Specific heat of the organic superconductor κ-(BEDT-TTF)2I3

We present high-resolution specific-heat, C, measurements on a large (4.47 mg) single crystal of κ-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$ from 0.25 to 20 K in zero and different magnetic fields. The electronic specific-heat coefficient in the normal state is extracted to γ=(18.9±1.5) mJ ${\mathrm{K}}^{\mathrm{-}2}$ ${\mathrm{mol}}^{\mathrm{-}1}$. For the ratio ΔC/(γ${\mathit{T}}_{\mathit{c}}$), where ΔC is the jump of C at ${\mathit{T}}_{\mathit{c}}$=3.4 K, a value of 1.6±0.2 consistent with the BCS prediction of 1.43 is found. The exact form of ΔC(T), the specific-heat difference between the superconducting and the normal state, however, deviates somewhat from the BCS dependence but might be explained by strong coupling. In magnetic fields applied perpendicular to the highly conducting b-c plane the height of the jump in C is strongly reduced and broadened with a concomitant reduction of ${\mathit{T}}_{\mathit{c}}$. In a field of B=0.5 T above ${\mathit{B}}_{\mathit{c}2}$ at low temperatures a hyperfine contribution to C is found which is larger than the value expected by nuclear hyperfine interaction alone.