Thermodynamics of anisotropic superconducting indium analyzed in terms of Eliashberg theory

Abstract

Experiments on the change of thermodynamic properties with impurity concentration in indium are presented, which cover the temperature range from 30 mK to the superconducting transition temperature $T_c$. From an analysis of the $T_c$ depression with impurity concentration and the temperature dependence of the thermodynamic critical fields in terms of Eliashberg theory, we conclude that the electron-phonon coupling anisotropy is large in indium. The mean-square anisotropy parameter 〈$a^2$〉 of the usual separable pairing potential is found to be 0.035–0.040. Particular attention is paid to the influence of various experimental error sources and the role of input parameters on the theoretical analysis. It is shown that the concept of coupling anisotropy and anisotropy removal by impurity scattering provides the only consistent description of the experimental results.