Phase behavior of the organic superconductors κ-(BEDT-TTF)2Cu[N(CN)2]X (X= Br and Cl) studied by ac calorimetry

Temperature dependence of heat capacities of organic superconductors, kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]X (X = Br and Cl), was measured by chopped-light ac calorimetry between 10 and 300 K. A small anomaly was clearly detected at the superconducting transition in the Br salt. For the Cl salt, a slight anomaly was observed at 45 K where the spin-density wave-like transition was previously suggested to occur, while there was no anomaly at the magnetic phase transition temperature to weak ferromagnetism around 22 K. In both salts, the steplike anomalies were observed around 100 K. The temperature of this anomaly depends on the measurement frequency. The anomalies are attributed to a glass transition caused by freezing of the motion of the ethylene moiety in BEDT-TTF molecule. The glass transition plays no important role in the electrical properties for the Cl salt, but does affect the electronic properties including superconducting properties for the Br salt.