Superconductivity of Ca exceeding 25 K at megabar pressures

The pressure dependence of the superconducting transition temperature T, 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.