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

TL;DR

This study uses density functional theory to analyze superconductivity in layered nitrides, revealing discrepancies between theoretical predictions and experimental results, and suggesting the need for improved models.

Contribution

It provides the first ab initio analysis of superconductivity in doped layered nitrides using SCDFT, highlighting limitations of current theoretical approaches.

Findings

Calculated Tc values are significantly lower than experimental values.

Theoretical Tc increases with doping, opposite to experimental trends.

Current models do not fully capture the superconducting mechanism in these materials.

Abstract

We present an ab initio analysis with density functional theory for superconductors (SCDFT) to understand the superconducting mechanism of doped layered nitrides \beta-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.