SUPERCURRENT DENSITY CORRELATION-FUNCTION OF YBA2CU3O7-DELTA GRAIN-BOUNDARY JOSEPHSON-JUNCTIONS

We have measured the thermal, noise induced slope Rp(B) of the current -voltage characteristics of YBa2Cu3O7-delta Grain Boundary Josephson J unctions (GBJs) for I-->0 in magnetic fields up to B = 12 T. From the measured Rp(B) dependences the magnetic field dependence of the critic al current I-c(B) was derived in the large magnetic field range betwee n 0 and 12 T. Finally, the supercurrent density correlation function w as obtained from the I-c(B) dependence using the Wiener-Khinchine rela tion. By introducing the concept of the bright (BGCF) and dark ground correlation function (DGCF) a spatial filtering of the information on the supercurrent density distribution was obtained. The investigation of the BGCFs and the comparison to DGCFs gives clear evidence that sup ercurrent density has strong inhomogeneities on small length scales in the nanometer regime.