NbTi: a nontrivial puzzle for the conventional theory of superconductivity

TL;DR

This study uses first-principles calculations to investigate superconductivity in NbTi, revealing complex effects like anharmonicity and disorder that challenge existing theoretical models.

Contribution

It provides the first ab-initio analysis of NbTi superconductivity, highlighting the importance of anharmonic effects and disorder, and exposing limitations of current computational methods.

Findings

Anharmonic effects stabilize phonons in NbTi's bcc phase.

Coulomb interactions reduce Tc by over 20%.

Lattice disorder significantly affects Tc predictions.

Abstract

We present the first $ab$-$initio$ study of superconductivity in NbTi, the workhorse for many applications. Despite its apparent simplicity, NbTi turns out to be a major challenge for computational superconductivity. In fact, anharmonic effects are crucial to obtain dynamically stable phonons for the ordered bcc phase, unstable at the harmonic level, and beyond-Morel Anderson effects in the Coulomb interaction reduce the Tc by more than 20%. Lattice disorder causes an additional large discrepancy in $T_c$ compared to experiment. Our results imply that a quantitative description of technologically-relevant superconductors requires methodological developments beyond the current standards.