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

Abstract: In this work, we review two different generalizations of a quantum geometric tensor (QGT) in two-band non-Hermitian systems and apply the formalism to the system of microcavity exciton polaritons. In particular, we extend the existing method of measuring the QGT that uses the pseudospins in photonic and polaritonic systems. We find that both forms of the generalized QGT can be expressed in terms of the exciton-polariton pseudospin components, which can be experimentally measured. We then present the generalized QGT components, i.e. the quantum metric and Berry curvature, for an exemplar non-Hermitian exciton-polariton system. Our simulations of the wave packet dynamics in this exciton-polariton system show that the right-right Berry curvature gives a more accurate description of the anomalous Hall drift.

Abstract: In easy-plane antiferromagnets, the nature of the elementary excitations of the spin system is captured by the precession of the magnon pseudospin around its equilibrium pseudofield, manifesting itself in the magnon Hanle effect. Here, we investigate the impact of growth-induced changes in the magnetic anisotropy on this effect in the antiferromagnetic insulator $\alpha$-Fe$_2$O$_3$ (hematite). To this end, we compare the structural, magnetic, and magnon-based spin transport properties of $\alpha$-Fe$_2$O$_3$ films with different thicknesses grown by pulsed laser deposition in molecular and atomic oxygen atmospheres. While in films grown with molecular oxygen a spin-reorientation transition (Morin transition) is absent down to $10\,$K, we observe a Morin transition for those grown by atomic-oxygen-assisted deposition, indicating a change in magnetic anisotropy. Interestingly, even for a $19\,$nm thin $\alpha$-Fe$_2$O$_3$ film grown with atomic oxygen we still detect a Morin transition at $125\,$K. We characterize the magnon Hanle effect in these $\alpha$-Fe$_2$O$_3$ films via all-electrical magnon transport measurements. The films grown with atomic oxygen show a markedly different magnon spin signal from those grown in molecular oxygen atmospheres. Most importantly, the maximum magnon Hanle signal is significantly enhanced and the Hanle peak is shifted to lower magnetic field values for films grown with atomic oxygen. These observations suggest a change of magnetic anisotropy for $\alpha$-Fe$_2$O$_3$ films fabricated by atomic-oxygen-assisted deposition resulting in an increased oxygen content in these films. Our findings provide new insights into the possibility to fine-tune the magnetic anisotropy in $\alpha$-Fe$_2$O$_3$ and thereby to engineer the magnon Hanle effect.

Abstract: In superlattices of twisted semiconductor monolayers, tunable moir\'e potentials emerge, trapping excitons into periodic arrays. In particular, spatially separated interlayer excitons are subject to a deep potential landscape and they exhibit a permanent dipole providing a unique opportunity to study interacting bosonic lattices. Recent experiments have demonstrated density-dependent transport properties of moir\'e excitons, which could play a key role for technological applications. However, the intriguing interplay between exciton-exciton interactions and moir\'e trapping has not been well understood yet. In this work, we develop a microscopic theory of interacting excitons in external potentials allowing us to tackle this highly challenging problem. We find that interactions between moir\'e excitons lead to a delocalization at intermediate densities and we show how this transition can be tuned via twist angle and temperature. The delocalization is accompanied by a modification of optical moir\'e resonances, which gradually merge into a single free exciton peak. The predicted density-tunability of the supercell hopping can be utilized to control the energy transport in moir\'e materials.

Abstract: Topological insulators and superconductors have attracted considerable attention, and many different theoretical tools have been used to gain insight into their properties. Here we investigate how perturbations can spread through exemplary one-dimensional topological insulators and superconductors using out-of-time ordered correlators. Out-of-time ordered correlators are often used to consider how information becomes scrambled during quantum dynamics. The wavefront of the out-of-time ordered correlator can be ballistic regardless of the underlying system dynamics, and here we confirm that for topological free fermion systems the wavefront spreads linearly at a characteristic butterfly velocity. We pay special attention to the topologically protected edge states, finding that "information" can become trapped in the edge states and essentially decoupled from the bulk, surviving for relatively long times. We consider different models with multiple possible edge states coexisting on a single edge.

