Superconductivity and Nontrivial Band Topology in NbN Polytypes

Niobium nitride is a good superconductor in its cubic structure (d-NbN) with Tc = 17 K [1]. Recent experiments on hexagonal NbN (e-NbN) revealed the existence of superconductivity with a Tc of 12 K [2]. WC-type NbN possesses topological properties with three fold band crossings [3]. Here we study the electronic band structure and superconductivity in these NbN polytypes by performing density functional calculations. Interestingly, our results reveal that all three NbN polytypes are topological metals. Specifically, d-NbN and e -NbN are, respectively, type-II and type-I Dirac metals, while WC-NbN is a topological metal with triply degenerate nodes. Our results also show that the electron-phonon coupling in d-NbN is much stronger than in e-NbN and WC-NbN. This results in a much higher superconducting transition temperature (T_c = 18 K) than in e-NbN and WC-NbN. Our findings thus suggest that the NbN polytypes would provide valuable opportunities for studying exotic phenomena arising from the interplay between superconductivity and band topology.
[1] M. W. Williams et al., J. Phys. Chem. Solids 28, 333 (1967).
[2] Y. Zou et al., Sci. Rep. 6, 22330 (2016).
[3] G. Chang et al., Sci. Rep. 7, 1688 (2017).