Topological Dirac Nodal-net Fermions in AlB2-type TiB2 and ZrB2
Abstract
Based on first-principles calculations and effective model analysis, a Dirac nodal-net semimetal state is recognized in AlB2-type TiB2 and ZrB2 when spin-orbit coupling (SOC) is ignored. Taking TiB2 as an example, there are several topological excitations in this nodal-net structure including triple point, nexus, and nodal link, which are protected by coexistence of spatial-inversion symmetry and time reversal symmetry. This nodal-net state is remarkably different from that of IrF4, which requires sublattice chiral symmetry. In addition, linearly and quadratically dispersed two-dimensional surface Dirac points are identified as having emerged on the B-terminated and Ti-terminated (001) surfaces of TiB2 respectively, which are analogous to those of monolayer and bilayer graphene.
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