Abstract: A sine-Gordon breather enhances the heat transfer in a thermally biased long Josephson junction. This solitonic channel allows for the tailoring of the local temperature throughout the system. Furthermore, the phenomenon implies a clear thermal fingerprint for the breather, and thus a 'non-destructive' breather detection strategy is proposed here. Distinct breathing frequencies result in morphologically different local temperature peaks, which can be identified in an experiment.

Abstract: Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based micro-focus angle resolved photoemission ($\mu$-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment. Our measurements unveil ladders of anisotropic, quantized subbands at energies that deviate from the scaling observed in conventional semiconductor quantum wells. We quantify the anisotropy of the effective masses and determine universal tight-binding parameters which provide an accurate description of the electronic structure for all thicknesses.

Abstract: Nanoscale fluid transport is typically pictured in terms of atomic-scale dynamics, as is natural in the real-space framework of molecular simulations. An alternative Fourier-space picture, that involves the collective charge fluctuation modes of both the liquid and the confining wall, has recently been successful at predicting new nanofluidic phenomena such as quantum friction and near-field heat transfer, that rely on the coupling of those fluctuations. Here, we study the charge fluctuation modes of a two-dimensional (planar) nanofluidic channel. Introducing confined response functions that generalize the notion of surface response function, we show that the channel walls exhibit coupled plasmon modes as soon as the confinement is comparable to the plasmon wavelength. Conversely, the water fluctuations remain remarkably bulk-like, with significant confinement effects arising only when the wall spacing is reduced to 7 A. We apply the confined response formalism to predict the dependence of the solid-water quantum friction and thermal boundary conductance on channel width for model channel wall materials. Our results provide a general framework for Coulomb interactions of fluctuating matter in nanoscale confinement.

Abstract: Moir\'e superlattices provide a highly tunable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators1-17 to moir\'e excitons7-10,18. Scanning tunneling microscopy has played a key role in probing microscopic behaviors of the moir\'e correlated ground states at the atomic scale1,11-15,19. Atomic-resolution imaging of quantum excited state in moir\'e heterostructures, however, has been an outstanding experimental challenge. Here we develop a novel photocurrent tunneling microscopy by combining laser excitation and scanning tunneling spectroscopy (laser-STM) to directly visualize the electron and hole distribution within the photoexcited moir\'e exciton in a twisted bilayer WS2 (t-WS2). We observe that the tunneling photocurrent alternates between positive and negative polarities at different locations within a single moir\'e unit cell. This alternating photocurrent originates from the exotic in-plane charge-transfer (ICT) moir\'e exciton in the t-WS2 that emerges from the competition between the electron-hole Coulomb interaction and the moir\'e potential landscape. Our photocurrent maps are in excellent agreement with our GW-BSE calculations for excitonic states in t-WS2. The photocurrent tunneling microscopy creates new opportunities for exploring photoexcited non-equilibrium moir\'e phenomena at the atomic scale.

Abstract: Chklovskii and Halperin theoretically predicted that a QPC between filling fractions $\nu=1$ and $1/3$ could act as a DC step-up transformer with an amplification factor of 3/2 which was observed recently in experiments. We revisit this problem in the context of AC transport in a bilayer quantum Hall (QH) setting. We show that the AC amplification is bounded by the DC limit of 3/2 in the presence of intra-layer electron-electron interactions alone, however, the possibility of having interlayer interactions open up a new avenue for amplification beyond the DC limit. This amplification can be understood in terms of displacement current due to the presence of ambient gate electrodes. We further show that AC conductance depicts resonances and anti-resonances resulting purely from interlayer interactions at certain magic frequencies.

Abstract: The non-Hermitian skin effect (NHSE) has been intensely investigated over the past few years and has unveiled new topological phases, which have no counterparts in Hermitian systems. Here we consider the hybridization between the NHSE in an exciton-polariton waveguide and a localized defect mode. By tuning the non-Hermiticity, we find that the resulting ground-state of the system is both spatially extended and energetically separated from other modes in the system. When polariton lasing occurs in the system, we find an enhanced spatial coherence compared to regular waveguides, which is robust in the presence of disorder.