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

Abstract: Theoretical studies on spin-dependent transport through helical molecules with straight spiral geometry have received intense research interest in the past decade, however, the physics in circular helical molecules has still less been explored. In this work, we theoretically construct a circular single helix (CSH) possessing the chirality-induced spin-orbit coupling and contacting with two non-magnetic electrodes. Our theoretical calculations demonstrate that the spin-related transport in CSH exhibits the so-called chiral-induced spin selectivity (CISS) effect and more importantly, the CISS-reduced spin-dependent destructive quantum interference (DQI) also occurs in the CSH, without any external magnetic field or magnetic electrodes. Moreover, the changing of CSH length or the electrode positions exhibits specific patterns in the spin-polarized conductance. Particularly, the dephasing magnitude can adjust effectively these two spin-dependent effects to realize their coexistence. Additionally, the phase transition between the CISS-dependent constructive quantum interference (CQI) and DQI is also observed in the CSH. Our theoretical work puts forwards a new material plateau to explore the CISS and to exhibit the novel CISS-dependent CQI effect.

Abstract: We propose a continuum model for the theoretical study of hybridized moir\'e excitons in transition metal dichalcogenides heterobilayers, and we use a variational method to solve the exciton wavefunction and calculate the optical absorption spectrum. The exciton continuum model is built by the charge continuum model for electrons and holes in moir\'e superlattices, thereby preserving the moir\'e periodicity and lattice symmetry from the charge continuum model. The momentum-space shift of interlayer electron-hole distribution is included, and thus the indirect nature of interlayer excitons is described. Spin and valley degrees of freedom and related interactions are omitted in this model, except for the spin-orbit energy splitting of A and B excitons. In long moir\'e-wavelength and zero charge-transfer-coupling limits, the exciton model and the optical absorption formula can be reduced to the counterparts of an isolated exciton. This continuum model is applied to the simulation of optical absorption by hybridized moir\'e excitons in $\text{WSe}_2$/$\text{WS}_2$ and $\text{MoSe}_2$/$\text{WS}_2$ heterobilayers. Twist-angle and electric-field dependences of absorption spectra are studied. Calculated spectra are compared with experimental observations in the literature, and correspondences of signatures are found. The deficiency and the potential of the present model are discussed.

Abstract: The spin Hall effect (SHE) can generate a pure spin current by an electric current, which is promisingly used to electrically control magnetization. To reduce power consumption of this control, a giant spin Hall angle (SHA) in the SHE is desired in low-resistivity systems for practical applications. Here, critical spin fluctuation near the antiferromagnetic (AFM) phase-transition is proved as an effective mechanism to create an additional part of SHE, named as fluctuation spin Hall effect (FSHE). This FSHE enhances the SHA due to the AFM spin fluctuation between conduction electrons and local spins. We detect the FSHE with the inverse and direct spin Hall effect (ISHE and DSHE) set-up and their temperature (T) dependences in the Cr/MgO/Fe magnetic tunnel junctions (MTJs). The SHA is significantly enhanced when temperature is approached to the N\'eel temperature (T_N) and has a peak value of -0.34 at 200 K near T_N. This value is higher than the room-temperature value by 240% and comparable to that of heavy metals Ta and W. Furthermore, the spin Hall resistivity of Cr well fits the modeled T-dependence when T approaches T_N from low temperatures, implying the AFM spin fluctuation nature of strong SHA enhancement. Thus, this study demonstrates the critical spin fluctuation as a prospective way of increasing SHA and enriches the AFM material candidates for spin-orbitronic devices.

Abstract: We study the exciton states in the symmetrically biased dice model, the electronic structures of which have an isolated flat band between two dispersive bands. At 1/3 or 2/3 filling, the model describes a two-dimensional semiconductor with the band edge at two degenerate valleys. Because of qualitative changes in the eigenvectors resulting from the bias term, the interband transition between the flat band and a dispersive band is valley contrasting under circularly polarized light. In terms of an effective-mass model and a realistic electron-hole interaction, we numerically calculate the spectrum and wave functions of the intravalley excitons, which are treated as Wannier-Mott excitons. We also discuss the fine structures of the exciton spectrum induced by the intravalley and intervalley exchange interactions. The symmetrically biased dice model thereby proves to be a new platform for valley-contrasting optoelectronics.

Abstract: Plasma echo is a dramatic manifestation of plasma damping process reversibility. In this paper we calculate temporal and spatial plasma echoes in graphene in the acoustic plasmon regime when echoes dominate over plasmon emission. We show an extremely strong spatial echo response and discuss how electron collisions reduce the echo. We also discuss differences between various electron dispersions, and differences between semiclassical and quantum model of echoes.

