A possible explanation for the high superconducting Tc in bcc Ti at high pressure

TL;DR

This study investigates the high superconducting transition temperature in high-pressure bcc titanium, proposing that lattice vacancies enhance electron-phonon coupling, aligning theoretical predictions with experimental observations.

Contribution

It introduces the idea that Ti vacancies can explain the high Tc in high-pressure titanium, bridging the gap between theory and experiment.

Findings

Vacancies cause pressure-dependent phonon softening.

Vacancies significantly increase electron-phonon coupling.

Computed Tc's match experimental data when vacancies are considered.

Abstract

Motivated by unexpected reports of a 26 K superconducting transition in elemental titanium at high pressure, we carry out an accurate ab-initio study of its properties to understand the rationale for this observation. The critical superconducting temperatures (Tc's) predicted under the assumption of a phononic pairing mechanism are found to be significantly lower than those experimentally observed. We argue that this disagreement cannot be explained by an unconventional coupling, as previously suggested, or by the existence of competing metastable structural phases. As a physically meaningful hypothesis to reconcile experimental and theoretical results, we assume the presence of Ti vacancies in the lattice. Our first-principles calculations indeed show that lattice vacancies can cause pressure dependent phonon softening and substantially increase the electron-phonon coupling at high pressure, yielding computed Tc's in agreement with the experimental measurements over the full pressure range from 150 to 300 GPa. We expect the proposed Tc enhancement mechanism to occur on a general basis in simple high-symmetry metals for various types of defects.