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.
@article{arxiv.1705.00511,
title = {Topological Dirac Nodal-net Fermions in AlB$_2$-type TiB$_2$ and ZrB$_2$},
author = {Xing Feng and Changming Yue and Zhida Song and QuanSheng Wu and Bin Wen},
journal= {arXiv preprint arXiv:1705.00511},
year = {2018}
}