Abstract: Fast measurements of quantum devices is important in areas such as quantum sensing, quantum computing and nanodevice quality analysis. Here, we develop a superconductor-semiconductor multi-module microwave assembly to demonstrate charge state readout at the state-of-the-art. The assembly consist of a superconducting readout resonator interfaced to a silicon-on-insulator (SOI) chiplet containing quantum dots (QDs) in a high-$\kappa$ nanowire transistor. The superconducting chiplet contains resonant and coupling elements as well as $LC$ filters that, when interfaced with the silicon chip, result in a resonant frequency $f=2.12$~GHz, a loaded quality factor $Q=680$, and a resonator impedance $Z=470$~$\Omega$. Combined with the large gate lever arms of SOI technology, we achieve a minimum integration time for single and double QD transitions of 2.77~ns and 13.5~ns, respectively. We utilize the assembly to measure charge noise over 9 decades of frequency up to 500~kHz and find a 1/$f$ dependence across the whole frequency spectrum as well as a charge noise level of 4~$\mu$eV/$\sqrt{\text{Hz}}$ at 1~Hz. The modular microwave circuitry presented here can be directly utilized in conjunction with other quantum device to improve the readout performance as well as enable large bandwidth noise spectroscopy, all without the complexity of superconductor-semiconductor monolithic fabrication.

Abstract: Several experimental reports have described electrical power output by electronic devices that channel spin-polarized currents across paramagnetic centers. Phononic radiation have been proposed as the source of the engine's energy, though other hypotheses, such as quantum vacuum fluctuations, should also be examined. This paper is the first of a series which will address these hypotheses. Herein, we investigate the more basic hypothesis that quantum vacuum fluctuations power a quantum engine that converts entanglement energy into useful electrical work. The system under review is composed of two atom-level quantum dots that are tunnel-coupled and exhibit a magnetic exchange interaction. This working substance is connected in series with two ferromagnetic electrodes. The engine cycle comprises two strokes. The thermalizing stroke puts the system into equilibrium with the electrode baths, leading to a release of electrical energy into the leads and to an increase in the system entropy due to entanglement. Then the measurement stroke breaks the entanglement between the two quantum dots, thereby reducing its entropy while energizing it on average. Using a perturbative master equation approach, we analytically demonstrate the efficiency of the engine, and we study the cycle numerically to gain insight into the relevant parameters to maximize power. Although the possibility of harvesting energy from the quantum vacuum fluctuations and the interactions with the baths is proven on paper and confirmed by numerical experiments, the efficiency remains low and is unstable. Our results indicate that quantum vacuum fluctuations alone are unlikely to be the energy source in the the quantum spintronic engine experiments that have been reported thus far.

Abstract: Ribbons of two-dimensional lattices have properties depending sensitively on the morphology of the two edges. For regular ribbons with two parallel straight edges, the atomic chains terminating the two edges may have more than one choices for a general edge orientation. We enumerate the possible choices for zigzag dice lattice ribbons, which are regular ribbons of the dice lattice with edges parallel to a zigzag direction, and explore the relation between the edge morphologies and their electronic spectra. A formula is introduced to count the number of distinct edge termination morphologies for the regular ribbons, which gives 18 distinct edge termination morphologies for the zigzag dice lattice ribbons. For the pure dice model, because the equivalence of the two rim sublattices, the numerical spectra of the zigzag ribbons show qualitative degeneracies among the different edge termination morphologies. For the symmetrically biased dice model, we see a one-to-one correspondence between the 18 edge termination morphologies and their electronic spectra, when both the zero-energy flat bands and the dispersive or nonzero-energy in-gap states are considered. We analytically study several interesting features in the electronic spectra, including the number and wave functions of the zero-energy flat bands, and the analytical spectrum of novel in-gap states. The in-gap states of the zigzag dice lattice ribbons both exhibit interesting similarities and show salient differences when compared to the spectra of the zigzag ribbons of the honeycomb lattice.

Abstract: We show that four narrow zigzag dice lattice ribbons, which have the minimal widths among their separate categories, constitute a unique collection of systems to study physics related to one-dimensional Dirac cones and flat bands. In zero magnetic field, all three combinations, including only Dirac cones, only flat bands, coexisting Dirac cones and flat bands, are realized in the low-energy band structures of one or two of the four ribbons. In particular, we identify flat bands and Dirac cones corresponding to the edge states of wide ribbons. In a perpendicular magnetic field that gives half a flux quantum per elementary rhombus, two of the four minimal ribbons have fully pinched spectrum, and dynamical evolutions from initially localized wave packets always lead to compact Aharonov-Bohm (AB) cages. The experimental realizations of these narrow zigzag dice lattice ribbons, and the opportunities of exploring novel single-body and many-body physics therein are discussed.

Abstract: The infrared response of a system of two vibrational modes in a cavity is calculated by an effective non-Hermitian Hamiltonian derived by employing the nonequilibrium Green's functions (NEGF) formalism. Degeneracies of the Hamiltonian (exceptional points, EP) widely employed in theoretical analysis of optical cavity spectroscopies are used in an approximate treatment and compared with the full NEGF. Qualitative limitations of the EP treatment are explained by examining the approximations employed in the calculation.

Abstract: A system having macroscopic patches in different topological phases have no well-defined global topological invariant. To treat such a case, the quantities labeling different areas of the sample according to their topological state are used, dubbed local topological markers. Here we study their dynamics. We concentrate on two quantities, namely local Chern marker and on-site charge induced by an applied magnetic field. We demonstrate that the time-dependent local Chern marker is much more non-local object than equilibrium one. Surprisingly, in large samples driven out of equilibrium, it leads to a simple description of the local Chern marker's dynamics by a local continuity equation. Also, we argue that the connection between the local Chern marker and magnetic-field induced charge known in static holds out of equilibrium in some experimentally relevant systems as well. This gives a clear physical description of the marker's evolution and provides a simple recipe for experimental estimation of the topological marker's value.