Dynamics of vortices and quasiparticles in the mixed state of Bi 2Sr 2CaCu 2O y

Abstract

Local fluctuation of vortex density and velocity fluctuation of vortices under finite driven current were investigated in Bi 2Sr 2CaCu 2O y at various regions in the H– T plane. For the local-density noise, we found that the narrow-band noise (NBN) showed a definite spatial correlation in the flow direction, whereas the broad-band noise (BBN) showed a rather local nature in both directions. Thus, we concluded that the NBN was generated by the macroscopic defects, and the BBN was generated by the bulk pinning centers. Our experimental results suggest a gradual transition from the plastic flow to the uniform flow with increasing magnetic field, and the experimentally obtained “phase diagram” is in good agreement with the prediction of a numerical simulation. We also found that the conduction noise shows a similar behavior as the density noise. To discuss the electronic structure of quasiparticles at the vortex core, we measured surface impedance around the vortex-phase boundaries. A jump in the surface reactance was found around the vortex melting field, while there were no anomalies in the surface resistance. We speculate that this phenomenon is related to the d-wave superconductivity. In our surface impedance data, we did not find any indication of the phase transition suggested from the thermal conductivity data.