Record high $T_{\rm c}$ and robust superconductivity in transition metal $\delta$-Ti phase at megabar pressure

TL;DR

This study reports a record high superconducting transition temperature of 23.6 K in elemental titanium under high pressure, with findings supported by experiments and density functional theory calculations indicating electron-phonon coupling as the mechanism.

Contribution

It demonstrates the highest $T_c$ in elemental metals achieved under megabar pressures and elucidates the underlying electron-phonon coupling mechanism through theoretical calculations.

Findings

$T_c$ reaches 23.6 K at 144.9 GPa

Superconductivity remains robust between 144.9 and 183 GPa

Electron-phonon coupling explains the pressure dependence of $T_c$

Abstract

We report a record high superconducting transition temperature ($T_{\rm c}$) up to 23.6 K under high pressure in the elemental metal Ti, one of the top ten most abundant elements in Earth's crust. The $T_{\rm c}$ increases monotonically from 2.3 K at 40.3 GPa to 23.6 K at 144.9 GPa, which surpasses all known records from elemental metals reported so far. With further compression, a robust $T_{\rm c}$ of ~23 K is observed between 144.9 and 183 GPa in the $\delta$-Ti phase. The pressure-dependent $T_{\rm c}$ can be well described by the conventional electron-phonon coupling (EPC) mechanism. Density Functional Theory calculations show the Fermi nesting and the phonon softening of optical branches at the $\gamma$-Ti to $\delta$-Ti phase transition pressure enhance EPC, which results in the record high $T_{\rm c}$. We attribute the robust superconductivity in $\delta$-Ti to the apparent robustness of its strong EPC against lattice compression. These results provide new insight into exploring new high-$T_{\rm c}$ elemental metals and Ti-based superconducting alloys.