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

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 alpha-phase group V elements, we uncover the presence of a topological phase in alpha-Sb, which can be characterized by a spin Chern number C-s = 2, even though it is Z(2) trivial. Although alpha-As and Sb would thus be classified as trivial insulators within the classification schemes, we demonstrate the existence of a phase transition between alpha-As and Sb, which is induced by band inversions at two generic k points. Without spin-orbit coupling (SOC), alpha-As is a trivial insulator, while alpha-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 alpha-Sb with a high spin Chern number of C-s = 2. We further show that monolayer alpha-Sb exhibits either a gapless band structure or a gapless spin spectrum on its edges, as expected from topological considerations.