Specific heat of the ambient-pressure organic superconductorβ-di[bis(ethylenedithio)tetrathiafulvalene] triiodide [β-(BEDT-TTF)2I3]

We have measured the specific heat of single crystals of the ambient-pressure superconductor β-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$, abbreviated β-(ET${)}_2$ ${\mathrm{I}}_3$, from 0.7 to 18 K in zero and applied magnetic fields. No bulk specific-heat anomaly in the vicinity of $T_c$ was observed, although Meissner-effect measurements (H∥$c^{\mathrm{*}}$) performed on these same crystals yield a volume percent superconductivity of at least 25% at 0.8 K. We find a γ=24±3 mJ/mol ${\mathrm{K}}^2$, which is larger than the value for the only previously known ambient-pressure superconductor, the Se-based (TMTSF${)}_2$ ${\mathrm{ClO}}_4$ (TMTSF denotes tetramethyltetraselenafulvalene). This difference is due primarily to the greater density of S atoms in β-(ET${)}_2$ ${\mathrm{I}}_3$ compared to the density of Se atoms in (TMTSF${)}_2$ ${\mathrm{ClO}}_4$. The response of the low-temperature specific heat to fields up to 10 T aligned along $c^{\mathrm{*}}$ was 0 to ±3%, in sharp contrast to previous reports of a 20% increase in the specific heat of (TMTSF${)}_2$ ${\mathrm{ClO}}_4$ in a field of 2 T. No evidence for a high-field magnetic or insulating state similar to that observed in (TMTSF${)}_2$ ${\mathrm{ClO}}_4$ was found for β-(ET${)}_2$ ${\mathrm{I}}_3$. New results on the specific heat in applied fields for (TMTSF${)}_2$ ${\mathrm{ClO}}_4$ are presented.