High-resolution ac-calorimetry studies of the quasi-two-dimensional organic superconductor (formula presented)

We report high-resolution specific-heat measurements on the quasi-two-dimensional organic superconductor (formula presented) performed by using a sensitive ac-modulation technique. The main observations are (i) a discontinuity at (formula presented) 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 (formula presented) The latter finding is incompatible with an order parameter that vanishes at certain parts of the Fermi surface. The data for (formula presented) can be well described by a strong-coupling extension of the BCS theory - the (formula presented)-model - similar to what has been recently found for the (formula presented) salt [Elsinger et al., Phys. Rev. Lett. 84, 6098 (2000)].