Topological nature of in‐gap bound states in disordered large‐gap monolayer transition metal dichalcogenides

We propose a physical model based on disordered (a hole punched inside a material) monolayer transition metal dichalcogenides (TMDs) to demonstrate a large‐gap quantum valley Hall insulator. We find an emergence of bound states lying inside the bulk gap of the TMDs. They are strongly affected by spin–valley coupling, rest‐ and kinetic‐mass terms and the hole size. In addition, in the whole range of the hole size, at least two in‐gap bound states with opposite angular momentum, circulating around the edge of the hole, exist.Their topological insulator (TI) feature is analyzed by the Chern number, characterized by spacial distribution of their probabilities and confirmed by energy dispersion curves (energy vs. angular momentum). It not only sheds light on overcoming low‐temperature operating limitation of existing narrow‐gap TIs, but also opens an opportunity to realize valley‐ and spin‐qubits. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Magnetically Controlled Electronic Transport Properties of a Ferromagnetic Junction on the Surface of a Topological Insulator

We have investigated the transport properties of the Dirac fermions through a ferromagnetic barrier junction on the surface of a strong topological insulator.The current-voltage characteristic curve and the tunneling conductance are calculated theoretically.Two interesting transport features are predicted:observable negative differential conductances and linear conductances tunable from unit to nearly zero.These features can be magnetically manipulated simply by changing the spacial orientation of the magnetization.Our results may contribute to the development of high-speed switching and functional applications or electrically controlled magnetization switching.     

Role of the Radiation‐Reaction Electric Field in the Optical Response of Two‐Dimensional Crystals

A classical theory of a radiating two‐dimensional crystal is proposed and an expression for the radiation‐reaction electric field is derived. This field plays an essential role in connecting the microscopic electromagnetic fields acting on each dipole of the crystal to the macroscopic one, via the boundary conditions for the system. The expression of the radiative‐reaction electric field coincides with the macroscopic electric field radiating from the crystal and, summed to the incident electric field, generates the total macroscopic electric field.     

Scattering of Electrons between Edge and Two-Dimensional States of a Two-Dimensional Topological Insulator and the Conductivity of the Topological Insulator Strip in a Metallic State

JETP Letters - The lifetime of electrons on edge states of a two-dimensional topological insulator against the background of an allowed two-dimensional band has been determined. It has been shown...     

Spin Resonance in a Quantum Dot at the Edge of a Topological Insulator with the Inclusion of Continuum States

Journal of Experimental and Theoretical Physics - The dynamics of electronic states under the action of a periodic electric field applied to a quantum dot created by magnetic barriers at the...     

Non‐magnetic defects in the bulk of two‐dimensional topological insulators

We found that non‐magnetic defects in two‐dimensional topological insulators induce bound states of two kinds for each spin orientation: electron‐ and hole‐like states. Depending on the sign of the defect potential these states can be also of two kinds with different distribution of the electron density. The density has a maximum or minimum in the center. A surprising effect caused by the topological order is a singular dependence of the bound‐state energy on the defect potential.

     

Topological Phase Transition and Eigenstates Localization in a Generalized Non‐Hermitian Su–Schrieffer–Heeger Model

The topological properties of a generalized non‐Hermitian Su–Schrieffer–Heeger model are investigated and it is demonstrated that the non‐Hermitian phase transition and the non‐Hermitian skin effect can be induced by intra‐cell asymmetric coupling under open boundary conditions. Through investigating and calculating the non‐Hermitian winding number with generalized Brillouin zone theory, it is found that the present non‐Hermitian system has an exact bulk‐boundary correspondence relationship. Meanwhile, the non‐Hermitian winding number is used to characterize the non‐Hermitian phase transition and determine the phase transition boundary, and it is found that the non‐Hermitian phase transition is not completely induced by the asymmetric coupling strength. By means of the mean inverse participation ratio, the factors that affect the eigenstates localization are shown and it is revealed that large system size or large asymmetric coupling strength can leave the system in the localized state. Additionally, it is found that for the asymmetric coupling strength and the system size, the eigenstates localization is much more sensitive to the asymmetric coupling strength.     

Raman scattering studies of spin‐waves in hexagonal BaFe12O19

We present the results of polarized Raman spectroscopy of hexagonal BaFe₁₂O₁₉ single crystal. The spectra, recorded from 200 to 800 cm^–1 and 1100 to 1700 cm^–1 in the 20–250 K temperature range, are analyzed on the basis of both crystal vibrations and spin‐waves. In the low wavenumber range, the Γ‐point phonons are observed. In the high wavenumber range, phonon mixings are observed; more interestingly, four modes of spin‐waves are identified in hexagonal BaFe₁₂O₁₉. Both have not been studied previously. Our analyses of the spin‐waves provide an optical method for quantitatively estimating the spin exchange interactions in hexagonal BaFe₁₂O₁₉. The four strong exchange integrals are found to have the values of J_ce = 1.31 meV, J_ae = 1.36 meV, J_cd = 1.46 meV, and J_bd = 1.71 meV. Our results also indicate that at ~200 and ~80 K, there would be additional spin‐ordering transitions in hexagonal BaFe₁₂O₁₉. Copyright © 2012 John Wiley & Sons, Ltd.