The superconductive critical temperature and the electronic specific heat of impur 1e tin

Abstract

From measurements of the threshold magnetic field as a function of temperature for specimens of tin containing 0.04 to 2.0 at. per cent of various impurities we have obtained the superconductive critical temperature, T c , of these samples, as well as the temperature coefficient, γ, of the electronic specific heat of the normal phase. We find that the change in T c produced by the impurities consists of two parts. There is a decrease which is linearly proportional to the increase in residual resistance ratio and hence to the decrease in electronic mean free path. Superimposed on this decrease there is an increase in T c . Our results also show that the measurable changes in γ are positive for all solutes, indicating a minimum in the density of states at or near the Fermi energy. We believe that the second, positive part of the change in T c is due to the increase in γ, and find that the correlation between these quantities is in reasonable agreement with the recent theory of B ar-Deen, C ooper and S chrieffer. 19 During the course of our work we also obtained a value of γ for pure tin which agrees well with a recent calorimetric result. Measurements of the residual resistance ratio of our samples show, as expected, that below the solubility limit this ratio increases linearly with impurity concentration with a slope which depends on the atomic radius of the impurity as well as on its valence.