Low-temperature heat capacity measurements of κ-type organic superconductors under pressure

Thermodynamic measurements using single crystals of κ-type organic superconductors have been performed under pressure and with magnetic field. We have adopted the ac temperature modulation technique for detecting relative variation of heat capacity as a function of temperature, since usual adiabatic condition inevitable for getting absolute values of heat capacities is difficult to attain under pressure. We used small ruthenium oxide chips as a heater and a thermometer and adhered them on the both side of single crystal and put them inside the Cu–Be cramp type pressure cell. The broad hump structure around 9 K which may be attributable to the thermal anomaly related to the superconductive transition was observed in κ-(BEDT-TTF)2Cu(NCS)2. However, the worse sensitivity of ruthenium oxide sensor (100 Ω at room temperature) in this temperature region make it difficult to separate the peak from the large background contribution. In the case of κ-[(BEDT-TTF)0.9(BEDSe-TTF)0.1]2Cu[N(CN)2]Br, we have observed a sharp peak in the heat capacity at ambient pressure and its downward shift at 3 kbar. The application of magnetic fields in the parallel direction to the superconductive layers was found to suppress the peak structure. The behaviors are consistent with the typical nature of 2D superconductors and the electronic phase diagram of κ-type organic salts.