Superconductivity in sulfur-based organic superconductors: A volume property

We have shown by dc low-field magnetization measurements the existence of diamagnetic shielding and Meissner signals in the ambient-pressure organic superconductors, -(ET)2IBr2 and -(ET)2AuI2, below T=2.5 K and T=4.1 K, respectively [ET is bis(ethylenedithio)tetra- thiafulvalene]. This proves that superconductivity is a volume property in these compounds. We also measured the upper critical field Hc2 in the compound -(ET)2AuI2. Combined with the low-field data this yields a thermodynamically consistent picture using Ginzburg-Landau theory and a weak-coupling Bardeen-Cooper-Schrieffer gap. © 1986 The American Physical Society.