Superconductivity of barium with highest transition temperatures in metallic materials at ambient pressure

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.