Prediction of a room-temperature eight-coordinate two-dimensional topological insulator: penta-RuS4 monolayer

We predict a stable eight-coordinate two-dimensional RuS4 monolayer, with a trilayer S-Ru-S and pentagonal rings tiling configuration by first-principles calculations. This monolayer exhibits unique anisotropic quadratic energy dispersion with two Dirac points emerging at the high-symmetric G point. When the spin-orbit coupling is included, a large nontrivial energy gap (70 meV) appears near the Fermi level. The topological nature of RuS4 monolayer is also confirmed by the nontrivial Z(2) invariant and gapless edge states. A four-band tight-binding model is further proposed, which reveals that the topological states arise from the indirect interaction of the Ru-dxz and dyz orbitals.