Supercurrent Density Correlation Function of YBa2Cu3O7_δ Grain Boundary Josephson Junctions

We have measured the thermal noise induced slope Rp(B) of the current-voltage characteristics of YBa2Cu3O7_δ Grain Boundary Josephson Junctions (GBJs) for I →0 in magnetic fields up to B = 12 T. From the measured Rp(B) dependences the magnetic field dependence of the critical current Ic(B) was derived in the large magnetic field range between 0 and 12 T. Finally, the supercurrent density correlation function was obtained from the Ic(B) dependence using the Wiener-Khinchine relation. 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 supercurrent density has strong inhomogeneities on small length scales in the nanometer regime.