Thermodynamic properties of quasi-two-dimensional organic superconductors

High-resolution specific-heat data of the organic superconductor κ-(ET)2Cu(NCS)2 (Tc=9.3K) can reasonably well be described by strong-coupling BCS theory. In line with our previous results for three other organic superconductors the electronic contribution of the specific heat vanishes exponentially towards low temperatures providing evidence for a fully-gapped order parameter. The thermal conductivity κ of κ-(ET)2Cu[N(CN)2]Br increases strongly in the superconducting state below Tc=11.5K indicating the substantial scattering of phonons by quasiparticles in the normal state. Down to the lowest temperatures (T≈0.25K) no T3 dependence of κ is observed.