Superconductivity in the Indium-Tin System

Abstract

Transition temperatures as a function of composition have been measured throughout the In-Sn binary system. Samples `quenched' from the liquid state had higher transition temperatures than well-annealed samples, confirming the effect reported earlier for Hg-In compositions. The enhancement in ${T}_{c}$ was 2-3\ifmmode^\circ\else\textdegree\fi{}K for the most favorable compositions. It is argued that the effect arises from internal strains in the samples and not from short-range disorder. The equilibrium ${T}_{c}$ values vary from 3.4\ifmmode^\circ\else\textdegree\fi{}K (near pure In) to 6.6\ifmmode^\circ\else\textdegree\fi{}K ($\ensuremath{\beta}$ phase, 33.5 at.% Sn). Transition temperature within any particular phase is a strong function of valence electron concentration, but not a function of the type found in transition metal superconductors. In all phases ${T}_{c}$ increases with increasing valence electron concentration. The data in the In terminal solid solution are compared with the BCS formula, and with the theory of Morel and Anderson. A substantial variation of the electron-phonon interaction parameter with alloying is required if the data are to be described with the BCS formula. The variation in this parameter computed from the theory of Morel and Anderson is very much less than the observed variation in our case. In the tetragonal In phase a discontinuity in $\frac{d{T}_{c}}{\mathrm{dx}}$ is found at $x=8$ at.% Sn. A crystallographic phase change at this composition in unlikely, and the discontinuity is thought to be an electronic effect. An apparatus for rapid and convenient measurement of transition temperatures is described.