Dynamics of vortices and quasiparticles in the mixed state of Bi2Sr2CaCu2Oy

Local fluctuation of vortex density and velocity fluctuation of vortices un der finite driven current were investigated in Bi2Sr2CaCu2Oy at various reg ions in the H-T plane. For the local-density noise, we found that the narro w-band noise (NBN) showed a definite spatial correlation in the flow direct ion, whereas the broad-band noise (BBN) showed a rather local nature in bot h directions. Thus, we concluded that the NBN was generated by the macrosco pic defects, and the BBN was generated by the bulk pinning centers. Our exp erimental results suggest a gradual transition from the plastic flow to the uniform flow with increasing magnetic field, and the experimentally obtain ed "phase diagram" is in good agreement with the prediction of a numerical simulation. We also found that the conduction noise shows a similar behavio r as the density noise. To discuss the electronic structure of quasiparticl es at the vortex core, we measured surface impedance around the vortex-phas e boundaries. A jump in the surface reactance was found around the vortex m elting field, while there were no anomalies in the surface resistance. We s peculate that this phenomenon is related to the d-wave superconductivity. I n our surface impedance data, we did not find any indication of the phase t ransition suggested from the thermal conductivity data. (C) 2000 Elsevier S cience B.V. All rights reserved.