First-Principles Calculation on Geometric,Electronic and Optical Properties of Fully Fluorinated Stanene:a Large-Gap Quantum Spin Hall Insulator

The searches for large-gap quantum spin Hall insulators are important for both practical and fundamental inter- ests. In this work, we present a theoretical observation of the two-dimensional fully fluorinated stanene （SnF） by means of density functional theory. Remarkably, a significant spin-orbit coupling is observed for the SnF monolayer in the valence band at the F point, with a considerable indirect band gap of 278 meV. The direct gap of the SnF monolayer is at the F point, which is slightly larger by as much as 381 meV. In addition, the elastic modulus of the SnF monolayer is about 20J/m^2, which is comparable with the in-plane stiffness of black phos- phorus monolayer along the x-direction （~28.94 J/m^2）. Finally, the optical properties of stanene, SnF monolayer and stanene/SnF bilayer are calculated, in which the stanene/SnF bilayer is supposed to be an attractive sunlight absorber.

Quantum spin Hall insulators in chemically functionalized As(110)and Sb(110)films

We propose a new type of quantum spin Hall （QSH） insulator in chemically functionalized As （110） and Sb （110） film. According to first-principles calculations, we find that metallic As （110） and Sb （110） films become QSH insulators after being chemically functionalized by hydrogen （H） or halogen （C1 and Br） atoms. The energy gaps of the functionalized films range from 0.121 eV to 0.304 eV, which are sufficiently large for practical applications at room temperature. The energy gaps originate from the spin-orbit coupling （SOC）. The energy gap increases linearly with the increase of the SOC strength λ/λ0. The Z2 invariant and the penetration depth of the edge states are also calculated and studied for the functionalized films.

An Anderson Impurity Interacting with the Helical Edge States in a Quantum Spin Hall Insulator

Using the natural orbitals renormalization group（NORG）method,we investigate the screening of the local spin of an Anderson impurity interacting with the helical edge states in a quantum spin Hall insulator.It is found that there is a local spin formed at the impurity site and the local spin is completel.y screened by electrons in the quantum spin Hall insulator.Meanwhile,the local spin is screened dominantly by a single active natural orbital.We then show that the Kondo screening mechanism becomes transparent and simple in the framework of the natural orbitals formalism.We project the active natural orbital respectively into real space and momentum space to characterize its structure.We conilrm the spin-momentum locking property of the edge states based on the occupancy of a Bloch state on the edge to which the impurity couples.Furthermore,we study the dynamical property of the active natural orbital represented by the local density of states,from which we observe the Kondo resonance peak.

Gallium bismuth halide GaBi-X2 （X = I, Br, CI） monolayers with distorted hexagonal framework： Novel room- temperature quantum spin Hall insulators

Quantum spin Hall （QSH） insulators with a large topologically nontrivial bulk gap are crucial for future applications of the QSH effect. Among these, group III-V monolayers and their halides, which have a chair structure （regular hexagonal framework）, have been widely studied. Using first-principles calculations, we formulate a new structure model for the functionalized group III-V monolayers, which consist of rectangular GaBi-X2 （X = I, Br, C1） monolayers with a distorted hexagonal framework （DHF）. These structures have a far lower energy than the GaBi-X2 monolayers with a chair structure. Remarkably, the DHF GaBi-X2 monolayers are all QSH insulators, which exhibit sizeable nontrivial band gaps ranging from 0.17 to 0.39 eV. The band gaps can be widely tuned by applying different spin-orbit coupling strengths, resulting in a distorted Dirac cone.

Progress on 2D topological insulators and potential applications in electronic devices

Two-dimensional topological insulators(2DTIs)have attracted increasing attention during the past few years.New 2DTIs with increasing larger spin-orbit coupling(SOC)gaps have been predicted by theoretical calculations and some of them have been synthesized experimentally.In this review,the 2DTIs,ranging from single element graphene-like materials to bi-elemental transition metal chalcogenides(TMDs)and to multi-elemental materials,with different thicknesses,structures,and phases,have been summarized and discussed.The topological properties(especially the quantum spin Hall effect and Dirac fermion feature)and potential applications have been summarized.This review also points out the challenge and opportunities for future 2DTI study,especially on the device applications based on the topological properties.

MoTe2 is a good match for Gel by preserving quantum spin Hall phase

Quantum spin Hall （QSH） insulator is a new class of materials that is quickly becoming mainstream in condensed-matter physics. The main obstacle for the development of QSH insulators is that their strong interactions with substrates make them difficult to study experimentally. In this study, using density functional theory, we discovered that MoTe2 is a good match for a GeI monolayer. The thermal stability of a van der Waals GeI/MoTe2 heterosheet was examined via molecular-dynamics simulations. Simulated scanning tunneling microscopy revealed that the GeI monolayer perfectly preserves the bulked honeycomb structure of MoTe2. The GeI on MoTe2 was confirmed to maintain its topological band structure with a sizable indirect bulk bandgap of 0.24 eV by directly calculating the spin Chern number to be -1. As expected, the electron mobility of the GeI is enhanced by MoTe2 substrate restriction. According to deformation- potential theory with the effective-mass approximation, the electron mobility of GeI/MoTe2 was estimated as 372.7 cm^2·s^-1·V^-1 at 300 K, which is 20 times higher than that of freestanding GeI. Our research shows that traditional substrates always destroy the topological states and hinder the electron transport in QSH insulators, and pave way for the further realization and utilization of QSH insulators at room temperature.

Local engineering of topological phase in monolayer MoS2

Monolayer transition metal dichalcogenides(TMDCs) with the 1 T0 structure are a new class of large-gap two-dimensional(2 D) topological insulators, hosting topologically protected conduction channels on the edges. However, the 1 T0 phase is metastable compared to the 2 H phase for most of 2 D TMDCs, among which the 1 T0 phase is least favored in monolayer MoS2. Here we report a clean and controllable technique to locally induce nanometer-sized 1 T0 phase in monolayer 2 H-MoS2 via a weak Argon-plasma treatment,resulting in topological phase boundaries of high density. We found that the stabilization of 1 T0 phase arises from the concerted effects of S vacancies and the tensile strain. Scanning tunneling spectroscopy(STS) clearly reveals a spin-orbit band gap(~60 meV) and topologically protected in-gap states residing at the 1 T0-2 H phase boundary, which are corroborated by density-functional theory(DFT) calculations.The strategy developed in this work can be generalized to a large variety of TMDCs materials, with potentials to realize scalable electronics and spintronics with low dissipation.