High-resolution ac-calorimetry studies of the quasi-two- dimensional organic superconductor kappa-(BEDT- TTF)2Cu(NCS)2

We report high-resolution specific-heat measurements on the quasi-two-dimensional organic superconductor $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}(\mathrm{NCS}{)}_{2},$ 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 $T\ensuremath{\ll}{T}_{c}.$ The latter finding is incompatible with an order parameter that vanishes at certain parts of the Fermi surface. The data for $Tl~{T}_{c}$ can be well described by a strong-coupling extension of the BCS theory \char22{} the $\ensuremath{\alpha}$-model \char22{} similar to what has been recently found for the $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}$ salt [Elsinger et al., Phys. Rev. Lett. 84, 6098 (2000)].