around the magic angle.

Authors:Hanlin Fang, Qiaoling Lin, Yi Zhang, Joshua Thompson, Sanshui Xiao, Zhipei Sun, Ermin Malic, Saroj Dash, Witlef Wieczorek

Abstract: Transition metal dichalcogenide (TMD) heterobilayers provide a versatile platform to explore unique excitonic physics via properties of the constituent TMDs and external stimuli. Interlayer excitons (IXs) can form in TMD heterobilayers as delocalized or localized states. However, the localization of IX in different types of potential traps, the emergence of biexcitons in the high-excitation regime, and the impact of potential traps on biexciton formation have remained elusive. In our work, we observe two types of potential traps in a MoSe$_2$/WSe$_2$ heterobilayer, which result in significantly different emission behavior of IXs at different temperatures. We identify the origin of these traps as localized defect states and the moir{\'e} potential of the TMD heterobilayer. Furthermore, with strong excitation intensity, a superlinear emission behavior indicates the emergence of interlayer biexcitons, whose formation peaks at a specific temperature. Our work elucidates the different excitation and temperature regimes required for the formation of both localized and delocalized IX and biexcitons, and, thus, contributes to a better understanding and application of the rich exciton physics in TMD heterostructures.

Authors:Azmain A. Hossain, Haozhe Wang, David S. Catherall, Martin Leung, Harm C. M. Knoops, James R. Renzas, Austin J. Minnich

Abstract: Microwave loss in superconducting titanium nitride (TiN) films is attributed to two-level systems in various interfaces arising in part from oxidation and microfabrication-induced damage. Atomic layer etching (ALE) is an emerging subtractive fabrication method which is capable of etching with Angstrom-scale etch depth control and potentially less damage. However, while ALE processes for TiN have been reported, they either employ HF vapor, incurring practical complications; or the etch rate lacks the desired control. Further, the superconducting characteristics of the etched films have not been characterized. Here, we report an isotropic plasma-thermal TiN ALE process consisting of sequential exposures to molecular oxygen and an SF$_6$/H$_2$ plasma. For certain ratios of SF$_6$:H$_2$ flow rates, we observe selective etching of TiO$_2$ over TiN, enabling self-limiting etching within a cycle. Etch rates were measured to vary from 1.1 \r{A}/cycle at 150 $^\circ$C to 3.2 \r{A}/cycle at 350 $^\circ$C using ex-situ ellipsometry. We demonstrate that the superconducting critical temperature of the etched film does not decrease beyond that expected from the decrease in film thickness, highlighting the low-damage nature of the process. These findings have relevance for applications of TiN in microwave kinetic inductance detectors and superconducting qubits.

Authors:Sergio Leiva M., Jürgen Henk, Ingrid Mertig, Annika Johansson

Abstract: The spin Edelstein effect has proven to be a promising phenomenon to generate spin polarization from a charge current in systems without inversion symmetry. In recent years, a current-induced orbital magnetization, called orbital Edelstein effect, has been predicted for various systems with broken inversion symmetry, using the atom-centered approximation and the modern theory of orbital magnetization. In this work, we study the current-induced spin and orbital magnetization for a bilayer system with Rashba interaction, using the modern theory of orbital magnetization and Boltzmann transport theory in relaxation-time approximation. We found that the orbital effect can be significantly larger than the spin effect, depending on the model parameters. Furthermore, the Edelstein response can be enhanced, suppressed, and even reversed, depending on the relation of the effective Rashba parameters of each layer. A sign change of the orbital polarization is related to an interchange of the corresponding layer localization of the states.

Authors:Asuka Honma, Daichi Takane, Seigo Souma, Yongjian Wang, Kosuke Nakayama, Miho Kitamura, Koji Horiba, Hiroshi Kumigashira, Takashi Takahashi, Yoichi Ando, Takafumi Sato

Abstract: We have performed micro-focused angle-resolved photoemission spectroscopy on NdSb which exhibits the type-I antiferromagnetism below TN = 16 K. We succeeded in selectively observing the band structure for all the three types of single-q antiferromagnetic (AF) domains at the surface. We found that the two of three surfaces whose AF-ordering vector lies within the surface plane commonly show two-fold-symmetric surface states (SSs) around the bulk-band edges, whereas the other surface with an out-of-plane AF-ordering vector displays four-fold-symmetric shallow electronlike SS at the Brillouin-zone center. We suggest that these SSs commonly originate from the combination of the PT (space-inversion and time-reversal) symmetry breaking at the surface and the band folding due to the AF order. The present results pave a pathway toward understanding the relationship between the symmetry and the surface electronic states in antiferromagnets.

Authors:K. Iyer, J. Rech, T. Jonckheere, L. Raymond, B. Grémaud, T. Martin

Abstract: Photons are emitted or absorbed by a nano-circuit under both equilibrium and non-equilibrium situations. Here, we focus on the non-equilibrium situation arising due to a temperature difference between the leads of a quantum point contact, and study the finite frequency (colored) noise. We explore this delta-$T$ noise in the finite frequency regime for two systems: conventional conductors described by Fermi liquid scattering theory and the fractional quantum Hall system at Laughlin filling fractions, described by the chiral Luttinger liquid formalism. We study the emission noise, its expansion in the temperature difference (focusing on the quadratic component) as well as the excess emission noise defined with respect to a properly chosen equilibrium situation. The behavior of these quantities are markedly different for the fractional quantum Hall system compared to Fermi liquids, signalling the role of strong correlations. We briefly treat the strong backscattering regime of the fractional quantum Hall liquid, where a behavior closer to the Fermi liquid case is observed.

Authors:Jakub Kierdaszuk, Rafał Bożek, Tomasz Stefaniuk, Ewelina Możdzyńska, Karolina Piętak-Jurczak, Sebastian Złotnik, Vitaly Zubialevich, Aleksandra Przewłoka, Aleksandra Krajewska, Wawrzyniec Kaszub, Marta Gryglas-Borysiewicz, Andrzej Wysmołek, Johannes Binder, Aneta Drabińska

Abstract: Few-layer graphene deposited on semiconductor nanorods separated by undoped spacers has been studied in perspective for the fabrication of stable nanoresonators. We show that an applied bias between the graphene layer and the nanorod substrate affects the graphene electrode in two ways: 1) by a change of the carrier concentration in graphene and 2) by inducing strain, as demonstrated by the Raman spectroscopy. The capacitance of the investigated structures scales with the area of graphene in contact with the nanorods. Due to the reduced contact surface, the efficiency of graphene gating is one order of magnitude lower than for a comparable structure without nanorods. The shift of graphene Raman modes observed under bias clearly shows the presence of electrostatically-induced strain and only a weak modification of carrier concentration, both independent of number of graphene layers. A higher impact of bias on strain was observed for samples with a larger contact area between the graphene and the nanorods which shows perspective for the construction of sensors and nanoresonator devices.

Authors:Moumita Patra

Abstract: In an open quantum system having a channel in the form of loop geometry, the current inside the channel, namely circular current, and overall junction current, namely transport current, can be different. A quantum ring has doubly degenerate eigen energies due to periodic boundary condition that is broken in an asymmetric ring where the ring is asymmetrically connected to the external electrodes. Kramers' degeneracy and spin degeneracy can be lifted by considering non-zero magnetic field and spin-orbit interaction (SOI), respectively. Here, we find that symmetry breaking impacts the circular current density vs energy ($E$) spectra in addition to lifting the degeneracy. For charge and spin current densities, the corresponding effects are not the same. Under symmetry-breaking they may remain symmetric or anti-symmetric or asymmetric around $E = 0$ whereas the transmission function (which is proportional to the junction current density) vs energy characteristic remains symmetric around $E = 0$. This study leads us to estimate the qualitative nature of the circular current and the choices of Fermi-energy/chemical potential to have a net non-zero current. As a result, we may manipulate the system to generate pure currents of charge, spin, or both, which is necessary for any spintronic and electronic applications.

Authors:Aritra Sen, György Frank, Baksa Kolok, Jeroen Danon, András Pályi

Abstract: The spin of a single electron confined in a semiconductor quantum dot is a natural qubit candidate. Fundamental building blocks of spin-based quantum computing have been demonstrated in double quantum dots with significant spin-orbit coupling. Here, we show that spin-orbit-coupled double quantum dots can be categorised in six classes, according to a partitioning of the multi-dimensional space of their $g$-tensors. The class determines physical characteristics of the double dot, i.e., features in transport, spectroscopy and coherence measurements, as well as qubit control, shuttling, and readout experiments. In particular, we predict that the spin physics is highly simplified due to pseudospin conservation, whenever the external magnetic field is pointing to special directions (`magic directions'), where the number of special directions is determined by the class. We also analyze the existence and relevance of magic loops in the space of magnetic-field directions, corresponding to equal local Zeeman splittings. These results present an important step toward precise interpretation and efficient design of spin-based quantum computing experiments in materials with strong spin-orbit coupling.

Authors:Chakradhar Rangi, Ka-Ming Tam, Juana Moreno

Abstract: Non-Hermitian topological phases have gained immense attention due to their potential to unlock novel features beyond Hermitian bounds. PT-symmetric (Parity Time-reversal symmetric) non-Hermitian models have been studied extensively over the past decade. In recent years, the topological properties of general non-Hermitian models, regardless of the balance between gains and losses, have also attracted vast attention. Here we propose a non-Hermitian second-order topological (SOT) insulator that hosts gapless corner states on a two-dimensional quasi-crystalline lattice (QL). We first construct a non-Hermitian extension of the Bernevig-Hughes-Zhang (BHZ) model on a QL generated by the Amman-Beenker (AB) tiling. This model has real spectra and supports helical edge states. Corner states emerge by adding a proper Wilson mass term that gaps out the edge states. We propose two variations of the mass term that result in fascinating characteristics. In the first variation, we obtain a purely real spectra for the second-order topological phase. In the latter, we get a complex spectra with corner states localized at only two corners. Our findings pave a path to engineering exotic SOT phases where corner states can be localized at designated corners.

Authors:R. C. Bento Ribeiro, J. H. Correa, L. S. Ricco, I. A. Shelykh, M. A. Continentino, A. C. Seridonio, M. Minissale, G. L. Lay, M. S. Figueira

Abstract: We theoretically investigate the possibility of obtaining Majorana zero modes (MZMs) in penta-silicene nanoribbons (p-SiNRs) with induced \textit{p}-wave superconductivity. The model explicitly considers an external magnetic field perpendicularly applied to the nanoribbon plane, as well as an extrinsic Rashba spin-orbit coupling (RSOC), in addition to the first nearest neighbor hopping term and \textit{p}-wave superconducting pairing. By analyzing the dispersion relation profiles, we observe the successive closing and reopening of the induced superconducting gap with a single spin component, indicating a spin-polarized topological phase transition (TPT). Correspondingly, the plots of the energy spectrum versus the chemical potential reveal the existence of zero-energy states with a preferential spin orientation characterized by nonoverlapping wave functions localized at opposite ends of the superconducting p-SiNRs. These findings strongly suggest the emergence of topologically protected, spin-polarized MZMs at the ends of the p-SiNRs with induced \textit{p}-wave superconducting pairing, which can be realized by proximitizing the nanoribbon with an \textit{s}-wave superconductor, such as lead. The proposal paves the way for silicene-based Majorana devices hosting multiple MZMs with a well-defined spin orientation, with possible applications in fault-tolerant quantum computing platforms and Majorana spintronics.

Authors:Juan Daniel Torres Luna, Sathish R. Kuppuswamy, Anton R. Akhmerov

Abstract: Braiding of Majorana states demonstrates their non-Abelian exchange statistics. One implementation of braiding requires control of the pairwise couplings between all Majorana states in a trijunction device. In order to have adiabaticity, a trijunction device requires the desired pair coupling to be sufficently large and the undesired couplings to vanish. In this work, we design and simulate of a trijunction device in a two-dimensional electron gas with a focus on the normal region that connects three Majorana states. We use an optimisation approach to find the operational regime of the device in a multi-dimensional voltage space. Using the optimization results, we simulate a braiding experiment by adiabatically coupling different pairs of Majorana states without closing the topological gap. We then evaluate the feasibility of braiding in a trijunction device for different shapes and disorder strengths.

Authors:Mengmeng Wu, Xiao Liu, Renfei Wang, Yoon Jang Chung, Adbhut Gupta, Kirk W. Baldwin, Loren Pfeiffer, Xi Lin, Yang Liu

Abstract: Measurement is the foundation of science, and is a subtle concept especially in quantum mechanics, where the action of detection interacts with the quantum system perturbatively. The property of a quantum system is captured from the stimulated evolution of either the system or the detecting reservoir. Transport measurement, which applies an electric field and studies the migration of charged particles, i.e. the current, is the most widely used technique. In ultra-high mobility two-dimensional systems, transport measurement reveals fruitful quantum phenomena such as the quantum Hall effect, the Aharonov-Bohm oscillation and ballistic trajectory of quasiparticles, the microwave induced zero resistance, the interference of quasiparticles, etc. The general assumption that the quantum phase remains unchanged with a sufficiently small probing current, unfortunately, is rarely examined experimentally. In this work, we probe the ultra-high mobility two-dimensional electron system via its interaction with a propagating surface acoustic wave and observe that the system becomes more incompressible when hosting a current.

