Cn -symmetric higher-order topological crystalline insulators in atomically thin transition metal dichalcogenides

Based on first-principles calculations and symmetry analysis, we predict atomically thin (1-N layers) 2H group-VIB transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se, Te) are large-gap higher-order topological crystalline insulators (HOTCIs) protected by C3 rotation symmetry. We explicitly demonstrate the nontrivial topological indices and the existence of hallmark corner states with quantized fractional charge for these familiar TMDs with a large bulk optical band gap (1.64-1.95eV for the monolayers), which would facilitate the experimental verification and exploration of the HOTCI states. Furthermore, we find that the well-defined corner states exist in the triangular finite-size flakes with armchair edges of the atomically thin (1-N layers) 2H group-VIB TMDs, and the corresponding quantized fractional charge is the number of layers N modulo three. The fractional corner charge will double when spin degree of freedom is taken into account.