Bulk superconducting specific-heat anomaly in $\beta$-di[bis (ethylenedithio) tetrathiafulvalene] diiodoaurate [$\beta -{\left(\mathrm{ET}\right)}_2\mathrm{Au}{\mathrm{I}}_2$]

In order to determine how universal the lack of a specific-heat anomaly at $T_c$ is in the $\beta -{\left(\mathrm{ET}\right)}_2{X}_3$ [$\beta -{\left({\mathrm{C}}_{10}{\mathrm{S}}_8{\mathrm{H}}_8\right)}_2{X}_3$, where $X_3$ is the linear anion] ambient-pressure organic superconductors [no anomaly has been found in $\beta -{\left(\mathrm{ET}\right)}_2\mathrm{Au}{\mathrm{I}}_2$], we have measured the low-temperature specific heat of $\beta -{\left(\mathrm{ET}\right)}_2\mathrm{Au}{\mathrm{I}}_2$, with $T_c=5\mathrm{}$ K. Using a high-precision, low-addendum specific-heat platform we have found a distinct, although somewhat broadened, anomaly at $T_c$ in $\beta -{\left(\mathrm{ET}\right)}_2\mathrm{Au}{\mathrm{I}}_2$ indicative of bulk superconductivity. Also, $\frac{C}{T} (T\to 0)$ extrapolates, within the accuracy of our measurements, to zero, indicative of a fully opened gap in the electronic energy levels.