Spin Orbit Coupling Gap and Indirect Gap in Strain-Tuned Topological Insulator-Antimonene.

Recently, searching large-bulk band gap topological insulator (TI) is under intensive study. Through k·P theory and first-principles calculations analysis on antimonene, we find that α-phase antimonene can be tuned to a 2D TI under an in-plane anisotropic strain and the magnitude of direct bulk band gap (SOC gap) depends on the strength of spin-orbit coupling (SOC) which is strain-dependent. As the band inversion of this TI accompanies with an indirect band gap, the TI bulk band gap is the indirect band gap, not the SOC gap. SOC gap can be enhanced by increasing strain, whereas the indirect band gap can be closed by increasing strain, such that large bulk band gap are forbidden. With the k·P theory analysis on antimonene, we know how to avoid such an indirect band gap. In case of indirect band gap avoided, the SOC gap could become the bulk band gap of a TI which can be enhanced by strain. Thus our theoretical analysis can help searching large bulk band gap TI.