Phase Behavior of Organic Superconductors, kappa-(BEDT-TTF)2Cu[N(CN)2]X (X = Br and Cl), Studied by ac Calorimetry

Hiroki AKUTSU, Kazuya SAITO and Michio SORAI

Temperature dependence of heat capacities of organic superconductors, kappa-(BEDT-TTF)2Cu[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 SDW 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 step-like 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.
(Phys. Rev. B, 61, 4346-4352 (2000))

Glass Transition in the Organic Superconductor with the Highest-Tc under Ambient Pressure, kappa-(ET)2Cu[N(CN)2]Br

Kazuya SAITO, Hiroki AKUTSU and Michio SORAI

A glass transition due to a freezing of intramolecular motion is reported for kappa-(ET)2Cu[N(CN)2]Br around 105 K on the basis of heat capacity spectroscopy utilizing chopped-light ac calorimetry on single crystal around 10 Hz. The glass transition twmperature well corresponds to the temperature below which the physical properties depends on the thermal history (e.g., cooling rate or annealing). The history dependence is reasonably resolved by taking into account the glass transition.
(Solid State Commun., 111, 471-475 (1999))

Metal-Insulator, SDW and Glass Transitions in (DMET)2BF4 and (DMET)2ClO4 Studied by ac Calorimetry

Hiroki AKUTSU, Kazuya SAITO, Yasuhisa YAMAMURA, Koichi KIKUCHI, Hiroyuki NISHIKAWA, Isao IKEMOTO, and Michio SORAI

Temperature dependence of the heat capacities of two organic conductors, (DMET)2BF4 and (DMET)2ClO4 was measured by chopped-light ac calorimetry between 10 and 300 K. A "Mott-like" nature of the metal-insulator transition around 32 K was confirmed by the absence of thermal anomaly. A magnetic transition showed around 18 K (or 19 K) an anomaly, the smallness of which is consistent with a suggested SDW formation. Besides the anomaly due to the SDW transition, a frequency-dependent step in heat capacity was detected around 110 K. The step-like anomaly is attributed to a glass transition due to the freezing of the intramolecular motion of the ethylene group in the BEDT-TTF moiety within DMET molecules.
(J. Phys. Soc. Jpn., 68, 1968-1974 (1999))

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