Superconductivity of calcium in phase VI

► Thermodynamic parameters of the superconducting state that induces in calcium in phase VI. ► The high critical temperature and Coulomb pseudopotential. ► The non BCS value of the ratio 2 Δ/ k B T C . ► The large electron effective mass. The properties of the superconducting state of calcium in phase VI were analyzed. By using the imaginary axis Eliashberg equations it has been shown, that the Coulomb pseudopotential reaches the high value equal to 0.215. In the considered case, the critical temperature is not properly described by the Allen–Dynes formula and it should be calculated with an use of the modified expression. In the paper the exact solutions of the Eliashberg equations on the real axis were also obtained. On this basis it was stated, that the effective potential of the electron–electron interaction is attractive for the frequencies lower or equal to the maximum phonon frequency. Then, the dimensionless parameter 2 Δ(0)/ k B T C = 4.10 was calculated. In the last step it has been proven, that the ratio of the electron effective mass to the bare electron mass is high and reaches its maximum equal to 2.36 for the critical temperature.