Quantum spin Hall insulators and quantum valley Hall insulators of BiX/SbX (X = H, F, Cl, and Br) monolayers with a record bulk band gap

TL;DR

This paper predicts large-gap 2D topological insulators BiX/SbX monolayers with record bulk gaps up to 1.08 eV, confirmed by first-principles calculations, and explores their stability and valleytronic properties for room-temperature applications.

Contribution

It introduces a new class of 2D topological insulators with record bulk gaps, confirmed by first-principles calculations and effective Hamiltonian analysis.

Findings

Bulk gaps up to 1.08 eV in BiX/SbX monolayers.

Confirmation of topological nature via Z2 index.

Stability of honeycomb structures at 600 K.

Abstract

Large bulk band gap is critical for application of the quantum spin Hall (QSH) insulator or two dimensional (2D) topological insulator (TI) in spintronic device operating at room temperature (RT). Based on the first-principles calculations, here we predict a group of 2D topological insulators BiX/SbX (X = H, F, Cl, and Br) monolayers with extraordinarily large bulk gaps from 0.32 to a record value of 1.08 eV. These giant-gaps are entirely due to the result of strong spin-orbit interaction related to px and py orbitals of Bi/Sb atoms around the two valley K and K' of honeycomb lattice, which is different significantly from the one consisted of pz orbital just like in graphene/silicene. The topological characteristic of BiX/SbX monolayers is confirmed by the calculated nontrivial Z2 index and an explicit construction of the low energy effective Hamiltonian in these systems. We show that the honeycomb structures of BiX monolayers remain stable even at a temperature of 600 K. These features make the giant-gap TIs BiX/SbX monolayers an ideal platform to realize many exotic phenomena and fabricate new quantum devices operating at RT. Furthermore, biased BiX/SbX monolayers become a quantum valley Hall insulator, showing valley-selective circular dichroism.