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High-temperature Superconductivity in Layered Nitrides β-Li$_x$MNCl (M = Ti, Zr, Hf): Insights from Density-functional Theory for Superconductors

We present an ab initio analysis with density functional theory for superconductors (SCDFT) to understand the superconducting mechanism of doped layered nitrides β-Li$_x$MNCl (M=Ti, Zr, and Hf). The current version of SCDFT is based on the Migdal-Eliashberg theory and has been shown to reproduce accurately experimental superconducting-transition temperatures Tc of a wide range of phonon-mediated superconductors. In the present case, however, our calculated Tc$\leq$4.3 K (M=Zr) and $\leq$10.5 K (M=Hf) are found to be less than a half of the experimental Tc. In addition, Tc obtained in the present calculation increases with the doping concentration x, opposite to that observed in the experiment. Our results indicate that we need to consider some elements missing in the present SCDFT based on the Migdal-Eliashberg theory.