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Electronic Transport In Mesoscopic Systems

A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

We develop the topological band theory for systems described by non-Hermitian Hamiltonians, whose energy spectra are generally complex. After generalizing the notion of gapped band structures to the non-Hermitian case, we classify ``gapped'' bands in one and two dimensions by explicitly finding their topological invariants. We find nontrivial generalizations of the Chern number in two dimensions, and a new classification in one dimension, whose topology is determined by the energy dispersion rather than the energy eigenstates. We then study the bulk-edge correspondence and the topological phase transition in two dimensions. Different from the Hermitian case, the transition generically involves an extended intermediate phase with complex-energy band degeneracies at isolated ``exceptional points'' in momentum space. We also systematically classify all types of band degeneracies.

Topological Band Theory for Non-Hermitian Hamiltonians

Theorists have discovered topological insulators that are insulating in their interior and on their surfaces but have conducting channels at corners or along edges.

(d - 2)-dimensional edge states of rotation symmetry protected topological states

Axions are weakly interacting particles of low mass, and were postulated more than 30 years ago in the framework of the Standard Model of particle physics. Their existence could explain the missing dark matter of the Universe. However, despite intensive searches, axions have yet to be observed. Here we show that magnetic fluctuations of topological insulators couple to the electromagnetic fields exactly like the axions, and propose several experiments to detect this dynamical axion field. In particular, we show that the axion coupling enables a nonlinear modulation of the electromagnetic field, leading to attenuated total reflection. We propose a new optical-modulator device based on this principle. Axions are hypothetical particles that might play an important part in particle physics, astrophysics and cosmology. So far they have eluded observation, but theoretical work now predicts that axion physics might be explored in condensed-matter systems known as topological insulators.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-001051.pdf

Dynamical Axion Field in Topological Magnetic Insulators

In this paper, we investigate the realization of topological Anderson insulators in electric circuits. A disordered Haldane model is constructed through electric circuit networks composed of capacitors and inductors, where the disorder is introduced through the random induction of the grounding inductors. Based on the noncommutative geometry method and transport calculations, we confirm that such kind of disorder can drive a phase transition from a normal insulator to a topological Anderson insulator. Besides, such a disorder also possesses unique characteristics which are absent for the usual Anderson disorder. Therefore distinct features are exhibited by the topological Anderson transition in electric circuits. Finally, the topological Anderson insulator in circuits holds additional advantages for microelectronic technology that can be easily detected by measuring the quantized transmission coefficients and the edge state wave functions.

Topological Anderson insulator in electric circuits

We study the characterization and realization of higher-order topological Anderson insulator (HOTAI) in non-Hermitian systems, where the non-Hermitian mechanism ensures extra symmetries as well as gain and loss disorder. We illuminate that the quadrupole moment Qxy can be used as the real space topological invariant of non-Hermitian higher-order topological insulator (HOTI). Based on the biorthogonal bases and non-Hermitian symmetries, we prove that Qxy can be quantized to 0 or 0.5. Considering the disorder effect, we find the disorder-induced phase transition from normal insulator to non-Hermitian HOTAI. Furthermore, we elucidate that the real space topological invariant Qxy is also applicable for systems with the non-Hermitian skin effect. Our work enlightens the study of the combination of disorder and non-Hermitian HOTI.

Topological Anderson insulator in two-dimensional non-Hermitian systems

https://iopscience.iop.org/article/10.1088/1674-1056/ab4582/pdf

Disorder induced phase transition in magnetic higher-order topological insulator: A machine learning study

We study the characterization and realization of higher-order topological Anderson insulator (HOTAI) in non-Hermitian systems, where the non-Hermitian mechanism ensures extra symmetries as well as gain and loss disorder. We illuminate that the quadrupole moment ${Q}_{xy}$ can be used as the real space topological invariant of non-Hermitian higher-order topological insulator (HOTI). Based on the biorthogonal bases and non-Hermitian symmetries, we prove that ${Q}_{xy}$ can be quantized to 0 or 0.5. Considering the disorder effect, we find the disorder-induced phase transition from normal insulator to non-Hermitian HOTAI. Furthermore, to clarify the universality of real-space topological invariant in non-Hermitian systems, we elucidate that ${Q}_{xy}$ is also applicable for samples with the non-Hermitian skin effect. Our work enlightens the study of the combination of disorder and non-Hermitian HOTI.

Real-space topological invariant and higher-order topological Anderson insulator in two-dimensional non-Hermitian systems

In this paper, we study an Anderson-localization-induced quantized transport in disordered Chern insulators (CIs). By investigating the disordered CIs with a step potential, we find that the chiral interface states emerge along the interfaces of the step potential, and the energy range for such quantized transport can be manipulated through the potential strength. Furthermore, numerical simulations on the case with a multistep potential demonstrate that such a chiral state can be spatially shifted by varying the Fermi energy, and the energy window for quantized transport is greatly enlarged. Experimentally, such a chiral interface states can be realized by imposing transverse electric field, in which the energy window for quantized transport is much broader than the intrinsic band gap of the corresponding CI. These phenomena are quite universal for disordered CIs due to the direct phase transition between the CI and the normal insulator.

Zhi-Qiang Zhang

Papers

Topological Anderson insulator in electric circuits

Topological Anderson insulator in two-dimensional non-Hermitian systems

Disorder induced phase transition in magnetic higher-order topological insulator: A machine learning study

Real-space topological invariant and higher-order topological Anderson insulator in two-dimensional non-Hermitian systems

Chiral interface states and related quantized transport in disordered Chern insulators