Site sensitive measurement of local fluctuation of driven vortex density in Bi2Sr2CaCu2Oy

Noise in the local vortex density was investigated in a site-sensitive mann er, which means we can measure the cross-correlation function, together wit h the auto-correlation function, in the directions both parallel and perpen dicular to the flow direction. We found that plastic flow was realized just when the vortices started moving, whereas it changes in to the coherent fl ow with further increasing driving current. The total features of our resul ts, including the variation as a function of temperature and dependence of the cross-correlation on the direction relative to the flow direction, agre e with a recent computer simulation by Olson et al. very well. We also meas ured the conduction noise in the same material, which enables us to discuss both the density-density correlation and velocity-velocity correlation in a comparative way.