High-resolution ac-calorimetry studies of the quasi-two-dimensional organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 -: art. no. 140509

We report high-resolution specific-heat measurements on the quasi-two-dimensional organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2), performed by using a sensitive ac-modulation technique. The main observations are (i) a discontinuity at T-c much in excess of what is expected for a weak-coupling BCS superconductor and (ii) a quasiparticle contribution to the specific heat with an exponentially weak temperature dependence at T<T-c. The latter finding is incompatible with an order parameter that vanishes at certain parts of the Fermi surface. The data for Tless than or equal toT(c) can be well described by a strong-coupling extension of the BCS theory - the alpha-model - similar to what has been recently found for the kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br salt [Elsinger , Phys. Rev. Lett. 84, 6098 (2000)].