Doubled Quantum Spin Hall Effect in α-Antimonene

For a two-dimensional time-reversal symmetric system, the quantum spin Hall phase is assumed to be the same as the Z₂ topological insulator phase in the existing literature because the spin Chern number C_s and the Z₂ invariant yield the same topological classification. Here, by investigating the topological 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 Z₂ trivial. Our analysis shows that α-As and Sb share the same band representations at high-symmetry points (HSPs) and would thus both be classified as trivial insulators in terms of topological quantum chemistry (TQC) and symmetry indicators (SIs). However, we demonstrate the existence of a phase transition between α-As and Sb, which is induced by band inversions at two generic <i>k</i> points. In the absence of 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 Dirac points and induces a nontrivial Berry curvature, endowing α-Sb a doubled quantum spin Hall insulator 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.