Authors:Lorenzo Lombardi, Alessandro Kovtun, Sebastiano Mantovani, Giulio Bertuzzi, Laura Favaretto, Cristian Bettini, Vincenzo Palermo, Manuela Melucci, Marco Bandini

Abstract: We present an environmentally benign methodology for the covalent functionalization (arylation) of reduced graphene oxide (rGO) nanosheets with arylazo sulfones. A variety of tagged aryl units were conveniently accommodated at the rGO surface via visible light irradiation of suspensions of carbon nanostructured materials in aqueous media. Mild reaction conditions, absence of photosensitizers, functional group tolerance and high atomic fractions (XPS analysis) represent some of the salient features characterizing the present methodology. Control experiments for the mechanistic elucidation (Raman analysis) and chemical nanomanipulation of the tagged rGO surfaces are also reported.

Authors:T. -K. Hsiao, P. Cova Fariña, S. D. Oosterhout, D. Jirovec, X. Zhang, C. J. van Diepen, W. I. L. Lawrie, C. -A. Wang, A. Sammak, G. Scappucci, M. Veldhorst, E. Demler, L. M. K. Vandersypen

Abstract: Quantum systems with engineered Hamiltonians can be used as simulators of many-body physics problems to provide insights beyond the capabilities of classical computers. Semiconductor gate-defined quantum dot arrays have emerged as a versatile platform for quantum simulation of generalized Fermi-Hubbard physics, one of the richest playgrounds in condensed matter physics. In this work, we employ a germanium 4$\times$2 quantum dot array and show that the naturally occurring long-range Coulomb interaction can lead to exciton formation and transport. We tune the quantum dot ladder into two capacitively-coupled channels and exploit Coulomb drag to probe the binding of electrons and holes. Specifically, we shuttle an electron through one leg of the ladder and observe that a hole is dragged along in the second leg under the right conditions. This corresponds to a transition from single-electron transport in one leg to exciton transport along the ladder. Our work paves the way for the study of excitonic states of matter in quantum dot arrays.

Authors:O. M. Bahrova

Abstract: The dissertation is devoted to the study of new fundamental phenomena which emerge due to electromechanical coupling in mesoscopic systems based on movable quantum dot.

Authors:Maryam Mansoury, Vram Mughnetsyan, Aram Manaselyan, Albert Kirakosyan, Vidar Gudmundsson, Vigen Aziz-Aghchegala

Abstract: In this paper a comparative study of the electronic and magnetic properties of quasi-two-dimensional electrons in an artificial graphene-like superlattice composed of circular and elliptical quantum dots is presented. A complete orthonormal set of basis wave functions, which has previously been constructed in the frame of the Coulomb gauge for the vector potential has been implemented for calculation of the energy dispersions, the Hofstadter spectra, the density of states and the orbital magnetization of the considered systems, taking into account both the translational symmetry of the superlattice and the wave function phase-shifts due to the presence of a transverse external magnetic field. Our calculations indicate a topological change in the miniband structure due to the ellipticity of the quantum dots, and non-trivial modifications of the electron energy dispersion surfaces in reciprocal space with the change of the number of magnetic flux quanta through the unit cell of the superlattice. The ellipticity of the QDs leads to an opening of a gap and considerable modifications of the Hofstadter spectrum. The orbital magnetization is shown to reveal significant oscillations with the change of the magnetic flux. The deviation from the circular geometry of quantum dots has a qualitative impact on the dependencies of the magnetization on both the magnetic flux and the temperature.

Authors:M. Zubair, P. Vasilopoulos, M. Tahir

Abstract: We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity induced spin-orbit coupling (SOC) using a low-energy Hamiltonian, with different types of symmetry breaking terms, obtained from a four-band, first and second nearest-neighbour tight-binding (TB) one. The competition between different perturbation terms leads to inverted SOC bands. Further, we study the effect of symmetry breaking terms on ac and dc transport by evaluating the corresponding conductivities within linear response theory. The scattering-independent part of the valley-Hall conductivity, as a function of the Fermi energy $E_{F}$, is mostly negative in the ranges $-\lambda_{R}\leqslant E_{F}$ and $E_{F}\geqslant\lambda_{R}$ when the strength $\lambda_{R}$ of the Rashba SOC increases except for a very narrow region around $E_{F}=0$ in which it peaks sharply upward. The scattering-dependent diffusive conductivity increases linearly with electron density, is directly proportional to $\lambda_{R}$ in the low- and high-density regimes, but weakens for $\lambda_{R}=0$. We investigate the optical response in the presence of a SOC-tunable band gap for variable $E_{F}$. An interesting feature of this SOC tuning is that it can be used to switch on and off the Drude-type intraband response. Furthermore, the ac conductivity exhibits interband responses due to the Rashba SOC. We also show that the valley-Hall conductivity changes sign when $E_F$ is comparable to $\lambda_R$ and vanishes at higher values of $E_F$. It also exhibits a strong dependence on temperature and a considerable structure as a function of the frequency.

Authors:Christina Psaroudaki, Gil Refael

Abstract: We explore the statistical properties of energy transfer in ensembles of doubly-driven Random- Matrix Floquet Hamiltonians, based on universal symmetry arguments. The energy pumping efficiency distribution P(E) is associated with the Hamiltonian parameter ensemble and the eigenvalue statistics of the Floquet operator. For specific Hamiltonian ensembles, P(E) undergoes a transition that cannot be associated with a symmetry breaking of the instantaneous Hamiltonian. The Floquet eigenvalue spacing distribution indicates the considered ensembles constitute generic nonintegrable Hamiltonian families. As a step towards Hamiltonian engineering, we develop a machine-learning classifier to understand the relative parameter importance in resulting high conversion efficiency. We propose Random Floquet Hamiltonians as a general framework to investigate frequency conversion effects in a new class of generic dynamical processes beyond adiabatic pumps.

Authors:M. Zubair, P. Vasilopoulos, M. Tahir

Abstract: We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity-induced spin-orbit coupling $\lambda_{v}$, sublattice potential $\Delta$, and an off-resonant circularly polarized light of frequency $\Omega$ that renormalizes $\Delta$ to $\bar{\Delta}_{\eta p} = \Delta +\eta p \Delta_{\Omega} $ with $\eta$ and $p$ the valley and polarization indices, respectively, and $ \Delta_{\Omega} $ the gap due to the off-resonant circularly polarized light. Using a low-energy Hamiltonian we find that the interplay between different perturbation terms leads to inverted spin-orbit coupled bands. At high $\Omega$ we study the band structure and dc transport using the Floquet theory and linear response formalism, respectively. We find that the inverted band structure transfers into the direct band one when the off-resonant light is present. The valley-Hall conductivity behaves as an even function of the Fermi energy in the presence and absence of this light. At $\Delta_{\Omega}$ = $\lambda_{v}$ - $\Delta$ a transition occurs from the valley-Hall phase to the anomalous Hall phase. In addition, the valley-Hall conductivity switches sign when the polarization of the off-resonant light changes. The valley polarization vanishes for $\Delta_{\Omega}$ = 0 but it is finite for $\Delta_{\Omega}$ $\neq$ 0 and reflects the lifting of the valley degeneracy of the energy levels, for $\Delta_{\Omega} \neq 0$, when the off-resonant light is present. The corresponding spin polarization, present for $\Delta_{\Omega}$ = 0, increases for $\Delta_{\Omega}$ $\neq$ 0. Further, pure $K$ or $K^{\prime}$ valley polarization is generated when $\Delta_{\Omega}$ changes sign. Also, the charge Hall conductivity is finite for $\Delta_{\Omega}\neq 0$ and changes sign when the handedness of the light polarization changes.

Authors:Qun Yang, Jiewen Xiao, Iñigo Robredo, Maia G. Vergniory, Binghai Yan, Claudia Felser

Abstract: The interplay between chirality and topology nurtures many exotic electronic properties. For instance, topological chiral semimetals display multifold chiral fermions which manifest nontrivial topological charge and spin texture. They are an ideal playground for exploring chirality-driven exotic physical phenomena. In this work, we reveal a monopole-like orbital-momentum locking texture on the three-dimensional Fermi surfaces of topological chiral semimetals with B20 structures (e.g., RhSi and PdGa). This orbital texture enables a large orbital Hall effect (OHE) and a giant orbital magnetoelectric (OME) effect in the presence of current flow. Different enantiomers exhibit the same OHE which can be converted to the spin Hall effect by spin-orbit coupling in materials. In contrast, the OME effect is chirality dependent and much larger than its spin counterpart. Our work reveals the crucial role of orbital texture for understanding OHE and OME effect in topological chiral semimetals and paves the path for applications in orbitronics, spintronics and enantiomer recognition.

Authors:C. Reichhardt, C. J. O. Reichhardt

Abstract: We investigate the topological defect populations for superconducting vortices and magnetic skyrmions on random pinning substrates under driving amplitudes that are swept at different rates or suddenly quenched. When the substrate pinning is sufficiently strong, the system exhibits a nonequilibrium phase transition at a critical drive into a more topologically ordered state. We examine the number of topological defects that remain as we cross the ordering transition at different rates. In the vortex case, the system dynamically orders into a moving smectic, and the Kibble-Zurek scaling hypothesis gives exponents consistent with directed percolation. Due to their strong Magnus force, the skyrmions dynamically order into an isotropic crystal, producing different Kibble-Zurek scaling exponents that are more consistent with coarsening. We argue that in the skyrmion crystal, the topological defects can both climb and glide, facilitating coarsening, whereas in the vortex smectic state, the defects cannot climb and coarsening is suppressed. We also examine pulsed driving across the ordering transition and find that the defect population on the ordered side of the transition decreases with time as a power law, indicating that coarsening can occur across nonequilibrium phase transitions. Our results should be general to a wide class of nonequilibrium systems driven over random disorder where there are well-defined topological defects.

Authors:Abhikbrata Sarkar, Zhanning Wang, Mathew Rendell, Nico W. Hendrickx, Menno Veldhorst, Giordano Scappucci, Mohammad Khalifa, Joe Salfi, Andre Saraiva, A. S. Dzurak, A. R. Hamilton, Dimitrie Culcer

Abstract: In this work we present a comprehensive theory of spin physics in planar Ge hole quantum dots in an in-plane magnetic field, where the orbital terms play a dominant role in qubit physics, and provide a brief comparison with experimental measurements of the angular dependence of electrically driven spin resonance. We focus the theoretical analysis on electrical spin operation, phonon-induced relaxation, and the existence of coherence sweet spots. We find that the choice of magnetic field orientation makes a substantial difference for the properties of hole spin qubits. Furthermore, although the Schrieffer-Wolff approximation can describe electron dipole spin resonance (EDSR), it does not capture the fundamental spin dynamics underlying qubit coherence. Specifically, we find that: (i) EDSR for in-plane magnetic fields varies non-linearly with the field strength and weaker than for perpendicular magnetic fields; (ii) The EDSR Rabi frequency is maximized when the a.c. electric field is aligned parallel to the magnetic field, and vanishes when the two are perpendicular; (iii) The Rabi ratio $T_1/T_\pi$, i.e. the number of EDSR gate operation per unit relaxation time, is expected to be as large as $5{\times}10^5$ at the magnetic fields used experimentally; (iv) The orbital magnetic field terms make the in-plane $g$-factor strongly anisotropic in a squeezed dot, in excellent agreement with experimental measurements; (v) The coherence sweet spots do not exist in an in-plane magnetic field, as the orbital magnetic field terms expose the qubit to all components of the defect electric field. These findings will provide a guideline for experiments to design ultrafast, highly coherent hole spin qubits in Ge.

Authors:Mikhail Kostylev

Abstract: We carried out numerical simulations of propagation of spin waves (magnons in quantum language) in a yttrium-iron garnet film. The numerical model is based on an original formalism. We demonstrated that a potential barrier for magnons, created by an Oersted field of a dc current flowing through a wire sitting on top of the film, is able to act as an electrically controlled partly transparent mirror for the magnons. We found that the mirror transparency can be set to 50% by properly adjusting the current strength, thus creating a semi-transparent mirror. A strong Hong-Ou-Mandel Effect for single magnons is expected in this configuration. The effect must be seen as two single magnons, launched simultaneously into the film from two transducers located from the opposite sides of the mirror, creating a two-microwave-photon state at the output port of one of the transducers. The probability of seeing those two-photon states at the output port of either transducer must be the same for both transducers.

Authors:P. P. Abrantes, W. J. M. Kort-Kamp, F. S. S. Rosa, C. Farina, F. A. Pinheiro, Tarik P. Cysne

Abstract: We investigate the spontaneous emission lifetime of a quantum emitter near a substrate coated with phosphorene under the influence of uniaxial strain. We consider both electric dipole and magnetic dipole-mediated spontaneous transitions from the excited to the ground state. The modeling of phosphorene is performed by employing a tight-binding model that goes beyond the usual low-energy description. We demonstrate that both electric and magnetic decay rates can be strongly tuned by the application of uniform strain, ranging from a near-total suppression of the Purcell effect to a remarkable enhancement of more than 1300% due to the high flexibility associated with the puckered lattice structure of phosphorene. We also unveil the use of strain as a mechanism to tailor the most probable decay pathways of the emitted quanta. Our results show that uniaxially strained phosphorene is an efficient and versatile material platform for the active control of light-matter interactions thanks to its extraordinary optomechanical properties.

