Superconductivity in the Mercury-Indium Alloy System

Abstract

The superconducting transition temperatures of a large number of Hg-In alloys have been measured and the variation of $T_c$ with composition determined over the entire range. Variation of $T_c$ with number of valence electrons/atom within any particular metallurgical phase is quite strong, $T_c$ in general increasing with decreasing number of valence electrons. The maximum and minimum values of $T_c$ occurring in the system are 4.5 and 3.1°K at approximately 80 and 20 at.% indium, respectively. HgIn, the only stoichiometric compound, has a transition temperature of 3.81°K. The observed composition dependence of $T_c$ is not what one would expect either from Matthias's empirical rules or from the simple BCS formula, assuming a nearly free electron model for the density of states. In one phase a discontinuity in $T_c$ and its composition dependence is found which cannot be correlated with a change in any lattice property and apparently arises from a discontinuous change in electronic structure. In connection with the problem of the effect of one phase upon superconductivity in another with which it is in intimate contact, there is evidence that the transition temperature of HgIn can be lowered a few tenths of a degree if large quantities of a lower transition temperature phase are also present. Electrical conductivity data, which were obtained in the course of the superconductivity measurements by an induced-current method, made possible also the determination of residual resistivity over the entire composition range.