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

Abstract

We report a record high superconducting transition temperature (Tc) 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 Tc 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 Tc of ∼23 K is observed between 144.9 and 183 GPa in the δ-Ti phase. The pressure-dependent Tc 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 γ-Ti to δ-Ti phase transition pressure enhance EPC, which results in the record high Tc. We attribute the robust superconductivity in δ-Ti to the apparent robustness of its strong EPC against lattice compression. These results provide new insight into exploring new high-Tc elemental metals and Ti-based superconducting alloys.