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

We predict a stable eight-coordinate two-dimensional RuS 4 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 Γ point. When the spin-orbit coupling is included, a large nontrivial energy gap (70 meV) appears near the Fermi level. The topological nature of RuS 4 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- d xz and d yz orbitals.