T-c up to 23.6 K and robust superconductivity in the transition metal delta-Ti phase at megabar pressure

We report a high superconducting transition temperature T-c up to 23.6 K, renewing the highest value in transition metals, under high pressure in the elemental metal Ti, one of the top ten most abundant elements in the earth's crust. The T-c increases monotonically from 2.3 K at 40.3 GPa to 23.6 K at 144.9 GPa. With further compression, a robust T-c of similar to 23 K is observed between 144.9 and 183 GPa in the delta-Ti phase. The pressure-dependent T-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-enhanced EPC, which results in the high T-c. We attribute the robust superconductivity in delta-Ti to the apparent robustness of its strong EPC against lattice compression. These results provide insight into exploring high-T-c elemental metals and Ti-based superconducting alloys.