Authors:E. E. Vdovin, M. T. Greenaway, Yu. N. Khanin, S. V. Morozov, O. Makarovsky, A. Patanè, A. Mishchenko, S. Slizovskiy, V. I. Fal'ko, A. K. Geim, K. S. Novoselov, L. Eaves

Abstract: Insights into the fundamental properties of graphene's Dirac-Weyl fermions have emerged from studies of electron tunnelling transistors in which an atomically thin layer of hexagonal boron nitride (hBN) is sandwiched between two layers of high purity graphene. Here, we show that when a single defect is present within the hBN tunnel barrier, it can inject electrons into the graphene layers and its sharply defined energy level acts as a high resolution spectroscopic probe of electron-electron interactions in graphene. We report a magnetic field dependent suppression of the tunnel current flowing through a single defect below temperatures of $\sim$ 2 K. This is attributed to the formation of a magnetically-induced Coulomb gap in the spectral density of electrons tunnelling into graphene due to electron-electron interactions.

Authors:Marcela Derli, E. Novais

Abstract: We present a novel theoretical approach to incorporate electronic interactions in the study of two-dimensional topological insulators. By exploiting the correspondence between edge state physics and entanglement spectrum in gapped topological systems, we deconstruct the system into one-dimensional channels. This framework enables a simple and elegant inclusion of fermionic interactions into the discussion of topological insulators. We apply this approach to the Kane-Mele model with interactions and magnetic impurities.

Authors:Wenwen Liu, Hanyu Wang, Biao Yang, Shuang Zhang

Abstract: With their non-Abelian topological charges, real multi-bandgap systems challenge the conventional topological phase classifications. As the minimal sector of multi-bandgap systems, real triple degeneracies (RTPs), which serves as real "Weyl points", lay the foundation for the research on real topological phases. However, experimental observation of RTP and physical systems with global band configuration consisting of multiple RTPs in crystals has not been reported. In this study, we employ Euler number to characterize RTPs, establish their connection with both Abelian and non-Abelian charges, and provide experimental evidence for the existence of RTPs in photonic meta-crystals. By considering RTPs as the basic elements, we further propose the concept of a topological compound, akin to a chemical compound, where we find that certain phases are not topologically allowed. The topological classification of RTPs in crystals demonstrated in our work plays a similar role as the "no-go" theorem in the Weyl system.

Authors:Akiyoshi Yamada, Yuki Fuseya

Abstract: The three-dimensional magneto-conductivity tensor was derived in a gauge invariant form based on the Kubo formula considering the quantum effect under a magnetic field, such as the Landau quantization and the quantum oscillations. We analytically demonstrated that the quantum formula of the magneto-conductivity can be obtained by adding a quantum oscillation factor to the classical formula. This result establishes the quantum--classical correspondence, which has long been missing in magnetotransport phenomena. Moreover, we found dissipative-to-dissipationless crossover in the Hall conductivity by paying special attention to the analytic properties of thermal Green's function. Finally, by calculating the magnetoresistance of semimetals, we identified a phase shift in quantum oscillation originating from the dissipationless transport predominant at high fields.

Authors:Bogdan R. Bułka

Abstract: We simulated the radiative response of the cavity quantum electrodynamics (QED) inductively coupled to the ring pierced by magnetic flux, and analyzed its spectral dependence to get insight into persistent current dynamics. Current fluctuations in the ring induce changes in the microwave resonator: shifting the resonant frequency and changing its damping. We use the linear response theory and calculate the current response function by means of the Green function technique. Our model contains two quantum dots which divide the ring into two arms with different electron transfers. There are two opposite (symmetric and asymmetric) components of the persistent current, which interplay can be observed in the response functions. The resonator reflectance shows characteristic shifts in the dispersive regime and avoided crossings at the resonance points. The magnitude of the resonator frequency shift is greater for coupling to the arm with higher transparency. Fluctuations of the symmetric component of the persistent current are relevant for a wide range of the Aharovov-Bohm phase $\phi$, while the asymmetric component becomes dominant close to $\phi\approx \pi$ (when the total persistent current changes its orientation)

Authors:J. C. G. Henriques, J. Fernández-Rossier

Abstract: Nanographene triangulenes with a S = 1 ground state have been used as building blocks of antiferromagnetic Haldane spin chains realizing a symmetry protected topological phase. By means of inelastic electron spectroscopy, it was found that the intermolecular exchange contains both linear and non-linear interactions, realizing the bilinear-biquadratic Hamiltonian. Starting from a Hubbard model, and mapping it to an interacting Creutz ladder, we analytically derive these effective spin-interactions using perturbation theory, up to fourth order. We find that for chains with more than two units other interactions arise, with same order-of-magnitude strength, that entail second neighbor linear, and three-site non-linear exchange. Our analytical expressions compare well with experimental and numerical results. We discuss the extension to general S = 1 molecules, and give numerical results for the strength of the non-linear exchange for several nanographenes. Our results pave the way towards rational design of spin Hamiltonians for nanographene based spin chains.

Authors:Talieh S. Ghiasi, Michael Borst, Samer Kurdi, Brecht G. Simon, Iacopo Bertelli, Carla Boix-Constant, Samuel Mañas-Valero, Herre S. J. van der Zant, Toeno van der Sar

Abstract: Magnetic imaging using nitrogen-vacancy (NV) spins in diamonds is a powerful technique for acquiring quantitative information about sub-micron scale magnetic order. A major challenge for its application in the research on two-dimensional (2D) magnets is the positioning of the NV centers at a well-defined, nanoscale distance to the target material required for detecting the small magnetic fields generated by magnetic monolayers. Here, we develop a diamond 'dry-transfer' technique, akin to the state-of-the-art 2D-materials assembly methods, and use it to place a diamond micro-membrane in direct contact with the 2D interlayer antiferromagnet CrSBr. We harness the resulting NV-sample proximity to spatially resolve the magnetic stray fields generated by the CrSBr, present only where the CrSBr thickness changes by an odd number of layers. From the magnetic stray field of a single uncompensated ferromagnetic layer in the CrSBr, we extract a monolayer magnetization of $M_\mathrm{CSB}$ = 0.46(2) T, without the need for exfoliation of monolayer crystals or applying large external magnetic fields. The ability to deterministically place NV-ensemble sensors into contact with target materials and detect ferromagnetic monolayer magnetizations paves the way for quantitative analysis of a wide range of 2D magnets assembled on arbitrary target substrates.

Authors:Nishchhal Verma, Daniele Guerci, Raquel Queiroz

Abstract: Recent experiments have confirmed the presence of interlayer excitons in the ground state of transition metal dichalcogenide (TMD) bilayers. The interlayer excitons are expected to show remarkable transport properties when they undergo Bose condensation. In this work, we demonstrate that quantum geometry of Bloch wavefunctions plays an important role in the phase stiffness of the Interlayer Exciton Condensate (IEC). Notably, we identify a geometric contribution that amplifies the stiffness, leading to the formation of a robust condensate with an increased BKT temperature. Our results have direct implications for the ongoing experimental efforts on interlayer excitons in materials that have non-trivial geometry. We provide quantitative estimates for the geometric contribution in TMD bilayers through a realistic continuum model with gated Coulomb interaction, and find that the substantially increased stiffness allows for an IEC to be realized at amenable experimental conditions.

Authors:Minxing Xu, Dongil Shin, Paolo M. Sberna, Roald van der Kolk, Andrea Cupertino, Miguel A. Bessa, Richard A. Norte

Abstract: For decades, mechanical resonators with high sensitivity have been realized using thin-film materials under high tensile loads. Although there have been remarkable strides in achieving low-dissipation mechanical sensors by utilizing high tensile stress, the performance of even the best strategy is limited by the tensile fracture strength of the resonator materials. In this study, a wafer-scale amorphous thin film is uncovered, which has the highest ultimate tensile strength ever measured for a nanostructured amorphous material. This silicon carbide (SiC) material exhibits an ultimate tensile strength of over 10 GPa, reaching the regime reserved for strong crystalline materials and approaching levels experimentally shown in graphene nanoribbons. Amorphous SiC strings with high aspect ratios are fabricated, with mechanical modes exceeding quality factors 10^8 at room temperature, the highest value achieved among SiC resonators. These performances are demonstrated faithfully after characterizing the mechanical properties of the thin film using the resonance behaviors of free-standing resonators. This robust thin-film material has significant potential for applications in nanomechanical sensors, solar cells, biological applications, space exploration and other areas requiring strength and stability in dynamic environments. The findings of this study open up new possibilities for the use of amorphous thin-film materials in high-performance applications.

Authors:Daiju Terasawa

Abstract: One-dimensional quantized conductance is derived from the electrons in a homogeneous electric field by calculating the traveling time of the accelerated motion and the number of electrons in the one-dimensional region. As a result, the quantized conductance is attributed to the finite time required for ballistic electrons to travel a finite length. In addition, this model requires no Joule heat dissipation, even if the conductance is a finite value, because the electric power is converted to kinetic energy of electrons. Furthermore, the relationship between the non-equilibrium source-drain bias $V_\mathrm{sd}$ and wavenumber $k$ in a one-dimensional conductor is shown as $k \propto \sqrt{V_\mathrm{sd}}$. This correspondence accounts for the wavelength of the coherent electron flows emitted from a quantum point contact. Furthermore, it explains the anomalous $0.7 \cdot 2e^2/h$ ($e$ is the elementary charge, and $h$ is the Plank's constant) conductance plateau as a consequence of the perturbation gap at the crossing point of the wavenumber-directional-splitting dispersion relation. We propose that this splitting is caused by the Rashba spin-orbit interaction induced by the potential gradient of the quantum well at quantum point contacts.

Authors:Ragheed Alhyder, Alberto Cappellaro, Mikhail Lemeshko, Artem G. Volosniev

Abstract: We demonstrate the possibility of a coupling between the magnetization direction of a ferromagnet and the tilting angle of adsorbed achiral molecules. To illustrate the mechanism of the coupling, we analyze a minimal Stoner model that includes Rashba spin-orbit coupling due to the electric field on the surface of the ferromagnet. The proposed mechanism allows us to study magnetic anisotropy of the system with an extended Stoner-Wohlfarth model, and argue that adsorbed achiral molecules can change magnetocrystalline anisotropy of the substrate. Our research's aim is to motivate further experimental studies of the current-free chirality induced spin selectivity effect involving both enantiomers.

Authors:Youssef Jeyar, Minggang Luo, Kevin Austry, Brahim Guizal, Yi Zheng, H. B. Chan, Mauro Antezza

Abstract: We investigate the Casimir-Lifshitz force (CLF) between two identical graphene strip gratings, laid on finite dielectric substrate. By using the scattering matrix (S-matrix) approach derived from the Fourier Modal Method with local basis functions (FMM-LBF), we fully take into account the high-order electromagnetic diffractions, the multiple scattering and the exact 2D feature of the graphene strips. We show that the non-additivity, which is one of the most interesting features of the CLF in general, is significantly high and can be modulated in situ without any change in the actual material geometry, by varying the graphene chemical potential. This study can open the deeper experimental exploration of the non-additive features of CLF with micro- or nano-electromechanical graphene-based systems.

Authors:Michael Lau, Wolfgang Häusler, Michael Thorwart

Abstract: We show that a Skyrmion in a classical bipartite antiferromagnetic lattice can be spatially displaced in a controlled manner by externally applied spin waves. We reveal the relation between the Skyrmion motion and the spin wave properties. To this end, we derive a classical spin wave formalism which is tailored to the antiferromagnetic two-dimensional square lattice. The antiferromagnetic spin waves can be classified into two types with respect to their polarization, with two modes each. The circularly polarized spin waves oscillate with different amplitudes in the respective sublattices and induce a Skyrmion Hall effect. The two modes are symmetric under sublattices exchange and determine the overall sign of the Hall angle. Linearly polarized spin waves oscillate elliptically, however, with the same amplitude on each sublattice. These accelerate the Skyrmion solely into their own propagation direction. The two modes are symmetric under component x-y exchange and impact Bloch- or N\'eel Skyrmions differently. Our results indicate possible technical applications of spin-wave driven Skyrmion motion. As one example we propose a racetrack where spin waves pump Skyrmions along the track in antiferromagnets.

