Influence of lattice instability on the superconducting properties of “La 3S 4”

Abstract

The normal state properties (the electronic specific heat constant, Debye temperature and electrical resistivity) and superconducting state properties [the superconducting transition temperature, T c, and the upper critical field at 0 K, H c 2 (0)] have been studied in the La 3S 4-La 2S 3 system. The superconducting properties and the electronic specific heat constant exhibit the maximum values in the alloy with the lowest sulfur content that does not undergo a low temperature crystallographic transformation. At lower sulfur contents the alloys exhibit a cubic to tetragonal transformation at ∼80 K with a serious degradation in their superconducting properties, especially H c 2 (0). These alloys clearly illustrate that materials which are almost but not quite unstable are good superconductors, relative to the more stable compositions.