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

1.用超声测量方法研究了有机超导体kappa- (BEDT-TTF) _2Cu (NCS) _2的涡旋态。纵声速的场依赖性明显下降。倾斜的量与余角成正比，其中余角是声音传播方向与磁场之间的夹角。声速异常用超导层间约瑟夫森耦合的热波动来解释。这种异常是由于该盐的Gruneisen参数较大造成的。β - (BEDT-TTF) _2I_3中超导转变的弹性异常通过低温退火已经知道β - (BEDT-TTF) _2I_3表现为第二次超导转变。到目前为止，这种盐中两种超导性的起源还没有弄清楚。在本研究项目中，我们可以观察到退火前2 K和退火后6 K的声速异常。声速异常可以用热力学关系的框架来解释。有机超导体的大单晶生长通过对（BEDT-TTF） _2I_3在一氯苯中多晶行为的平衡分析，研究了其晶体生长的机理。我们的研究清楚地表明，（BEDT-TTF） _2I_3晶体在溶剂中分解为中性的BEDT-TTF和I_2，处于平衡状态。结果表明，α - (BEDT-TTF) _2I_3在室温下比β盐更稳定。然而，在较低的温度下，-盐比-盐更稳定。这一事实使我们可以通过溶解度和生长温度来控制盐的多晶型。我们成功地生长出了α - (BEDT-TTF) _2I_3、β - (BEDT-TTF) _2I_3、kappa- (BEDT-TTF) _2Cu (NCS) _2和kappa- (BEDT-TTF) _2Cu [N (CN) _2] Br的大单晶。

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.