Authors:L. S. Sirkina, E. A. Muljarov

Abstract: We demonstrate a strong influence of the phonon environment on the coherent dynamics of the quantum dot (QD)-cavity system in the quantum strong coupling regime. This regime is implemented in the nonlinear QD-cavity QED and can be reliably measured by heterodyne spectral interferometry. We present a semi-analytic asymptotically exact path integral-based approach to the nonlinear optical response of this system, which includes two key ingredients: Trotter's decomposition and linked-cluster expansion. Applied to the four-wave-mixing optical polarization, this approach provides access to different excitation and measurement channels, as well as to higher-order optical nonlinearities and quantum correlators. Furthermore, it allows us to extract useful analytic approximations and analyze the nonlinear optical response in terms of quantum transitions between phonon-dressed states of the anharmonic Jaynes-Cummings (JC) ladder. Being well described by these approximations at low temperatures and small exciton-cavity coupling, the exact solution deviates from them for stronger couplings and higher temperatures, demonstrating remarkable non-Markovian effects, spectral asymmetry, and strong phonon renormalization of the JC ladder.

Authors:Alexei Bissonnette, Nicolas Delnour, Andrew Mckenna, Hichem Eleuch, Michael Hilke, Richard MacKenzie

Abstract: We consider a Su-Schrieffer-Heeger chain to which we attach a semi-infinite undimerized chain (lead) to both ends. We study the effect of the openness of the SSH model on its properties. A representation of the infinite system using an effective Hamiltonian allows us to examine its low-energy states in more detail. We show that, as one would expect, the topological edge states hybridize as the coupling between the systems is increased. As this coupling grows, these states are suppressed, while a new type of edge state emerges from the trivial topological phase. These new states, referred to as phase-inverted edge states, are localized low-energy modes very similar to the edge states of the topological phase. Interestingly, localization occurs on a new shifted interface, moving from the first (and last) site to the second (and second to last) site. This suggests that the topology of the system is strongly affected by the leads, with three regimes of behavior. For very small coupling the system is in a well-defined topological phase; for very large coupling it is in the opposite phase; in the intermediate region, the system is in a transition regime.

Authors:Raffael L. Klees, Daniel Gresta, Jonathan Sturm, Laurens W. Molenkamp, Ewelina M. Hankiewicz

Abstract: The unambiguous identification of Majorana zero modes (MZMs) is one of the most outstanding problems of condensed matter physics. Thermal transport provides a detection tool that is sensitive to these chargeless quasiparticles. We study thermoelectric transport between metallic leads transverse to a Josephson junction. The central double quantum dot hosts conventional or topological Andreev states that depend on the phase difference $\phi$. We show that the presence of MZMs can be identified by a significant amplification of both the electrical and thermal conductance at $\phi \approx \pi$ as well as the Seebeck coefficient at $\phi \approx 0$. We further investigate the robustness of our results against Cooper pair splitting processes.

Authors:D. I. Martínez Moreno, J. Negro, L. M. Nieto

Abstract: Symmetries associated with the Hamiltonian describing bilayer graphene subjected to a constant magnetic field perpendicular to the plane of the bilayer are calculated using polar coordinates. These symmetries are then applied to explain some fundamental properties, such as the spectrum and the integer pseudo-spin character of the eigenfunctions. The probability and current densities of the bilayer Hamiltonian have also been calculated in polar coordinates and shown to be gauge invariant and scalar under generalized rotations. We also define appropriate coherent states of this system as eigenfunctions, with complex eigenvalues, of a suitable chose annihilation operator. In this framework, symmetries are also useful to show the meaning of the complex eigenvalue in terms of expected values. The local current density of these coherent states is shown to exhibit a kind of radial component interference effect, something that has gone unnoticed until now. Some of these results that have just been exposed are graphically illustrated throughout the manuscript.

Authors:Mahan Mohseni, Hassan Allami, Daniel Miravet, David J. Gayowsky, Marek Korkusinski, Pawel Hawrylak

Abstract: We present here a theory of Majorana excitons, photo-excited conduction electron-valence band hole pairs, interacting with Majorana Fermions in a Kitaev chain of semiconductor quantum dots embedded in a nanowire. Using analytical tools and exact diagonalisation methods we identify the presence of Majorana Zero Modes in the nanowire absorption spectra.

Authors:Felipe Recabal, Felipe Herrera

Abstract: Stationary coherence in small conducting arrays has been shown to influence the transport efficiency of electronic nanodevices. Model schemes that capture the interplay between electron delocalization and system-reservoir interactions on the device performance are therefore important for designing next-generation nanojunctions powered by quantum coherence. We use a Lindblad open quantum system approach to obtain the current-voltage characteristics of small-size networks of interacting conducting sites subject to radiative and non-radiative interactions with the environment, for experimentally-relevant case studies. Lindblad theory is shown to reproduce recent measurements of negative conductance in single-molecule junctions using a biased two-site model driven by thermal fluctuations. For array sites with conducting ground and excited orbitals in the presence of radiative incoherent pumping, we show that Coulomb interactions that otherwise suppress charge transport can be overcome to produce light-induced currents. We also show that in nanojunctions having asymmetric transfer rates between the array and electrical contacts, an incoherent driving field can induce photocurrents at zero bias voltage whose direction depend on the type or orbital delocalization established between sites. Possible extensions of the theory are discussed.

Authors:Binghao Guo, Wangqian Miao, Victor Huang, Alexander C. Lygo, Xi Dai, Susanne Stemmer

Abstract: We report a topological phase transition in quantum-confined cadmium arsenide (Cd3As2) thin films under an in-plane Zeeman field when the Fermi level is tuned into the topological gap via an electric field. Symmetry considerations in this case predict the appearance of a two-dimensional Weyl semimetal (2D WSM), with a pair of Weyl nodes of opposite chirality at charge neutrality that are protected by space-time inversion (C2T) symmetry. We show that the 2D WSM phase displays unique transport signatures, including saturated resistivities on the order of h/e^2 that persist over a range of in-plane magnetic fields. Moreover, applying a small out-of-plane magnetic field, while keeping the in-plane field within the stability range of the 2D WSM phase, gives rise to a well-developed odd integer quantum Hall effect, characteristic of degenerate, massive Weyl fermions. A minimal four-band k.p model of Cd3As2, which incorporates first-principles effective g factors, qualitatively explains our findings.

Authors:Richard Hess, Henry F. Legg, Daniel Loss, Jelena Klinovaja

Abstract: In this Reply we respond to the comment by Das Sarma and Pan [1] on Hess et al., Phys. Rev. Lett. 130, 207001, "Trivial Andreev Band Mimicking Topological Bulk Gap Reopening in the Nonlocal Conductance of Long Rashba Nanowires" [2]. First, we note that Das Sarma and Pan reproduce the key results of Hess et al., substantiating that our findings are entirely valid. Next, we clarify the incorrect statement by Das Sarma and Pan that the main result of Hess et al. requires a "contrived periodic pristine system", pointing out the extensive discussion of positional disorder in the Hess et al. We also demonstrate that nonlocal conductance features are generically reduced by disorder. This applies to both an Andreev band and to a genuine topological bulk gap reopening signature (BRS). In fact, the suppression of nonlocal conductance of a genuine BRS by disorder was discussed in, e.g., Pan, Sau, Das Sarma, PRB 103, 014513 (2021) [3]. We conclude that, contrary to the claims of Das Sarma and Pan, the minimal model of Hess et al. is relevant to current realistic nanowire devices where only a few overlapping ABSs would be required to mimic a BRS.

Authors:Yihang Zeng, Q. Shi, A. Okounkova, Dihao Sun, K. Watanabe, T. Taniguchi, J. Hone, C. R. Dean, J. I. A. Li

Abstract: The low-temperature phase diagram of a Bosonic system is predicted to contain an exotic quantum phase, called a supersolid, that is defined by broken translational symmetry and off-diagonal long-range order. This unique combination of properties enables a seemingly paradoxical scenario where a bosonic solid exhibits dissipationless mass flow. However, despite decades of extensive efforts, experimental realization of such a supersolid phase remains elusive. In this work we report experimental observation of a superfluid-to-insulating transition in the bosonic system of spatially indirect excitons in double layer graphene. Utilizing a variety of transport methods to characterize the superfluid-insulator phase boundary as a function of both density and temperature suggests the insulator to be a solid phase driven by repulsive dipole-dipole interactions in the dilute limit. The exciton solid exhibits a unique melting transition, with the high-temperature phase recovering a hallmark transport signature of off-diagonal long-range order, perfect Coulomb drag. The reentrant superfluid-like behaviour could indicate the low temperature solid also corresponds to a quantum coherent phase.

Authors:Selma Franca, Adolfo G. Grushin

Abstract: In chiral crystals crystalline symmetries can protect multifold fermions, pseudo-relativistic masless quasiparticles that have no high-energy counterparts. Their realization in transition metal mono-silicides has exemplified their intriguing physical properties, such as long Fermi arc surface states and unusual optical responses. Recent experimental studies on amorphous transition metal mono-silicides suggest that topological properties may survive beyond crystals, even though theoretical evidence is lacking. Motivated by these findings, we theoretically study a tight-binding model of amorphous transition metal mono-silicides. We find that topological properties of multifold fermions survive in the presence of structural disorder that converts the semimetal into a diffusive metal. We characterize this topological diffusive metal phase with the spectral localizer, a real-space topological indicator that we show can signal multifold fermions. Our findings showcase how topological properties can survive in disordered metals, and how they can be uncovered using the spectral localizer.

Authors:G. Catarina, J. C. G. Henriques, A. Molina-Sánchez, A. T. Costa, J. Fernández-Rossier

Abstract: We provide a comprehensive theory of magnetic phases in two-dimensional triangulene crystals, using both Hubbard model and density functional theory (DFT) calculations. We consider centrosymmetric and non-centrosymmetric triangulene crystals. In all cases, DFT and mean-field Hubbard model predict the emergence of broken-symmetry antiferromagnetic (ferrimagnetic) phases for the centrosymmetric (non-centrosymmetric) crystals. This includes the special case of the [4,4]triangulene crystal, whose non-interacting energy bands feature a gap with flat valence and conduction bands. We show how the lack of contrast between the local density of states of these bands, recently measured via scanning tunneling spectroscopy, is a natural consequence of a broken-symmetry N\'eel state that blocks intermolecular hybridization. Using random phase approximation, we also compute the spin wave spectrum of these crystals, including the recently synthesized [4,4]triangulene crystal. The results are in excellent agreement with the predictions of a Heisenberg spin model derived from multi-configuration calculations for the unit cell. We conclude that experimental results are compatible with an antiferromagnetically ordered phase where each triangulene retains the spin predicted for the isolated species.

Authors:Manuel Vilas-Varela, Francisco Romero-Lara, Alessio Vegliante, Jan Patrick Calupitan, Adrián Martínez, Lorenz Meyer, Unai Uriarte-Amiano, Niklas Friedrich, Dongfei Wang, Natalia E. Koval, María E. Sandoval-Salinas, David Casanova, Martina Corso, Emilio Artacho, Diego Peña, Jose Ignacio Pascual

Abstract: Triangulenes are open-shell triangular graphene flakes with total spin increasing with their size. In the last years, on-surface-synthesis strategies have permitted fabricating and engineering triangulenes of various sizes and structures with atomic precision. However, direct proof of the increasing total spin with their size remains elusive. In this work, we report the combined in-solution and on-surface synthesis of a large nitrogen-doped triangulene (aza-[5]-triangulene) and the detection of its high spin ground state on a Au(111) surface. Bond-resolved scanning tunneling microscopy images uncovered radical states distributed along the zigzag edges, which were detected as weak zero-bias resonances in scanning tunneling spectra. These spectral features reveal the partial Kondo screening of a high spin state. Through a combination of several simulation tools, we find that the observed distribution of radical states is explained by a quintet ground state (S = 2), instead of the expected quartet state (S = 3/2), confirming the positively charged state of the molecule on the surface. We further provide a qualitative description of the change of (anti)aromaticity introduced by N-substitution, and its role in the charge stabilization on a surface, resulting in a S = 2 aza-[5]-triangulene on Au(111).

Authors:Yangyu Guo, Mauricio Gómez Viloria, Riccardo Messina, Philippe Ben-Abdallah, Samy Merabia

Abstract: The understanding of extreme near-field heat transport across vacuum nanogaps between polar dielectric materials remains an open question. In this work, we present a molecular dynamic simulation of heat transport across MgO-MgO nanogaps, together with a consistent comparison with the continuum fluctuational-electrodynamics theory using local dielectric properties. The dielectric function is computed by Green-Kubo molecular dynamics with the anharmonic damping properly included. As a result, the direct atomistic modeling shows significant deviation from the continuum theory even up to a gap size of few nanometers due to non-local dielectric response from acoustic and optical branches as well as phonon tunneling. The lattice anharmonicity is demonstrated to have a large impact on the energy transmission and thermal conductance, in contrast to its moderate effect reported for metallic vacuum nanogaps. The present work thus provides further insight into the physics of heat transport in the extreme near-field regime between polar materials, and put forward a methodology to account for anharmonic effects.

Authors:Dominik Szombathy, Miklós Antal Werner, Cătălin Paşcu Moca, Örs Legeza, Assaf Hamo, Shahal Ilani, Gergely Zaránd

Abstract: The collective tunneling of a a Wigner necklace - a crystalline state of a small number of strongly interacting electrons confined to a suspended nanotube and subject to a double well potential - is theoretically analyzed and compared with experiments in [Shapir $\textit{et al.}$, Science $\textbf {364}$, 870 (2019)]. Density Matrix Renormalization Group computations, exact diagonalization, and instanton theory provide a consistent description of this very strongly interacting system, and show good agreement with experiments. Experimentally extracted and theoretically computed tunneling amplitudes exhibit a scaling collapse. Collective quantum fluctuations renormalize the tunneling, and substantially enhance it as the number of electrons increases.

