Prediction of two-dimensional d-block elemental materials with normal honeycomb, triangular-dodecagonal, and square-octagonal structures from first principles.

By first-principles calculations, we investigated the electronic structures and magnetic properties of several tetravalent transition-metal monolayers with normal honeycomb, triangular-dodecagonal, and square-octagonal structures by considering the effects of spin-orbit coupling and electronic strong correlation of d orbitals. For both standard and corrected approaches, spin-polarized Dirac points contributed by d states appear in the monolayers with hexagonal lattice (honeycomb and 3–12 lattices), but for 4–8 lattices, Dirac points disappear, demonstrating that specific symmetries are required for forming Dirac cones. By adding the on-site Coulomb repulsion, the electronic correlation of d orbital is enhanced and thus the electronic localization increases, aggravating the spin splitting. For Hf3-12, the coexistence of massless Dirac fermions and massive heavy fermions is found. Moreover, the spin-orbit coupling destroys the degeneracy of two bands at K points, and the largest gap opening of 214 meV appears in Hf4-8 due to both Coulomb repulsion and spin-orbit coupling. Our results demonstrate that the spin splitting and gap opening depend on the lattice symmetry, bond length, electronic strong correlation, and spin-orbit coupling. These predicted structures provide new choices in synthesizing two-dimensional transition-metal materials, which has the potential applications in spintronic devices, quantum computation, hydrogen storage, and catalytic chemistry. [ABSTRACT FROM AUTHOR]