High spin-Chern-number insulator in α-antimonene with a hidden topological phase

Abstract For a time-reversal symmetric system, the quantum spin Hall phase is assumed to be the same as the {Z}₂ topological insulator phase in the existing literature. The spin Chern number {C}_s is presumed to yield the same topological classification as the {Z}₂ invariant. Here, by investigating the electronic structures of monolayer α-phase group V elements, we uncover the presence of a topological phase in α-Sb, which can be characterized by a spin Chern number {C}_s = 2, even though it is {Z}₂ trivial. Although α-As and Sb would thus be classified as trivial insulators within the classification schemes, we demonstrate the existence of a phase transition between α-As and Sb, which is induced by band inversions at two generic k points. Without spin–orbit coupling (SOC), α-As is a trivial insulator, while α-Sb is a Dirac semimetal with four Dirac points (DPs) located away from the high-symmetry lines. Inclusion of the SOC gaps out the DPs and induces a nontrivial Berry curvature, endowing α-Sb with a high spin Chern number of {C}_s = 2. We further show that monolayer α-Sb exhibits either a gapless band structure or a gapless spin spectrum on its edges, as expected from topological considerations.