Location-Sensitive Measurement of the Local Fluctuation of Driven Vortex Density in Bi$_2$Sr$_2$CaCu$_2$O$_y$

Abstract

To investigate the dynamics of driven vortices in superconductors, noise in the local vortex density was investigated in the mixed state of a high-$T_c$ superconductor, Bi$_2$Sr$_2$CaCu$_2$O$_y$, using a two-dimensional electron gas (2DEG) micro-Hall probe array. We studied the cross-correlation function, together with the auto-correlation function, both parallel and perpendicular to the direction of flow of the vortices. The broadband noise (BBN) did not have large spatial correlations. This suggests that the BBN is due to the fluctuation of the local vortex density generated by bulk pinning centers under the area of each probe. On the other hand, the narrow-band noise (NBN) (with the the time scale of the transit time of vortices) had large translational correlations. These definitely shows that the NBN was generated by semimacroscopic imperfections like the surface, and macroscopic line defects {\it etc.}. In relation to the dynamic phase diagram, large BBN was observed when the vortices started moving. The dependence of the spatial correlations on the direction of the array relative to the direction of the driving current suggested that plastic flow was present when the large BBN was observed. The gross features of our data agree well with some of the theoretically proposed dynamical phase diagrams of vortices in superconductors.