Authors:Damian Bouwmeester, Talieh S. Ghiasi, Gabriela Borin Barin, Klaus Müllen, Pascal Ruffieux, Roman Fasel, Herre S. J. van der Zant

Abstract: Atomically precise graphene nanoribbons (GNRs) are predicted to exhibit exceptional edge-related properties, such as localized edge states, spin polarization, and half-metallicity. However, the absence of low-resistance nano-scale electrical contacts to the GNRs hinders harnessing their properties in field-effect transistors. In this paper, we make electrical contact with 9-atom-wide armchair GNRs using superconducting alloy MoRe as well as Pd (as a reference), which are two of the metals providing low-resistance contacts to carbon nanotubes. We take a step towards contacting a single GNR by fabrication of electrodes with a needle-like geometry, with about 20 nm tip diameter and 10 nm separation. To preserve the nano-scale geometry of the contacts, we develop a PMMA-assisted technique to transfer the GNRs onto the pre-patterned electrodes. Our device characterizations as a function of bias-voltage and temperature, show a thermally-activated gate-tunable conductance in the GNR-MoRe-based transistors.

Authors:Hugo Merbouche Institute for Applied Physics, University of Muenster, Muenster, Germany, Ping Che Unité Mixte de Physique CNRS, Thales, Université Paris-Saclay, Gif-sur-Yvette, France, Titiksha Srivastava SPEC, CEA, CNRS, Université Paris-Saclay, Gif-sur-Yvette, France, Nathan Beaulieu LabSTICC UMR 6285, Université de Bretagne Occidentale, Brest, France, Jamal Ben Youssef LabSTICC UMR 6285, Université de Bretagne Occidentale, Brest, France, Manuel Muñoz Instituto de Tecnologías Físicas y de la Información, Massimiliano d'Aquino Department of Electrical Engineering and ICT, University of Naples Federico II, Italy, Claudio Serpico Department of Electrical Engineering and ICT, University of Naples Federico II, Italy, Grégoire de Loubens SPEC, CEA, CNRS, Université Paris-Saclay, Gif-sur-Yvette, France, Paolo Bortolotti Unité Mixte de Physique CNRS, Thales, Université Paris-Saclay, Gif-sur-Yvette, France, Abdelmadjid Anane Unité Mixte de Physique CNRS, Thales, Université Paris-Saclay, Gif-sur-Yvette, France, Sergej O. Demokritov Institute for Applied Physics, University of Muenster, Muenster, Germany, Vladislav E. Demidov Institute for Applied Physics, University of Muenster, Muenster, Germany

Abstract: We study experimentally the processes of parametric excitation in microscopic magnetically saturated disks of nanometer-thick Yttrium Iron Garnet. We show that, depending on the relative orientation between the parametric pumping field and the static magnetization, excitation of either degenerate or non-degenerate magnon pairs is possible. In the latter case, which is particularly important for applications associated with the realization of computation in the reciprocal space, a single-frequency pumping can generate pairs of magnons whose frequencies correspond to different eigenmodes of the disk. We show that, depending on the size of the disk and the modes involved, the frequency difference in a pair can vary in the range 0.1-0.8 GHz. We demonstrate that in this system, one can easily realize a practically important situation where several magnon pairs share the same mode. We also observe the simultaneous generation of up to six different modes using a fixed-frequency monochromatic pumping. Our experimental findings are supported by numerical calculations that allow us to unambiguously identify the excited modes. Our results open new possibilities for the implementation of reciprocal-space computing making use of low damping magnetic insulators.

Authors:Troy Dion, Kilian D. Stenning, Alex Vanstone, Holly H. Holder, Rawnak Sultana, Ghanem Alatteili, Victoria Martinez, Mojtaba Taghipour Kaffash, Takashi Kimura, Hidekazu Kurebayashi, Will R. Branford, Ezio Iacocca, Benjamin M. Jungfleisch, Jack C. Gartside

Abstract: Strongly-interacting nanomagnetic arrays are ideal systems for exploring the frontiers of magnonic control. They provide functional reconfigurable platforms and attractive technological solutions across storage, GHz communications and neuromorphic computing. Typically, these systems are primarily constrained by their range of accessible states and the strength of magnon coupling phenomena. Increasingly, magnetic nanostructures have explored the benefits of expanding into three dimensions. This has broadened the horizons of magnetic microstate spaces and functional behaviours, but precise control of 3D states and dynamics remains challenging. Here, we introduce a 3D magnonic metamaterial, compatible with widely-available fabrication and characterisation techniques. By combining independently-programmable artificial spin-systems strongly coupled in the z-plane, we construct a reconfigurable 3D metamaterial with an exceptionally high 16N microstate space and intense static and dynamic magnetic coupling. The system exhibits a broad range of emergent phenomena including ultrastrong magnon-magnon coupling with normalised coupling rates of $\frac{\Delta \omega}{\gamma} = 0.57$ and magnon-magnon cooperativity up to C = 126.4, GHz mode shifts in zero applied field and chirality-selective magneto-toroidal microstate programming and corresponding magnonic spectral control.

Authors:Jorge Estrada-Álvarez, Francisco Domínguez-Adame, Elena Díaz

Abstract: Viscous flow of interacting electrons in two dimensional materials features a bunch of exotic effects. A model resembling the Navier-Stokes equation for classical fluids accounts for them in the so called hydrodynamic regime. This regime occurs when electron-electron collisions are frequent enough. We performed a detailed analysis of the hydrodynamic requirements and found three new routes to achieve viscous electron flow: favouring frequent inelastic collisions, the application of a magnetic field or a high-frequency electric field. More reflective edges of the material further span the range of validity of the above conditions. Our results show that the conventional requirement of frequent electron-electron collisions is too restrictive, and, as a consequence, materials and phenomena to be described using hydrodynamics are widened. We discuss recent experiments regarding Poiseuille-like flows, superballistic conduction and negative resistances as signatures for viscous flow onset. We conclude that these usual signatures of viscous electron flow are achieved by following alternative meta-hydrodynamic routes.

Authors:Ádám Bácsi, Luka Pavešić, Rok Žitko

Abstract: We present a theoretical study of a system consisting of a superconducting island and two quantum dots, a possible platform for building qubits and Cooper pair splitters, in the regime where each dot hosts a single electron and, hence, carries a magnetic moment. We focus on the case where the dots are coupled to overlapping superconductor states and we study whether the spins are ferromagnetically or antiferromagnetically aligned. We show that if the total number of particles is even, the spins align antiferromagnetically in the flatband limit, i.e., when the bandwidth of the superconductor is negligibly small, but ferromagnetically if the bandwidth is finite and above some value. If the total number of particles is odd, the alignment is ferromagnetic independently from the bandwidth. This implies that the results of the flatband limit are applicable only within restricted parameter regime for realistic superconducting qubit systems and that some care is required in applying simplified models to devices such as Cooper pair splitters.

Authors:D. Kuiri, M. P. Nowak

Abstract: Hybrid Josephson junctions realized on a two-dimensional electron gas are considered promising candidates for developing topological elements that are easily controllable and scalable. Here, we theoretically study the possibility of the detection of topological superconductivity via the non-local spectroscopy technique. We show that the non-local conductance is related to the system band structure, allowing probe of the gap closing and reopening related to the topological transition. We demonstrate that the topological transition induces a change in the sign of the non-local conductance at zero energy due to the change in the quasiparticle character of the dispersion at zero momentum. Importantly, we find that the tunability of the superconducting phase difference via flux in hybrid Josephson junctions systems is strongly influenced by the strength of the Zeeman interaction, which leads to considerable modifications in the complete phase diagram that can be measured under realistic experimental conditions.

Authors:Athanasios Tsintzis, Rubén Seoane Souto, Karsten Flensberg, Jeroen Danon, Martin Leijnse

Abstract: The possibility to engineer artificial Kitaev chains in arrays of quantum dots coupled via narrow superconducting regions has emerged as an attractive way to overcome the disorder issues that complicate the realization and detection of topological superconducting phases in other platforms. Although a true topological phase would require long chains, already a two-site chain realized in a double quantum dot can be tuned to points in parameter space where it hosts zero-energy states that seem identical to the Majorana bound states that characterize a topological phase. These states were named "poor man's Majorana bound states" (PMMs) because they lack formal topological protection. In this work, we propose a roadmap for next-generation experiments on PMMs. The roadmap starts with experiments to characterize a single pair of PMMs by measuring the Majorana quality, then moves on to initialization and readout of the parity of a PMM pair, which allows measuring quasiparticle poisoning times. The next step is to couple two PMM systems to form a qubit. We discuss measurements of the coherence time of such a qubit, as well as a test of Majorana fusion rules in the same setup. Finally, we propose and analyse three different types of braiding-like experiments which require more complex device geometries. Our conclusions are supported by calculations based on a realistic model with interacting and spinful quantum dots, as well as by simpler models to gain physical insight. Our calculations show that it is indeed possible to demonstrate nonabelian physics in minimal two-site Kitaev chains despite the lack of a true topological phase. But our findings also reveal that doing so requires some extra care, appropriately modified protocols and awareness of the details of this particular platform.

Authors:S. Tacchi, R. Silvani, M. Kuepferling, A. Fernandez Scarioni, S. Sievers, H. W. Schumacher, E. Darwin, M. -A. Syskaki, G. Jakob, M. Klaui, G. Carlotti

Abstract: Despite the huge recent interest towards chiral magnetism related to the interfacial Dzyaloshinskii Moriya interaction (iDMI) in layered systems, there is a lack of experimental data on the effect of iDMI on the spin waves eigenmodes of laterally confined nanostructures. Here we exploit Brillouin Light Scattering (BLS) to analyze the spin wave eigenmodes of non-interacting circular and elliptical dots, as well as of long stripes, patterned starting from a Pt(3.4 nm)/CoFeB(0.8 nm) bilayer, with lateral dimensions ranging from 100 nm to 400 nm. Our experimental results, corroborated by micromagnetic simulations based on the GPU-accelerated MuMax3 software package, provide evidence for a strong suppression of the frequency asymmetry between counter-propagating spin waves (corresponding to either Stokes or anti-Stokes peaks in BLS spectra), when the lateral confinement is reduced from 400 nm to 100 nm, i.e. when it becomes lower than the light wavelength. Such an evolution reflects the modification of the spin wave character from propagating to stationary and indicates that the BLS based method of quantifying the i-DMI strength from the frequency difference of counter propagating spin waves is not applicable in the case of magnetic elements with lateral dimension below about 400 nm.

Authors:Ran Xue, Max Beer, Inga Seidler, Simon Humpohl, Jhih-Sian Tu, Stefan Trellenkamp, Tom Struck, Hendrik Bluhm, Lars R. Schreiber

Abstract: The connectivity within single carrier information-processing devices requires transport and storage of single charge quanta. Our all-electrical Si/SiGe shuttle device, called quantum bus (QuBus), spans a length of 10 $\mathrm{\mu}$m and is operated by only six simply-tunable voltage pulses. It operates in conveyor-mode, i.e. the electron is adiabatically transported while confined to a moving QD. We introduce a characterization method, called shuttle-tomography, to benchmark the potential imperfections and local shuttle-fidelity of the QuBus. The fidelity of the single-electron shuttle across the full device and back (a total distance of 19 $\mathrm{\mu}$m) is $(99.7 \pm 0.3)\,\%$. Using the QuBus, we position and detect up to 34 electrons and initialize a register of 34 quantum dots with arbitrarily chosen patterns of zero and single-electrons. The simple operation signals, compatibility with industry fabrication and low spin-environment-interaction in $^{28}$Si/SiGe, promises spin-conserving transport of spin qubits for quantum connectivity in quantum computing architectures.

Authors:M. D. Bouman, J. H. Mentink

Abstract: Understanding the speed limits for the propagation of magnons is of key importance for the development of ultrafast spintronics and magnonics. Recently, it was predicted that in 2D antiferromagnets, spin correlations can propagate faster than the highest magnon velocity. Here we gain deeper understanding of this supermagnonic effect based on time-dependent Schwinger boson mean-field theory. We find that the supermagnonic effect is determined by the competition between propagating magnons and a localized quasi-bound state, which is tunable by lattice coordination and quantum spin value $S$, suggesting a new scenario to enhance magnon propagation.

