Spin valley and giant quantum spin Hall gap of hydrofluorinated bismuth nanosheet

Heng Gao,Feng Miao,Wei Ren,Tao Hu,Alessandro Stroppa,Xinran Wang,Wei Wu,Baigeng Wang

doi:10.1038/s41598-018-25478-6

Abstract

Spin-valley and electronic band topological properties have been extensively explored in quantum material science, yet their coexistence has rarely been realized in stoichiometric two-dimensional (2D) materials. We theoretically predict the quantum spin Hall effect (QSHE) in the hydrofluorinated bismuth (Bi2HF) nanosheet where the hydrogen (H) and fluorine (F) atoms are functionalized on opposite sides of bismuth (Bi) atomic monolayer. Such Bi2HF nanosheet is found to be a 2D topological insulator with a giant band gap of 0.97 eV which might host room temperature QSHE. The atomistic structure of Bi2HF nanosheet is noncentrosymmetric and the spontaneous polarization arises from the hydrofluorinated morphology. The phonon spectrum and ab initio molecular dynamic (AIMD) calculations reveal that the proposed Bi2HF nanosheet is dynamically and thermally stable. The inversion symmetry breaking together with spin-orbit coupling (SOC) leads to the coupling between spin and valley in Bi2HF nanosheet. The emerging valley-dependent properties and the interplay between intrinsic dipole and SOC are investigated using first-principles calculations combined with an effective Hamiltonian model. The topological invariant of the Bi2HF nanosheet is confirmed by using Wilson loop method and the calculated helical metallic edge states are shown to host QSHE. The Bi2HF nanosheet is therefore a promising platform to realize room temperature QSHE and valley spintronics.

Highlights

The research of atomically thin two-dimensional (2D) materials has been a forefront topic in condensed matter physics since the successful exfoliation of single layer graphene[1]
We have found that the Bi2HF nanosheet is a 2D topological insulator with a bulk band gap of 0.97 eV which can host room-temperature quantum spin Hall effect (QSHE)
Unlike the Bi monolayer and fully hydrogenated or fluorinated Bi monolayer[27], the structure of Bi2HF nanosheet is noncentrosymmetric owing to the H and F codecoration on opposite sides of Bi monolayer which breaks inversion symmetry and induces a net out-of-plane dipole moment

Summary

Introduction

The research of atomically thin two-dimensional (2D) materials has been a forefront topic in condensed matter physics since the successful exfoliation of single layer graphene[1]. To achieve room temperature functionality the theoretical and experimental searches of QSHE host with large bulk band gaps become critically important[29,30]. For this purpose, we propose a hydrofluorinated Bi-based 2D structure namely Bi2HF nanosheet which intrinsically hosts topological band and valley properties. To figure out the coupling of spin and valley, we have used the first principles calculations and an effective Hamiltonian model to investigate the interplay between the intrinsic polarization and SOC It shows an in-plane Rashba-type spin texture and an out-of-plane spin splitting of the band structure of Bi2HF nanosheet. The Bi2HF nanosheet is discovered as a promising platform to realize room temperature QSHE and valley spintronics

Methods

Results

Conclusion