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

Abstract

A large bulk band gap is critical for the application of quantum spin Hall (QSH) insulators or two-dimensional (2D) topological insulators (TIs) in spintronic devices operating at room temperature (RT). On the basis of first-principles calculations, we predicted a group of 2D TI BiX/SbX (X=H, F, Cl and Br) monolayers with extraordinarily large bulk gaps from 0.32 eV to a record value of 1.08 eV. These giant-gaps are entirely due to the result of the strong spin-orbit interaction related to the px and py orbitals of the Bi/Sb atoms around the two valleys K and K′ of the honeycomb lattice, which is significantly different from that consisting of the pz orbital as 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 demonstrate that the honeycomb structures of BiX monolayers remain stable even at 600 K. Owing to these features, the giant-gap TIs BiX/SbX monolayers are 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, exhibiting valley-selective circular dichroism. Chinese researchers have identified new materials with record ‘bulk bandgaps’ ideal for generating stable, quantum-based devices. Jinbo Yang from Peking University and co-workers performed first-principles calculations to investigate two-dimensional topological insulators — recently discovered crystals with an insulating core and surface ‘quantum spin Hall states’, which can move electrons without energy loss. The quantum features of topological insulators normally emerge only at temperatures close to absolute zero, but the team's computations revealed that ultrathin bismuth- and antimony-based films arranged in graphene-like, honeycomb frameworks can produce quantum effects much closer to room temperature thanks to giant energy gaps (0.32–1.08 eV) between the materials' conduction and valence bands. These gaps, which result from strong electron spin–orbit interactions, stabilize the surface states against interference from thermally activated carriers and make these materials intriguing targets for future experiments. 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 were predicated. These giant-gaps result from the strong spin-orbit interaction related to px and py orbitals of Bi/Sb atoms around the two valleys K and K′. The honeycomb structures of BiX monolayers remain stable even at a temperature of 600 K. The electric field-biased BiX/SbX monolayers become quantum valley Hall insulators, showing valley-selective circular dichroism. These features make the BiX/SbX monolayers an ideal platform to realize many exotic phenomena and fabricate new quantum devices.