Authors:Florian Höhe, Ciprian Padurariu, Brecht I. C Donvil, Lukas Danner, Joachim Ankerhold, Björn Kubala

Abstract: Synchronization is a widespread phenomenon encountered in many natural and engineered systems with nonlinear classical dynamics. How synchronization concepts and mechanisms transfer to the quantum realm and whether features are universal or platform specific are timely questions of fundamental interest. Here, we present a new approach to model incoherently driven dissipative quantum systems susceptible to synchronization within the framework of Josephson photonics devices, where a dc-biased Josephson junction creates (non-classical) light in a microwave cavity. The combined quantum compound constitutes a self-sustained oscillator with a neutrally stable phase. Linking current noise to the full counting statistics of photon emission allows us to capture phase diffusion, but moreover permits phase locking to an ac-signal and mutual synchronization of two such devices. Thereby one can observe phase stabilization leading to a sharp emission spectrum as well as unique photon emission statistics revealing shot noise induced phase slips. Two-time perturbation theory is used to obtain a reduced description of the oscillators phase dynamics in form of a Fokker-Planck equation in generalization of classical synchronization theories.

Authors:F. Urban, F. Giubileo, A. Grillo, L. Iemmo, G. Luongo, M. Passacantando, T. Foller, L. Madauß, E. Pollmann, M. P. Geller, D. Oing, M. Schleberger, A. Di Bartolomeo

Abstract: We study the effect of electric stress, gas pressure and gas type on the hysteresis in the transfer characteristics of monolayer molybdenum disulfide (MoS2) field effect transistors. The presence of defects and point vacancies in the MoS2 crystal structure facilitates the adsorption of oxygen, nitrogen, hydrogen or methane, which strongly affect the transistor electrical characteristics. Although the gas adsorption does not modify the conduction type, we demonstrate a correlation between hysteresis width and adsorption energy onto the MoS2 surface. We show that hysteresis is controllable by pressure and/or gas type. Hysteresis features two well-separated current levels, especially when gases are stably adsorbed on the channel, which can be exploited in memory devices.

Authors:Davide Girardi, Simone Finizio, Claire Donnelly, Guglielmo Rubini, Sina Mayr, Valerio Levati, Simone Cuccurullo, Federico Maspero, Jörg Raabe, Daniela Petti, Edoardo Albisetti

Abstract: Spin waves are collective perturbations in the orientation of the magnetic moments in magnetically ordered materials. Their rich phenomenology is intrinsically three dimensional, from the trajectory of the spin precession during their propagation, to the profiles of the spin-wave mode throughout the volume of the magnetic system. This gives rise to novel complex phenomena with high potential for applications in the field of magnonics. However, the three-dimensional imaging of spin waves, key to understanding and harnessing these phenomena, has so far not been possible. Here, we image the three-dimensional dynamics of spin waves excited in a synthetic antiferromagnet, with nanoscale spatial resolution and sub-ns temporal resolution, using time-resolved magnetic laminography. In this way, we map the distribution of the spin-wave modes throughout the volume of the structure, revealing unexpected depth-dependent profiles originating from the interlayer dipolar interaction. We experimentally demonstrate the existence of complex three-dimensional interference patterns, and analyze them via micromagnetic modelling. We find that these patterns are generated by the superposition of spin waves with non-uniform amplitude profiles, and that their features can be controlled by tuning the composition and structure of the magnetic system. Our results open unforeseen possibilities for the study of complex spin-wave modes and their interaction within nanostructures, and for the generation and manipulation of three-dimensional spin-wave landscapes for the design of novel functions in magnonic devices.

Authors:Erwin Mönch, Stefan Hubmann, Ivan Yahniuk, Sophia Schweiss, Vasily V. Bel'kov, Leonid E. Golub, Robin Huber, Jonathan Eroms, Kenji Watanabe, Takashi Taniguchi, Dieter Weiss, Sergey D. Ganichev

Abstract: We report on the observation of a nonlinear intensity dependence of the terahertz radiation induced ratchet effects in bilayer graphene with asymmetric dual grating gate lateral lattices. These nonlinear ratchet currents are studied in structures of two designs with dual grating gate fabricated on top of encapsulated bilayer graphene and beneath it. The strength and sign of the photocurrent can be controllably varied by changing the bias voltages applied to individual dual grating subgates and the back gate. The current consists of contributions insensitive to the radiation's polarization state, defined by the orientation of the radiation electric field vector with respect to the dual grating gate metal stripes, and the circular ratchet sensitive to the radiation helicity. We show that intense terahertz radiation results in a nonlinear intensity dependence caused by electron gas heating. At room temperature the ratchet current saturates at high intensities of the order of hundreds to several hundreds of kWcm$^{-2}$. At $T = 4 {\rm K}$, the nonlinearity manifests itself at intensities that are one or two orders of magnitude lower, moreover, the photoresponse exhibits a complex dependence on the intensity, including a saturation and even a change of sign with increasing intensity. This complexity is attributed to the interplay of the Seebeck ratchet and the dynamic carrier density redistribution, which feature different intensity dependencies and a nonlinear behavior of the sample's conductivity induced by electron gas heating. Our study demonstrates that graphene-based asymmetric dual grating gate devices can be used as terahertz detectors at room temperature over a wide dynamic range, spanning many orders of magnitude of terahertz radiation power. Therefore, their integration together with current-driven read-out electronics is attractive for the operation with high-power pulsed sources.

Authors:Manuel Längle, Kenichiro Mizohat, Clemens Mangler, Alberto Trentino, Kimmo Mustonen, E. Harriet Åhlgren, Jani Kotakoski

Abstract: Van der Waals atomic solids of noble gases on metals at cryogenic temperatures were the first experimental examples of two-dimensional systems. Recently such structures have also been created on under encapsulation by graphene, allowing studies at elevated temperatures through scanning tunneling microscopy. However, for this technique, the encapsulation layer often obscures the actual arrangement of the noble gas atoms. Here, we create Kr and Xe clusters in between two suspended graphene layers, and uncover their atomic structure through direct imaging with transmission electron microscopy. We show that small crystals (N<9) arrange as expected based on the simple non-directional van der Waals interaction. Crystals larger than this show some deviations for the outermost atoms, possibly enabled by deformations in the encapsulating graphene lattice. We further discuss the dynamics of the clusters within the graphene sandwich, and show that while all Xe clusters with up to at least N=51 remain solid, Kr clusters with already N~16 turn occasionally fluid under our experimental conditions with an estimated pressure of ca. 0.3 GPa. This study opens a way for the so-far unexplored frontier of encapsulated two-dimensional van der Waals solids with exciting possibilities for condensed matter physics research that expands from quantum structures to biological applications.

Authors:Hu Jinbing, Zhuang Songlin, Yang Yi

Abstract: The topology of the Brillouin zone, foundational in topological physics, is always assumed to be a torus. We theoretically report the construction of Brillouin real projective plane ($\mathrm{RP}^2$) and the appearance of quadrupole insulating phase, which are enabled by momentum-space nonsymmorphic symmetries stemming from $\mathbb{Z}_2$ synthetic gauge fields. We show that the momentum-space nonsymmorphic symmetries quantize bulk polarization and Wannier-sector polarization nonlocally across different momenta, resulting in quantized corner charges and an isotropic binary bulk quadrupole phase diagram, where the phase transition is triggered by a bulk energy gap closing. Under open boundary conditions, the nontrivial bulk quadrupole phase manifests either trivial or nontrivial edge polarization, resulting from the violation of momentum-space nonsymmorphic symmetries under lattice termination. We present a concrete design for the $\mathrm{RP}^2$ quadrupole insulator based on acoustic resonator arrays and discuss its feasibility in optics, mechanics, and electrical circuits. Our results show that deforming the Brillouin manifold creates opportunities for realizing high-order band topology.

Authors:Jeffrey Rable, Jyotirmay Dwivedi, Nitin Samarth

Abstract: Nitrogen-vacancy (NV) centers in diamond offer a sensitive method of measuring the spatially localized dynamics of magnetization and associated spin textures in ferromagnetic materials. We use NV centers in a deterministically positioned nanodiamond to demonstrate off-resonant detection of GHz-scale microwave field driven oscillations of a single domain wall (DW). The technique exploits the enhanced relaxation of NV center spins due to the broadband stray fields generated by an oscillating DW pinned at an engineered defect in a lithographically patterned ferromagnetic nanowire. Discrepancies between the observed DW oscillation frequency and predictions from micromagnetic simulations suggest extreme sensitivity of DW dynamics to patterning imperfections such as edge roughness. These experiments and simulations identify potential pathways toward quantum spintronic devices that exploit current driven DWs as nanoscale microwave generators for qubit control, greatly increasing the driving field at an NV center and thus drastically reducing the {\pi} pulse time.

Authors:N. A. Stepanov, Mikhail A. Skvortsov

Abstract: We study the electron-phonon relaxation in the model of a granular metal film, where the grains are formed by regularly arranged potential barriers of arbitrary transparency. The relaxation rate of Debye acoustic phonons is calculated taking into account two mechanisms of electron-phonon scattering: the standard Frohlich interaction of the lattice deformation with the electron density and the interaction mediated by the displacement of grain boundaries dragged by the lattice vibration. At lowest temperatures, the electron-phonon cooling power follows the power-law temperature dependence typical for clean systems, but with the prefactor growing as the transparency of the grain boundaries decreases.

Authors:Kyoung-Min Kim, Moon Jip Park

Abstract: The use of moir\'e patterns to manipulate two-dimensional materials has facilitated new possibilities for controlling material properties. The moir\'e patterns in the two-dimensional magnets can cause peculiar spin texture, as shown by previous studies focused on twisted bilayer systems. In our study, we develop a theoretical model to investigate the magnetic structure of twisted trilayer magnets. Unlike the twisted bilayer, the twisted trilayer magnet has four different local stacking structures distinguished by the interlayer couplings between the three layers. Our results show that the complex interlayer coupling effects in the moir\'e superlattice can lead to the stabilization of rich magnetic domain structures; these structures can be significantly manipulated by adjusting the twist angle. Additionally, external magnetic fields can easily manipulate these domain structures, indicating potential applications in spintronics devices.

Authors:Udo Ausserlechner

Abstract: I discuss uniform, isotropic, plane, singly connected, electrically linear, regular symmetric Hall-plates with an arbitrary number of N peripheral contacts exposed to a uniform perpendicular magnetic field of arbitrary strength. In practice, the regular symmetry is the most common one. If the Hall-plates are mapped conformally to the unit disk, regular symmetry means that all contacts are equally large and all contacts spacings are equally large, yet the contacts spacings may have a different size than the contacts. Such Hall-plates do not change when they are rotated by 360{\deg}/N. Their indefinite conductance matrices are circulant matrices, whose complex eigenvalues are computable in closed form. These eigenvalues are used to discuss the Hall-output voltage, the maximum noise-efficiency, and Van-der-Pauw's method for measuring sheet resistances. For practical use, I report simple approximations for Hall-plates with four contacts and 90{\deg} symmetry with popular shapes like disks, rectangles, octagons, squares, and Greek crosses with and without rounded corners.

Authors:Sudin Ganguly, Suparna Sarkar, Kallol Mondal, Santanu K. Maiti

Abstract: The present work explores the potential for observing multiple reentrant localization behavior in a double-stranded helical (DSH) system, extending beyond the conventional nearest-neighbor hopping interaction. The DSH system is considered to have hopping dimerization in each strand, while also being subjected to a transverse electric field. The inclusion of an electric field serves the dual purpose of inducing quasiperiodic disorder and strand-wise staggered site energies. Two reentrant localization regions are identified: one exhibiting true extended behavior in the thermodynamic limit, while the second region shows quasi-extended characteristics with partial spreading within the helix. The DSH system exhibits three distinct single-particle mobility edges linked to localization transitions present in the system. The analysis in this study involves examining various parameters such as the single-particle energy spectrum, inverse participation ratio, local probability amplitude, and more. Our proposal, combining achievable hopping dimerization and induced correlated disorder, presents a unique opportunity to study phenomenon of reentrant localization, generating significant research interest.

Authors:Zhen-Tao Zhang, Bao-Long Liang, Zhen-Shan Yang

Abstract: Electron teleportation via two separate Majorana bound states(MBSs) is a manifestation of the non-locality of MBSs. A superconductor may host multiple separate or partial overlapping MBSs, and it is difficult to distinguish them. Here, we have studied the electron teleportation between two quantum dots via multiple MBSs in a superconductor island, two of which couple with the quantum dots. We find that in the absence of Majorana coupling, both elastic and inelastic electron transfers are allowed for specific system settings, and the extent to which the island state is changed after the teleportation relies on the initial state of the MBSs. In the presence of Majorana couplings, the elastic and inelastic teleportations are selective according to which pair of MBSs are coupled. Meanwhile, the cotuneling processes are distinct for different MBSs coupling types. In addition, we have investigated the effect of the asymmetry of the tunnelings to quantum dots on the transport. Our findings are meaningful for resolving transport signatures induced by topological MBSs and that stems from nontopological quasiparticle.

