Prediction of two-dimensional topological crystalline insulator in PbSe monolayer.

Two-dimensional (2D) topological crystalline insulator, a new class where states are protected by lattice symmetry instead of by time-reversal symmetry, is predicted in PbSe monolayer based on first-principles electronic structure calculations. A combination of strong spin-orbit interaction with quantum confinement effects in PbSe monolayer lead to a topological phase transition with an even number of band inversion momentum space points. We demonstrate that the PbSe nanostructure presents pairs of spin-polarized Dirac cones coming from the monolayer edges, where each Dirac pair presents a unique spin alignment, leading to a quantum spin Hall system. More importantly, due to the quantum confinement this 2D nanostructure presents larger band gap as compared to its parent narrow band gap trivial insulator bulk PbSe, favoring a room-temperature 2D band gap with crystalline-protected Dirac states at the edges, turning this system interesting to combine nontrivial topological states with nanoelectronic and spintronic applications.