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

摘要: 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) discontinuity at ${T}_{c}$ much in excess of what is expected for weak-coupling BCS and (ii) quasiparticle contribution to specific heat with an exponentially weak temperature dependence $T\ensuremath{\ll}{T}_{c}.$ latter finding incompatible order parameter that vanishes certain parts Fermi surface. data $Tl~{T}_{c}$ can be well described strong-coupling extension theory --- $\ensuremath{\alpha}$-model similar has been recently found $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}$ salt [Elsinger et al., Phys. Rev. Lett. 84, 6098 (2000)].