Authors:Tianjiao Li, Chenxi Tian, Atieh Moridi, Jingjie Yeo

Abstract: Due to their remarkable mechanical and chemical properties, Ti-Al based materials are attracting considerable interest in numerous fields of engineering, such as automotive, aerospace, and defense. With their low density, high strength, and resistance to corrosion and oxidation, these intermetallic alloys and compound metal-metallic composites have found diverse applications. The present study delves into the interfacial dynamics of these Ti-Al systems, particularly focusing on the behavior of Ti and Al atoms in the presence of TiAl$_3$ grain boundaries under experimental heat treatment conditions. Using a combination of Molecular Dynamics and Markov State Model analyses, we scrutinize the kinetic processes involved in the formation of TiAl$_3$. The Molecular Dynamics simulation indicates that at the early stage of heat treatment, the predominating process is the diffusion of Al atoms towards the Ti surface through the TiAl$_3$ grain boundaries. The Markov State Modeling identifies three distinct dynamic states of Al atoms within the Ti/Al mixture that forms during the process, each exhibiting a unique spatial distribution. Using transition timescales as a qualitative measure of the rapidness of the dynamics, it is observed that the Al dynamics is significantly less rapid near the Ti surface compared to the Al surface. Put together, the results offer a comprehensive understanding of the interfacial dynamics and reveals a three-stage diffusion mechanism. The process initiates with the premelting of Al, proceeds with the prevalent diffusion of Al atoms towards the Ti surface, and eventually ceases as the Ti concentration within the mixture progressively increases. The insights gained from this study could contribute significantly to the control and optimization of manufacturing processes for these high-performing Ti-Al based materials.

Authors:Somnath Koley, Jiabin Cui, Yossef. E. Panfil, Yonatan Ossia, Adar Levi, Einav Scharf, Lior Verbitsky, Uri Banin

Abstract: We investigate the charge re-distribution upon optical excitation of various necked homodimer CQDMs using single particle emission spectroscopy. By tuning the hybridization of the electron wavefunction at a fixed center-to-center distance through controlling the neck girth, we reveal two coupling limits. On one hand a connected-but-confined situation where neighbouring CQDs are weakly fused to each other manifesting a weak coupling regime, and on the other hand, a connected-and-delocalized situation, where the neck is filled beyond the facet size leading to a rod-like architecture manifesting strong-coupling. Either coupling regimes entrust distinct optical signatures clearly resolved at room temperature in terms of photoluminescence quantum yield, intensity time traces, lifetimes, and spectra of the neutral-exciton, charged-exciton, and biexciton states. The interplay between the radiative and non-radiative Auger decays of these states, turns emitted photons from the CQDMs in the weak-coupling regime highly bunched unlike CQD monomers, while the antibunching is regained at the strong-coupling regime. This behavior correlates with the hybridization energy being smaller than the thermal energy (kT approx. 25meV) at the weak-coupling limit (delta E approax.5-10meV), leading to exciton localization suppressing Auger decay. In the neck-filled architectures, the larger hybridization energy (delta E approx.20-30meV) leads to exciton delocalization while activating the fast charged and multi-exciton Auger decay processes. This work sets an analogy for the artificial molecule CQDMs with regular molecules, where the two distinct regimes of weak- and strong-coupling correspond to ionic- or covalent- type bonding, respectively.

Authors:Lior Asor, Jing Liu, Shuting Xiang, Nir Tessler, Anatoly I. Frenkel, Uri Banin

Abstract: Doped heavy metal-free III-V semiconductor nanocrystal quantum dots are of great interest both from the fundamental aspects of doping in highly confined structures, and from the applicative side of utilizing such building blocks in the fabrication of p-n homojunction devices. InAs nanocrystals, that are of particular relevance for short wave IR detection and emission applications, manifest heavy n-type character poising a challenge for their transition to p-type behavior. We present p-type doping of InAs nanocrystals with Zn-enabling control over the charge carrier type in InAs QDs field effect transistors. The post-synthesis doping reaction mechanism is studied for Zn precursors with varying reactivity. Successful p-type doping was achieved by the more reactive precursor, diethylzinc. Substitutional doping by Zn2+ replacing In3+ is established by X-ray absorption spectroscopy analysis. Furthermore, enhanced near IR photoluminescence is observed due to surface passivation by Zn as indicated from elemental mapping utilizing high resolution electron microscopy corroborated by X-ray photoelectron spectroscopy study. The demonstrated ability to control the carrier type, along with the improved emission characteristics, paves the way towards fabrication of optoelectronic devices active in the short wave IR region utilizing heavy-metal free nanocrystal building blocks.

Authors:J. Dastoor, D. M. Willerton, W. Reisner, G. Gervais

Abstract: We theoretically calculate the fundamental noise that is present in gaseous (dilute fluid) flow in channels in the classical and degenerate quantum regime, where the Fermi-Dirac and Bose- Einstein distribution must be considered. Results for both regimes are analogous to their electrical counterparts. The quantum noise is calculated for a two terminal system and is a complicated function of the thermal and shot noise with the thermal noise dominating when $2k_BT >> m\Delta P$ and vice versa. The cumulant generating function for mass flow, which generates all the higher order statistics related to our mass flow distribution, is also derived and is used to find an expression for the third cumulant of flow across a fluidic channel.

Authors:Sharareh Sayyad

Abstract: The emergence of chiral anomaly entails various fascinating phenomena such as anomalous quantum Hall effect and chiral magnetic effect in different branches of (non-)Hermitian physics. While in the single-particle picture, anomalous currents merely appear due to the coupling of massless particles with background fields, many-body interactions can also be responsible for anomalous transport in interacting systems. In this Letter, we study anomalous chiral currents in systems where interacting massless fermions with complex Fermi velocities are coupled to complex gauge fields. Our results reveal that incorporating non-Hermiticity and many-body interactions gives rise to additional terms in anomalous relations beyond their Hermitian counterparts. We further present that many-body corrections in the subsequent non-Hermitian chiral magnetic field or anomalous Hall effect are nonvanishing in nonequilibrium or inhomogeneous systems. Our results advance efforts in understanding the anomalous transport in interacting non-Hermitian systems.

Authors:Jan-Lucas Uslu, Taoufiq Ouaj, David Tebbe, Jo Henri Bertram, Marc Schütte, Kenji Watanabe, Takashi Taniguchi, Bernd Beschoten, Lutz Waldecker, Christoph Stampfer

Abstract: The most widely used method for obtaining high-quality two-dimensional materials is through mechanical exfoliation of bulk crystals. Manual identification of suitable flakes from the resulting random distribution of crystal thicknesses and sizes on a substrate is a time-consuming, tedious task. Here, we present a platform for fully automated scanning, detection, and classification of two-dimensional materials, the source code of which we make openly available. Our platform is designed to be accurate, reliable, fast, and versatile in integrating new materials, making it suitable for everyday laboratory work. The implementation allows a fully automized scanning and analysis of wafers with an average inference time of 100 ms for images of 2.3 Mpixels. The developed detection algorithm is based on a combination of the flakes' optical contrast toward the substrate and their geometric shape. We demonstrate that it is able to detect the majority of exfoliated flakes of various materials, with an average recall (AR50) between 66\% and 92\%. We also show that the algorithm can be trained with as few as five flakes of a given material, which we demonstrate for the examples of few-layer graphene, WSe$_2$, CrI$_3$, 1T-TaS$_2$ and hBN. Our platform has been tested over a two-year period, during which more than 10$^6$ images of multiple different materials were acquired by over 30 individual researchers.

Authors:Duilio De Santis, Claudio Guarcello, Bernardo Spagnolo, Angelo Carollo, Davide Valenti

Abstract: Noisy and ac forcing can cooperatively lead to the emergence of sine-Gordon breathers robust to dissipation. This phenomenon is studied, for both Neumann and periodic boundary conditions (NBC and PBC, respectively), at different values of the main system parameters, such as the noise intensity and the ac frequency-amplitude pair. In all the considered cases, nonmonotonicities of the probability of generating only breathers versus the noise strength are observed, implying that optimal noise ranges for the breather formation process exist. Within the latter scenarios, the statistics of the breathers' number, position, and amplitude are analyzed. The number of breathers is found to grow, on average, with the noise amplitude. The breathers' spatial distribution is sharply peaked at the system's edges for NBC, whereas it is essentially uniform for PBC. The average breather amplitude is dictated by the ac frequency-amplitude pair. Finally, a size analysis shows that the minimum system length for the generation mechanism is given by the typical breather half-width (width) in NBC (PBC).

Authors:Hassan Lamsaadi, Dorian Beret, Ioannis Paradisanos, Pierre Renucci, Delphine Lagarde, Xavier Marie, Bernhard Urbaszek, Ziyang Gan, Antony George, Kenji Watanabe, Takashi Taniguchi, Andrey Turchanin, Laurent Lombez, Nicolas Combe, Vincent Paillard, Jean-Marie Poumirol

Abstract: Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challenging task. Here we demonstrate that an atomically sharp TMD-ML lateral heterostructure (MoSe$_{2}$-WSe$_{2}$) transforms the isotropic exciton diffusion into a unidirectional excitonic flow through the junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a modified exciton transfer model, we show a discontinuity of the exciton density distribution on each side of the interface. We introduce the concept of exciton Kapitza resistance, by analogy with the interfacial thermal resistance referred to as Kapitza resistance. By comparing different heterostructures with or without top hexagonal boron nitride (hBN) layer, we deduce that the transport properties can be controlled, over distances far greater than the junction width, by the exciton density through near-field engineering and/or laser power density. This work provides a new approach for controlling the neutral exciton flow, which is key toward the conception of excitonic devices.

Authors:Bin Liu School of Materials Science and Physics, China University of Mining and Technology, Yang Li School of Materials Science and Physics, China University of Mining and Technology, Bin Yang School of Materials Science and Physics, China University of Mining and Technology, Xiaopeng Shen School of Materials Science and Physics, China University of Mining and Technology, Yuting Yang School of Materials Science and Physics, China University of Mining and Technology, Zhi Hong Hang School of Physical Science and Technology & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University Institute for Advanced Study, Soochow University, Motohiko Ezawa Department of Applied Physics, University of Tokyo

Abstract: The study of topological states has developed rapidly in electric circuits, which permits flexible fabrications of non-Hermitian systems by introducing non-Hermitian terms. Here, nonreciprocal coupling terms are realized by utilizing a voltage follower module in non-Hermitian topological electric circuits. We report the experimental realization of one- and two- dimensional non-Hermitian skin interface states in electric circuits, where interface states induced by non-Hermitian skin effects are localized at the interface of different domains carrying different winding numbers. Our electric circuit system provides a readily accessible platform to explore non-Hermitian-induced topological phases, and paves a new road for device applications.

Authors:Dmitry Zverevich, Alex Levchenko

Abstract: In two-dimensional electron systems, plasmons are gapless and long-lived collective excitations of propagating charge density oscillations. We study the fluctuation mechanism of plasmon-assisted transport in the regime of electron hydrodynamics. We consider pristine electron liquids where charge fluctuations are thermally induced by viscous stresses and intrinsic currents, while attenuation of plasmons is determined by the Maxwell mechanism of charge relaxation. We show that while the contribution of plasmons to the shear viscosity and thermal conductivity of a Fermi liquid is small, plasmon resonances in the bilayer devices enhance the drag resistance. In systems without Galilean invariance, fluctuation-driven contributions to dissipative coefficients can be described only in terms of hydrodynamic quantities: intrinsic conductivity, viscosity, and plasmon dispersion relation.

Authors:Radha Krishnan, Sangram Biswas, Yu-Ling Hsueh, Hongyang Ma, Rajib Rahman, Bent Weber

Abstract: Spins confined to atomically-thin semiconductors are being actively explored as quantum information carriers. In transition metal dichalcogenides (TMDCs), the hexagonal crystal lattice gives rise to an additional valley degree of freedom with spin-valley locking and potentially enhanced spin life- and coherence times. However, realizing well-separated single-particle levels, and achieving transparent electrical contact to address them has remained challenging. Here, we report well-defined spin states in a few-layer MoS$ _2$ transistor, characterized with a spectral resolution of $\sim{50~\mu}$eV at ${T_\textrm{el} = 150}$~mK. Ground state magnetospectroscopy confirms a finite Berry-curvature induced coupling of spin and valley, reflected in a pronounced Zeeman anisotropy, with a large out-of-plane $g$-factor of ${g_\perp \simeq 8}$. A finite in-plane $g$-factor (${g_\parallel \simeq 0.55-0.8}$) allows us to quantify spin-valley locking and estimate the spin-orbit splitting ${2\Delta_{\rm SO} \sim 100~\mu}$eV. The demonstration of spin-valley locking is an important milestone towards realizing spin-valley quantum bits.

Authors:Antti Ranni, Harald Havir, Subhomoy Haldar, Ville F. Maisi

Abstract: In this article we present an experimental study of microwave resonators made out of Josephson junctions. The junctions are embedded in a transmission line geometry so that they increase the inductance per length for the line. By comparing two devices with different input/output coupling strengths, we show that the coupling capacitors, however, add a significant amount to the total capacitance of the resonator. This makes the resonators with high coupling capacitance to act rather as lumped element resonators with inductance from the junctions and capacitance from the end sections. Based on a circuit analysis, we also show that the input and output couplings of the resonator are limited to a maximum value of $\omega_r Z_0 /4 Z_r$ where $\omega_r$ is the resonance frequency and $Z_0$ and $Z_r$ are the characteristic impedances of the input/output lines and the resonator respectively.

