The structure of thermal anomalies in association with superconductive transitions in κ-type BEDT-TTF superconductors is discussed. The κ-(BEDT-TTF)2X system has a typical example of dimer-Mott system with two-dimensional structure. The high Tc superconductive phase is neighboring to the antiferromagnetic insulating phase with a first order phase boundary. The transition temperature is the highest around the boundary region in superconductive phase and with the increase of pressure superconductivity is suppressed. The heat capacity peak shows a large peak with strong coupling behavior around the phase boundary region. However, it changes to weak coupling ones in the lower Tc sample. The consistency with the results of high-pressure calorimetry is discussed.
Fig. 1. Temperature dependence of electronic heat capacity around the superconductive transition temperature of κ-(BEDT-TTF)2Cu(NCS)2. The value of ΔCp / γTc is larger than BCS values and strong coupling behaviors are suggested from this data and other data already reported by several groups.
Fig. 2. Temperature dependence of heat capacity around the superconductive transition of κ-(BEDT-TTF)2Ag(CN)2H2O with relatively lower transition temperature than κ-(BEDT-TTF)2Cu(NCS)2.
Fig.3. Temperature dependence of superconductive transition of κ-(BEDT-TTF)2X system obtained using high pressure calorimeter.