Acoustic Phonon Properties of (BEDT-TTF) -based organic superconductors

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1.Vortex state in kappa- (BEDT-TTF) _2Cu (NCS) _2The vortex state of the organic superconductor kappa- (BEDT-TTF) _2Cu (NCS) _2 been investigated by ultrasonic measurements. The field dependence in the longitudinal sound velocity shows a remarkable dip. The amount to the dip is proportional to cosrheta, where rheta is the angle between the propagation direction of the sound and the magnetic field. The sound velocity anomaly is interpreted by the thermal fluctuation of the Josephson coupling between the superconducting layrs. Such anomaly is caused by a large Gruneisen parameter of this salt.2.Elastic anomaly at the superconducting transition in beta- (BEDT-TTF) _2I_3It has been known that beta- (BEDT-TTF) _2I_3 shows the second superconducting transition by low-temperature annealing. The origin of two types of superconductivity in this salt has not been made clear, so far. In this research project, we could observe the sound velocity anomalies at 2 K before annealing and at 6 K after annealing. The sound velocity anomalies could be interpreted in the frame of the thermodynamic relation.3.Large single crystal growth of organic superconductorsThe mechanism of crystal growth of (BEDT-TTF) _2I_3 has been investigated by the equilibrium analysis of polymorphic behavior in monochlorobenzene. Our study clearly indicates that (BEDT-TTF) _2I_3 crystals are decomposed to neutral BEDT-TTF and I_2 in the solvent, and that they are in the equilibrium state. It was found that alpha- (BEDT-TTF) _2I_3 is stabler that beta-salt at room temperature. However, beta-salt becomes stabler than alpha-salt at lower temperatures. This fact makes us to control tha polymorph of this salt by the solubility and the growth temperature. We could successfully grow large single crystals of alpha- (BEDT-TTF) _2I_3, beta- (BEDT-TTF) _2I_3, kappa- (BEDT-TTF) _2Cu (NCS) _2 and kappa- (BEDT-TTF) _2Cu [N (CN) _2] Br.