Abstract
Pressure-induced superconductivity often occurs following structural transition under hydrostatic pressure (P(HP)) but disappears after the pressure is released. In the alkali-earth metal barium, superconductivity appears after structural transformation from body-centered cubic structure to hexagonal-close-packed (hcp) structure at P(HP) = 5 GPa, and the superconducting transition temperature (T(c)) reaches a maximum of 5 K at P(HP) = 18 GPa. Furthermore, by stabilizing the low-temperature phase at P(HP) ~ 30 GPa, Tc reached a higher level of 8 K. Herein, we demonstrate a significantly higher T(c) superconductivity in Ba even at ambient pressure. This was made possible through severe plastic deformation of high-pressure torsion (HPT). In this HPT-processed Ba, we observed superconductivity at T(c) = 3 K and T(c) = 24 K in the quasi-stabilized hcp and orthorhombic structures, respectively. In particular, the latter T(c) represents the highest value achieved at ambient pressure among single-element superconducting metals, including intermetallics. The phenomenon is attributed to a strained high-pressure phase, stabilized by residual strains generated from lattice defects such as dislocations and grain boundaries. Significantly, the observed T(c) far exceeds predictions from DFT calculations under normal hydrostatic compressions. The study demonstrates the importance of utilizing high-pressure strained phases as quasi-stable superconducting states at ambient pressure.