Thermodynamic Measurements of Organic Superconductor κ-(BEDT-TTF)2Cu(NCS)2 under Pressure

We have measured heat capacities of organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 under several pressures. This compound shows superconductive transition around 10 K, and we have detected an anomaly of the superconductive transition at about 9.4 K under ambient pressure. The transition temperature shifts to lower temperatures under 2.5 kbar and 4 kbar. Simultaneously, the peak-size changes from large one to small one and the peak-shape changes from sharp one to broader one with the decrease of transition temperature. The differences of peak-size and peak-shape may be related to stability of superconductivity in the κ-(BEDT-TTF)2X phase diagram. We think these changes imply superconducting phase changes from strong coupling regime to weak coupling regime

(by Y.Muraoka & Y. Nakazawa)

	Fig. 1. Block diagram of the detection system of the high–pressure ac calorimetry.
	Fig. 2. Temperature dependence of the heat capacity of κ–(BEDT–TTF)2Cu(NCS)2 under ambient pressure obtained under 0 T, 2 T, and 7 T. The thermal anomaly under 0 T is suppressed with increasing magnetic field.
	Fig. 3. Temperature and external pressure dependence of ΔCpT−1 of κ–(BEDT–TTF)2Cu(NCS)2. ΔCpT−1 is determined by a difference of 0 T and 7 T data. It is to be noted that the size and the shape of ΔCpT−1 become broader under pressures.