Tc up to 23.6 K and robust superconductivity in the transition metal δ−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 $\ensuremath{\sim}23$ K is observed between 144.9 and 183 GPa in the $\ensuremath{\delta}$-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 $\ensuremath{\gamma}$-Ti to $\ensuremath{\delta}$-Ti phase transition pressure-enhanced EPC, which results in the high ${T}_{c}$. We attribute the robust superconductivity in $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Ti}$ 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.