First-principles study of the superconducting transition temperature Tc in compressed barium: structural identification based on the pressure dependence of Tc

ABSTRACT The superconducting transition temperature Tc of compressed barium (Ba) has been reported to increase with pressure (P) up to ≈8 K under low-temperature compression [Jackson et al., Phys. Rev. B 96, 184,514 (2017)]. However, those measurements were interpreted only through a previously established phase diagram without simultaneous structural determination. We calculated Tc for several candidate structures using first-principles methods, and compared them with the experimental trends. The experimental pressures are offset by ≈10 GPa in comparison to our results, likely due to non-equilibrium effects caused by compression at ≈10 K. After accounting for this offset, we argue that the data assigned to the Ba-VI phase cannot be explained by a single structure. The Tc(P) slope below ≈20 GPa corresponds to the stable hexagonal close-packed Ba-II phase, whereas above ≈20 GPa it follows Pnma-related metastable structures. These results demonstrate that the Tc(P) slope provides a reliable structural fingerprint for identifying the superconducting phases in compressed Ba.