Characterization of the high-pressure superconductivity in the Pnma phase of calcium

The thermodynamic parameters of the superconducting state in calcium under the pressure at 161 GPa have been calculated within the framework of the Eliashberg approach. It has been shown that the value of the Coulomb pseudopotential is high (0.24) and the critical temperature (25 K) should be determined from the modified Allen-Dynes formula. In addition, it has been found that the basic dimensionless ratios of the thermodynamic parameters significantly diverge from the BCS predictions, and take the following values: (i) The zero temperature energy gap to the critical temperature(R1) is equal to 4.01. (ii) The ratio R2 equals 2.17. (iii) The quantity R3=0.158. Finally, it has been proven that the electron effective mass is large and takes the maximum of 2.32*me at TC.