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

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