Topological Dirac states in transition-metal monolayers on graphyne.

Realizing topological Dirac states in two-dimensional (2D) magnetic materials is particularly important to spintronics. Here, we propose that such states can be obtained in a transition-metal (Hf) monolayer grown on a 2D substrate with hexagonal hollow geometry (graphyne). We find that the significant orbital hybridizations between Hf and C atoms can induce sizable magnetism and bring three Dirac cones at/around each high-symmetry K(K') point in the Brillouin zone. One Dirac cone is formed by pure spin-up electrons from the dz2 orbital of Hf, and the remaining two are formed by crossover between spin-up electrons from the dz2 orbital and spin-down electrons from the hybridization of the dxy/x2-y2 orbitals of Hf atoms and the pz orbital of C atoms. We also find that the spin-orbit coupling effect can open sizable band gaps for the Dirac cones. The Berry curvature calculations further show the nontrivial topological nature of the system with a negative Chern number C = -3, which is mainly attributed to the Dirac states. Molecular dynamics simulations confirm the system's thermodynamic stability approaching room temperature. The results provide a new avenue for realizing the high-temperature quantum anomalous Hall effect based on 2D transition-metals.

Giant conductance anisotropy in black phosphorene tuned by external electric field.

Based on the non-equilibrium Green's function method, the conductance anisotropy of black phosphorene has been studied under chemical potential and/or external electric field. The direction and magnitude of the conductance anisotropy strongly depend on the chemical potential, which are in good agreement with experimental observations. Furthermore, the magnitude of conductance anisotropy can be largely modulated by external electric field, which is one order of magnitude larger than previously reported. The directions of the maximum and minimum conductance can be reversed by external electric field, which shows that the former jumps from 24° to 90° and the latter jumps from 90° to 0° respectively. The results are fundamentally interesting and technologically promising in nano-electronics.