Alkali-metal-induced topological nodal line semimetal in layered XN2 (X = Cr, Mo, W)

Abstract

Based on first principles calculations and the K·p effective model, we propose that alkali metal deposition on the surface of hexagonal XN2 (X = Cr, Mo, W) nanosheets induces topologically nontrivial phases in these systems. When spin orbit coupling (SOC) is disregarded, the electron-like conduction band from N-pz orbitals can be considered to cross the hole-like valence band from X-dz2 orbitals, thereby giving rise to a topological nodal line state in lithium-functionalized XN2 sheets (Li2MoN2 and Li2WN2). Such band crossing is protected by the existence of mirror reflection and time reversal symmetry. More interestingly, the bands cross exactly at the Fermi level, and the linear dispersion regions of such band crossings extend to as high as 0.9 eV above the crossing. For Li2CrN2, the results reveal the emergence of a Dirac cone at the Fermi level. Our calculations show that lattice compression decreases the thickness of a Li2CrN2 nanosheet, leading to phase transition to a nodal line semimetal. The evolution of the band gap of Li2XN2 at the Γ point indicates that the nontrivial topological character of Li2XN2 nanolayers is stable over a large strain range. When SOC is included, the band crossing point is gapped out giving rise to quantum spin Hall states in Li2CrN2 nanosheets, while for Li2MoN2, the SOC-induced gap at the crossing points is negligible.