Specific heat in high magnetic field ofκ-di[bis(ethylenedithio)tetrathiafulvalene]- di(thiocyano)cuprate [κ-(ET)2Cu(NCS)2]: Evidence for strong-coupling superconductivity

We have measured the specific heat down to 1.5 K of a collection of high-quality single crystals of κ-(ET${)}_2$Cu(NCS${)}_2$ in fields parallel to the crystalline $a^{\mathrm{*}}$ direction to 12.5 T. In this first determination of γ, we find γ to be 25±3 mJ/${\mathrm{K}}^2$ mole. The anomaly in C at $T_c$ is quite distinct when compared to normal-state data obtained by suppressing $T_c$ with field. The value of ΔC/γT>2 obtained implies that this organic superconductor has strong coupling.