Photoinduced Topological Phase Transition and a Single Dirac-Cone State in Silicene

Silicene (a monolayer of silicon atoms) is a two-dimensional topological insulator (TI) that undergoes a topological phase transition to a band insulator under external electric field E(z). We investigate a photoinduced topological phase transition from a TI to another TI by changing its topological class by irradiating circular polarized light at fixed E(z). The band structure is modified by photon dressing with a new dispersion, where the topological property is altered. By increasing the intensity of light at E(z)=0, a photoinduced quantum Hall insulator is realized. Its edge modes are anisotropic chiral, in which the velocities of up and down spins are different. At E(z)>E(cr) with a certain critical field E(cr), a photoinduced spin-polarized quantum Hall insulator emerges. This is a new state of matter, possessing one Chern number and one-half spin-Chern numbers. We newly discover a single Dirac-cone state along a phase boundary. A distinctive hallmark of the state is that one of the two Dirac valleys is closed and the other open.