Spin-fluctuation-mediated d+id' pairing mechanism in doped β-MNCl (M=Hf,Zr) superconductors

We study two types of models for the superconducting layered nitride beta-MNCl (M=Hf,Zr); a single-band model on a triangular lattice, and a two-band model on a honeycomb lattice. We find that the former model does not suffice as an effective model while the latter one can be a good candidate. We propose from the study on the two-band model a possibility of spin-fluctuation-mediated d+id(')-wave superconductivity in the doped beta-MNCl. We show that the relatively high T-c obtained in the doped band insulator is a characteristic feature of the spin-fluctuation-mediated superconductivity on a honeycomb lattice. We also find that the gap anisotropy on the Fermi surface strongly increases upon increasing the doping concentration, and the interlayer hopping suppresses superconductivity. These results are in qualitative agreement with the experimental findings.