High-resolution ac-calorimetry studies on κ-(ET)2Cu(NCS)2: a strong-coupling superconductor with finite energy gap

We report high-resolution heat capacity measurements on κ-(ET)2Cu(NCS)2 utilising an ac-modulation technique. The quasi-particle contribution to the specific heat in the superconducting state, Ces, can be well described by a strong-coupling BCS model implying an exponentially weak temperature dependence at T⪡Tc. This result rules out an order parameter with zeroes on the Fermi surface.