Strain-induced Topological Phase Transitions in α-phase Pnictogen Monolayers

Monolayer (ML) α-phase pnictogens with strong spin-orbit coupling (SOC) are of significant interest due to their prominence of topologically non-trivial electronic structures and two-dimensional (2D) structure. For instance, 1ML α-Sb is a Dirac semi-metal predicted to host a High Spin Chern Insulating phase due to SOC, while 1ML α-Bi is a Z₂ topological insulator due to a band inversion at the Γ point. These 1ML black phosphorous structures are extremely susceptible to strain and buckling of the crystal lattice, which can change the electronic and topological properties of the film. Here we report strain induced topological phase transitions for both α-Sb and α-Bi monolayers and strain diagrams indicating the regime of these transitions. In addition, spontaneous space inversion symmetry breaking in the relaxation of α-Bi induces band degeneracy lifting, revealing an intrinsic Weyl semi-metallic state in this 2D system.