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, β-(ET${)}_2$ ${\mathrm{IBr}}_2$ and β-(ET${)}_2$ ${\mathrm{AuI}}_2$, 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 $H_{c2}$ in the compound β-(ET${)}_2$ ${\mathrm{AuI}}_2$. Combined with the low-field data this yields a thermodynamically consistent picture using Ginzburg-Landau theory and a weak-coupling Bardeen-Cooper-Schrieffer gap.