Interplay between p- and d- orbitals yields multiple Dirac states in one- and two-dimensional CrB4

Abstract

Theoretical evidence of the existence of six inequivalent and six threefold degenerate pairs of Dirac cones in the low-spectrum diagram of monolayered hexagonal CrB4 is provided. The four d-electrons of the Cr atom are yielded to the B sublattices creating an isoelectronic structure to graphene where the interplay between p- and d- orbitals leads to the appearance of additional Dirac states on both one- and two-dimensional geometries. Ab initio calculations show that, although spin-orbit interaction splits the cone-shaped valence and conduction bands, CrB4 is a semimetal with compensated electron-hole pockets. As the two-dimensional layer is shaped into finite-width ribbons, one actual and one symmetry-frustrated Dirac point are observed at the Fermi level, yielding massless fermions in a one-dimensional nano-structure with no topological insulating features. A rational explanation in terms of periodic boundary conditions across the ribbon axis is given to unveil the origin of the Dirac point.