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

Fri, 14 Apr 2023

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1.Anomalous non-Hermitian skin effect: the topological inequivalence of skin modes versus point gap

Authors:Gang-Feng Guo, Xi-Xi Bao, Han-Jie Zhu, Xiao-Ming Zhao, Lin Zhuang, Lei Tan, Wu-Ming Liu

Abstract: Non-Hermitian skin effect, the localization of an extensive number of eigenstates at the ends of the system, has greatly expanded the frontier of physical laws. It has long been believed that the present of skin modes is equivalent to the topologically nontrivial point gap of complex eigenvalues under periodic boundary conditions, and vice versa. However, we find that this concomitance can be broken, i.e., the skin modes can be present or absent whereas the point gap is topologically trivial or nontrivial, respectively, named anomalous non-Hermitian skin effect. This anomalous phenomenon arises when the unidirectional hopping amplitudes leading to the decoupling-like behaviors among subsystems are emergence. The emergence of the anomalous non-Hermitian skin effect is accompanied by the mutations of the open boundary energy spectrum, whose structure exhibits the multifold exceptional point and can not be recovered by continuum bands. Moreover, an experimental setup using circuits is proposed to simulate this novel quantum effect. Our results reveal the topologically inequivalent between skin modes and point gap. This new effect not only can give a deeper understanding of non-Bloch theory and the critical phenomenon in non-Hermitian systems, but may also inspire new applications such as in the sensors field.

2.Bulk-edge correspondence of Stiefel-Whitney and Euler insulators through the entanglement spectrum and cutting procedure

Authors:Ryo Takahashi, Tomoki Ozawa

Abstract: We propose a novel type of bulk-edge correspondence for two-dimensional Stiefel-Whitney insulators and Euler insulators, which are topological insulators protected by the $PT$ symmetry. We find that, although the energy spectrum under the open boundary condition is generally gapped, the entanglement spectrum is gapless when the Stiefel-Whitney or Euler class is nonzero. The robustness of the gapless spectrum for Stiefel-Whitney insulator can be understood through an emergent anti-unitary particle-hole symmetry. For the Euler insulators, we propose a conjecture, which is supported by our numerical calculation, that the Euler class is equal to the number of crossing in the entanglement spectrum, taking into account the degree of the crossings. We also discuss that these crossings of the entanglement spectrum are related to the gap closing points in the cutting procedure, which is the energy spectrum as the magnitude of the boundary hopping is varied.

3.Hall effect of ferro/antiferromagnetic wallpaper fermions

Authors:Koki Mizuno, Ai Yamakage

Abstract: Nonsymmorphic crystals can host characteristic double surface Dirac cones with fourfold degeneracy on the Dirac points, called wallpaper fermion, protected by wallpaper group symmetry. We clarify the charge and spin Hall effect of wallpaper fermions in the presence of the (anti)ferromagnetism.Based on a four-sublattice model, we construct the effective Hamiltonian of wallpaper fermions coupled with the ferromagnetic or antiferromagnetic moment.Both ferromagnetic and antiferromagnetic moments induce an energy gap for the wallpaper fermions, leading to quantized (spin) Hall conductivity. The ferromagnetic wallpaper fermion induces the Hall conductivity quantized into $e^2/h$, which is twice that for a single Dirac fermion on the surface of topological insulators. On the other hand, the spin Hall conductivity decays and reaches to be a finite value as the antiferromagnetic coupling increases. We also show that the results above are valid for a general model of wallpaper fermions from symmetry consideration.

4.Exciton dynamics in CdTe/CdZnTe quantum well

Authors:A. V. Mikhailov St. Petersburg State University, A. S. Kurdyubov St. Petersburg State University, E. S. Khramtsov St. Petersburg State University, I. V. Ignatiev St. Petersburg State University, B. F. Gribakin Univ. Montpellier, S. Cronenberger Univ. Montpellier, D. Scalbert Univ. Montpellier, M. R. Vladimirova Univ. Montpellier, R. André Institut Néel

Abstract: Exciton energy structure and population dynamics in a wide CdTe/CdZnTe quantum well are studied by spectrally-resolved pump-probe spectroscopy. Multiple excitonic resonances in reflectance spectra are observed and identified by solving numerically three-dimensional Schr\"odinger equation. The pump-probe reflectivity signal is shown to be dominated by the photoinduced nonradiative broadening of the excitonic resonances, while pump-induced exciton energy shift and reduction of the oscillator strength appear to be negligible. This broadening is induced by the reservoir of dark excitons with large in-plane wave vector, which are coupled to the the bright excitons states. The dynamics of the pump-induced nonradiative broadening observed experimentally is characterised by three components: signal build up on the scale of tens of picoseconds (i) and bi-exponential decay on the scale of one nanosecond (ii) and ten nanosecons (iii). Possible mechanisms of the reservoir population and depletion responsible for this behaviour are discussed.

5.Realizing Majorana zero modes in magnetic field-free InAs-Al nanowires with fewer growth constraints

Authors:Benjamin D Woods, Mark Friesen

Abstract: Common proposals for realizing topological superconductivity and Majorana zero modes in semiconductor-superconductor hybrids require large magnetic fields, which paradoxically suppress the superconducting gap of the parent superconductor. Although two-channel schemes have been proposed as a way to eliminate magnetic fields, geometric constraints make their implementation challenging, since the channels should be immersed in nearly antiparallel electric fields. Here, we propose an experimentally favorable scheme for realizing field-free topological superconductivity, in two-channel InAs-Al nanowires, that overcomes such growth constraints. Crucially, we show that antiparallel fields are not required, if the channels are energetically detuned. We compute topological phase diagrams for realistically modeled nanowires, finding a broad range of parameters that could potentially harbor Majorana zero modes. This work, therefore, solves a major technical challenge and opens the door to near-term experiments.