Specific heat measurements of the gap structure of the organic superconductors kappa-(ET)₂Cu[N(CN)₂]Br and kappa-(ET)₂Cu(NCS)₂

We present high resolution heat capacity measurements for the organic superconductors kappa-(ET)_2Cu[N(CN)_2]Br and kappa-(ET)_2Cu(NCS)_2 in fields up to 14 T. We use the high field data to determine the normal state specific heat and hence extract the behavior of the electronic specific heat C_{el} in the superconducting state in zero and finite fields. We find that in both materials for T/T_c<0.3, C_{el}(H=0)\sim T^2 indicating d-wave superconductivity. Our data are inconsistent with s-wave behavior, but may be fitted to a strong coupling d-wave model over the full temperature range.