Type-II Dirac nodal chain semimetal CrB₄

Dirac nodal line semimetals with topologically protected drumhead surface states have attracted intense theoretical and experimental attention over a decade. However, the study of type-II Dirac nodal line semimetals is rare, especially the type-II nodal chain semimetals have not been confirmed by experiment due to the lack of ideal material platform. In this study, based on symmetry analysis and the first-principles electronic structure calculations, we predict that CrB₄ is an ideal type-II Dirac nodal chain semimetal protected by the mirror symmetry. Moreover, there are two nodal rings protected by both space-inversion and time-reversal symmetries in CrB₄. More importantly, in CrB₄ the topologically protected drumhead surface states span the entire Brillouin zone at the Fermi level. Considering the fact that CrB₄ has been synthesized experimentally and the spin–orbit coupling is very weak, CrB₄ provides an ideal material platform for studying the exotic properties of type-II Dirac nodal chain semimetals in experiment.