Dynamics vs electronic states of vortex core of high- Tc superconductors investigated by high-frequency impedance measurement

Abstract

Dynamics of vortices reflects the electronic states of quasiparticles in the core. To understand this, we investigated the following three issues. (1) We investigated the complex surface impedance, Z s, of YBa 2Cu 3O y (YBCO) as a function of magnetic field, H. The total features were well expressed by the Coffey–Clem model. From the data, we estimated the viscosity and pinning frequency, which were found to be independent of frequency. In particular, the obtained viscosity definitely shows that the core of vortex of YBCO is moderately clean. (2) An anomaly found in the surface reactance at the first order transition (FOT) of vortex lattice was investigated in Bi 2Sr 2CaCu 2O y with various doping levels. As a result, the anomaly was found only in the samples exhibiting the FOT. On the other hand, we did not observe the anomaly in YBCO. These suggest that the anomaly is due to the change in the electronic states of the vortices characteristic of materials with very strong anisotropy. (3) We measured H dependence of both the thermal conductivity κ( H) and Z s( H) in exactly the same pieces of crystal. We could not find any anomaly in Z s( H) even at the onset of the plateau. This result suggests that the origin of the plateau in κ( H) is not a drastic phase transition but is rather a gradual crossover.