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.