Authors:Iskander Mukatayev, Gabriele D'Avino, Benoit Sklenard, Valerio Olevano, Jing Li

Abstract: In X-ray photoelectron spectroscopy (XPS), the injected hole interacts with the electronic polarization cloud induced by the hole itself, ultimately resulting in a lower binding energy. Such polarization effect can shift the core-level energy by more than 1 eV, as shown here by embedded many-body perturbation theory for the paradigmatic case of noble gas clusters made of Ar, Kr, or Xe. The polarization energy is almost identical for the different core-orbitals of a given atom, but it strongly depends on the position of the ionized atom in the cluster. An analytical formula is derived from classical continuum electrostatics, providing an effective and accurate description of polarization effects, which permits to achieve an excellent agreement with available experiments on noble gas clusters at a modest computational cost. Electronic polarization provides a crucial contribution to core levels absolute energies and chemical shifts.

Authors:A. V. Shumilin, T. S. Shamirzaev, D. S. Smirnov

Abstract: We propose a concept of quantum dot based light emitting diode that produces circularly polarized light due to the tuning of the exciton fine structure by magnetic field and electron nuclear hyperfine interaction. The device operates under injection of electrons and holes from nonmagnetic contacts in a small field of the order of milliteslas. Its size can be parametrically smaller than the light wavelength, and circular polarization degree of electroluminescence can reach 100%. The proposed concept is compatible with the micropillar cavities, which allows for the deterministic electrical generation of single circularly polarized photons.

Authors:Chao Shen, Wenkang Zhan, Kaiyao Xin, Manyang Li, Zhenyu Sun, Jian Tang, Zhaofeng Wu, Bo Xu, Zhongming Wei, Chao Zhao, Zhanguo Wang

Abstract: Self-assembled InAs/GaAs quantum dots (QDs) have properties highly valuable for developing various optoelectronic devices such as QD lasers and single photon sources. The applications strongly rely on the density and quality of these dots, which has motivated studies of the growth process control to realize high-quality epi-wafers and devices. Establishing the process parameters in molecular beam epitaxy (MBE) for a specific density of QDs is a multidimensional optimization challenge, usually addressed through time-consuming and iterative trial-and-error. Meanwhile, reflective high-energy electron diffraction (RHEED) has been widely used to capture a wealth of growth information in situ. However, it still faces the challenges of extracting information from noisy and overlapping images. Here, based on 3D ResNet, we developed a machine learning (ML) model specially designed for training RHEED videos instead of static images and providing real-time feedback on surface morphologies for process control. We demonstrated that ML from previous growth could predict the post-growth density of QDs, by successfully tuning the QD densities in near-real time from 1.5E10 cm-2 down to 3.8E8 cm-2 or up to 1.4 E11 cm-2. Compared to traditional methods, our approach, with in-situ tuning capabilities and excellent reliability, can dramatically expedite the material optimization process and improve the reproducibility of MBE growth, constituting significant progress for thin film growth techniques. The concepts and methodologies proved feasible in this work are promising to be applied to a variety of material growth processes, which will revolutionize semiconductor manufacturing for microelectronic and optoelectronic industries.

Authors:Yadav P. Kandel, Suraj Thapa Magar, Arjun Iyer, William H. Renninger, John M. Nichol

Abstract: Because of their small size, low loss, and compatibility with magnetic fields and elevated temperatures, surface acoustic wave resonators hold significant potential as future quantum interconnects. Here, we design, fabricate, and characterize GHz-frequency surface acoustic wave resonators with the potential for strong capacitive coupling to nanoscale solid-state quantum systems, including semiconductor quantum dots. Strong capacitive coupling to such systems requires a large characteristic impedance, and the resonators we fabricate have impedance values above 100 $\Omega$. We achieve such high impedance values by tightly confining a Gaussian acoustic mode. At the same time, the resonators also have low loss, with quality factors of several thousand at millikelvin temperatures. These high-impedance resonators are expected to exhibit large vacuum electric-field fluctuations and have the potential for strong coupling to a variety of solid-state quantum systems.

Authors:Felipe Reyes-Osorio, Branislav K. Nikolic

Abstract: Understanding the origin of damping mechanisms in magnetization dynamics of metallic ferromagnets is a fundamental problem for nonequilibrium many-body physics of systems where quantum conduction electrons interact with localized spins assumed to be governed by the classical Landau-Lifshitz-Gilbert (LLG) equation. It is also of critical importance for applications as damping affects energy consumption and speed of spintronic and magnonic devices. Since the 1970s, a variety of linear-response and scattering theory approaches have been developed to produce widely used formulas for computation of spatially-independent Gilbert scalar parameter as the magnitude of the Gilbert damping term in the LLG equation. The largely-unexploited-for-this-purpose Schwinger-Keldysh field theory (SKFT) offers additional possibilities, such as rigorously deriving an extended LLG equation by integrating quantum electrons out. Here we derive such equation whose Gilbert damping for metallic ferromagnets in $d=1$-$3$ dimensions is nonlocal-i.e., dependent on position of all localized spins at a given time-and nonuniform, even if all localized spins are collinear and spin-orbit coupling (SOC) is absent. This is in sharp contrast to standard lore, where nonlocal damping is possible only if localized spins are noncollinear, while SOC is required to obtain a standard Gilbert damping scalar parameter for collinear localized spins. The same mechanism, which is physically due to retarded response of conduction electronic spins to the motion of localized spins, generates wavevector-dependent damping on spin waves, whereas nonzero SOC makes nonlocal damping anisotropic. Our analytical formulas, with their nonlocality being more prominent in low spatial dimensions $d \le 2$, are fully corroborated by numerically exact $d=1$ quantum-classical simulations.

Authors:Deyi Zhuo, Zi-Jie Yan, Zi-Ting Sun, Ling-Jie Zhou, Yi-Fan Zhao, Ruoxi Zhang, Ruobing Mei, Hemian Yi, Ke Wang, Moses H. W. Chan, Chao-Xing Liu, K. T. Law, Cui-Zu Chang

Abstract: An axion insulator is a three-dimensional (3D) topological insulator (TI), in which the bulk maintains the time-reversal symmetry or inversion symmetry but the surface states are gapped by surface magnetization. The axion insulator state has been observed in molecular beam epitaxy (MBE)-grown magnetically doped TI sandwiches and exfoliated intrinsic magnetic TI MnBi2Te4 flakes with an even number layer. All these samples have a thickness of ~10 nm, near the 2D-to-3D boundary. The coupling between the top and bottom surface states in thin samples may hinder the observation of quantized topological magnetoelectric response. Here, we employ MBE to synthesize magnetic TI sandwich heterostructures and find that the axion insulator state persists in a 3D sample with a thickness of ~106 nm. Our transport results show that the axion insulator state starts to emerge when the thickness of the middle undoped TI layer is greater than ~3 nm. The 3D hundred-nanometer-thick axion insulator provides a promising platform for the exploration of the topological magnetoelectric effect and other emergent magnetic topological states, such as the high-order TI phase.

Authors:Hannah L. Stern, Carmem M. Gilardoni, Qiushi Gu, Simone Eizagirre Barker, Oliver Powell, Xiaoxi Deng, Louis Follet, Chi Li, Andrew Ramsay, Hark Hoe Tan, Igor Aharonovich, Mete Atatüre

Abstract: Quantum networks and sensing require solid-state spin-photon interfaces that combine single-photon generation and long-lived spin coherence with scalable device integration, ideally at ambient conditions. Despite rapid progress reported across several candidate systems, those possessing quantum coherent single spins at room temperature remain extremely rare. Here, we report quantum coherent control under ambient conditions of a single-photon emitting defect spin in a a two-dimensional material, hexagonal boron nitride. We identify that the carbon-related defect has a spin-triplet electronic ground-state manifold. We demonstrate that the spin coherence is governed predominantly by coupling to only a few proximal nuclei and is prolonged by decoupling protocols. Our results allow for a room-temperature spin qubit coupled to a multi-qubit quantum register or quantum sensor with nanoscale sample proximity.

Authors:Wojciech J. Jankowski, Mohammedreza Noormandipour, Adrien Bouhon, Robert-Jan Slager

Abstract: We study the effect of disorder in systems having a non-trivial Euler class. As these recently proposed multi-gap topological phases come about by braiding non-Abelian charged band nodes residing between different bands to induce stable pairs within isolated band subspaces, novel properties that include a finite critical phase under the debraiding to a metal rather than a transition point and a modified stability may be expected when the disorder preserves the underlying $C_2\cal{T}$ or $\cal{P}\cal{T}$ symmetry on average. Employing elaborate numerical computations, we verify the robustness of associated topology by evaluating the changes in the average densities of states and conductivities for different types of disorders. Upon performing a scaling analysis around the corresponding quantum critical points we retrieve a universality for the localization length exponent of $\nu = 1.4 \pm 0.1$ for Euler-protected phases, relating to 2D percolation models. We generically find that quenched disorder drives Euler semimetals into critical metallic phases. Finally, we show that magnetic disorder can also induce topological transitions to quantum anomalous Hall plaquettes with local Chern numbers determined by the initial value of the Euler invariant.

Authors:Johnathon Rackham, Brittni Pratt, Dalton Griner, Dallin Smith, Yanping Cai, Roger G. Harrison, Alex Reid, Jeffrey Kortright, Mark K. Transtrum, Karine Chesnel

Abstract: We report nanoscale inter-particle magnetic orderings in self-assemblies of Fe3O4 nanoparticles (NPs), and the emergence of inter-particle antiferromagnetic (AF) (super-antiferromagnetic) correlations near the coercive field at low temperature. The magnetic ordering is probed via x-ray resonant magnetic scattering (XRMS), with the x-ray energy tuned to the Fe-L3 edge and using circular polarized light. By exploiting dichroic effects, a magnetic scattering signal is isolated from the charge scattering signal. The magnetic signal informs about nanoscale spatial orderings at various stages throughout the magnetization process and at various temperatures throughout the superparamagnetic blocking transition, for two different sizes of NPs, 5 and 11 nm, with blocking temperatures TB of 28 K and 170 K, respectively. At 300 K, while the magnetometry data essentially shows superparamagnetism and absence of hysteresis for both particle sizes, the XRMS data reveals the presence of non-zero (up to 9/100) inter-particle AF couplings when the applied field is released to zero for the 11 nm NPs. These AF couplings are drastically amplified when the NPs are cooled down below TB and reach up to 12/100 for the 5 nm NPs and 48/100 for the 11 nm NPs, near the coercive point. The data suggests that the particle size affects the prevalence of the AF couplings: compared to ferromagnetic (F) couplings, the relative prevalence of AF couplings at the coercive point increases from a factor ~ 1.6 to 3.8 when the NP size increases from 5 to 11 nm.

Authors:Jia Wang, Hui Hu, Xia-Ji Liu

Abstract: We have developed a microscopic many-body theory of two-dimensional coherent spectroscopy (2DCS) for a model of polarons in one-dimensional (1D) materials. Our theory accounts for contributions from all three processes: excited-state emission (ESE), ground-state bleaching (GSB), and excited-state absorption (ESA). While the ESE and GSB contributions can be accurately described using a Chevy's ansatz with one particle-hole excitation, the ESA process requires information about the many-body eigenstates involving two impurities. To calculate these double polaron states, we have extended the Chevy's ansatz with one particle-hole excitation. The validity of this ansatz was verified by comparing our results with an exact calculation using Bethe's ansatz. Our numerical results reveal that in the weak interaction limit, the ESA contribution cancels out the total ESE and GSB contributions, resulting in less significant spectral features. However, for strong interactions, the features of the ESA contribution and the combined ESE and GSB contributions remain observable in the 2DCS spectra. These features provide valuable information about the interactions between polarons. Additionally, we have investigated the mixing time dynamics, which characterize the quantum coherences of the polaron resonances. Overall, our theory provides a comprehensive framework for understanding and interpreting the 2DCS spectra of polarons in 1D materials, shedding light on their interactions and coherent dynamics.

Authors:YunHeng Chen, Lachlan Oberg, Johannes Flick, Artur Lozovoi, Carlos A. Meriles, Marcus W. Doherty

Abstract: The performance of semiconductor devices can be enhanced by understanding and characterizing the bound states of deep defects. However, simulating these states using conventional $\textit{ab initio}$ techniques, which are limited by small cell volumes, proves challenging due to their extended nature spanning tens of nanometers. Here, we present a semi-$\textit{ab initio}$ method for modeling the bound states of deep defects in semiconductors, applied to the negatively charged nitrogen vacancy (NV$^-$) center in diamond. We employ density functional theory (DFT) calculations to construct accurate potentials for an effective mass model and allowing us to unveil the structure of the bound hole states. We also develop a model to calculate the non-radiative capture cross-sections, which agrees with experiment within one order of magnitude. Finally, we present the first attempt at constructing the photoionization spectrum of NV$^0\rightarrow$ NV$^-$ + bound hole, showing that the electronic transitions of the bound holes can be distinguished from phonon sidebands. This approach can be adapted to other deep defects in semiconductors.

Authors:Lina Johnsen Kamra, Simran Chourasia, G. A. Bobkov, V. M. Gordeeva, I. V. Bobkova, Akashdeep Kamra