Superconductivity of Ca Exceeding 25 K at Megabar Pressures

The pressure dependence of the superconducting transition temperature T c in calcium was measured up to 161 GPa. T c increased significantly with pressure and reached 25 K at 161 GPa, which is the highest observed T c for all elements. Compared with the result obtained in a recent structural experiment, T c increases within the simple cubic structure phase and becomes rather stable but still increases in the Ca-IV and Ca-V phases.