Orbital-active Dirac materials from the symmetry principle

AbstractDirac materials, starting with graphene, have drawn tremendous research interest in the past decade. Instead of focusing on the p_z p z orbital as in graphene, we move a step further and study orbital-active Dirac materials, where the orbital degrees of freedom transform as a two-dimensional irreducible representation of the lattice point group. Examples of orbital-active Dirac materials occur in a broad class of systems, including transition-metal-oxide heterostructures, transition-metal dichalcogenide monolayers, germanene, stanene, and optical lattices. Different systems are unified based on symmetry principles. The band structure of orbital-active Dirac materials features Dirac cones at K(K') K ( K ′ ) and quadratic band touching points at Γ, regardless of the origin of the orbital degrees of freedom. In the strong anisotropy limit, i.e., when the π-bonding can be neglected, flat bands appear due to the destructive interference. These features make orbital-active Dirac materials an even wider playground for searching for exotic states of matter, such as the Dirac semi-metal, ferromagnetism, Wigner crystallization, quantum spin Hall state, and quantum anomalous Hall state.