Analysis of the critical current density in grain boundary Josephson junctions on a nanometer scale

The critical current I_c of YBa₂Cu₃O_7-δ bicrystal grain boundary Josephson junctions GBJs) have been measured in igh magnetic fields up to B=12 T. Even at the highest applied magnetic fields, I_c varied periodically with varying magnetic field. By comparing the envelope of the measured I_c(B) dependencies to model calculations we obtain information on the spatial distribution of the supercurrent density j_c on a length scale down to below 1 nm. Our analysis gives clear evidence that j_c has spatial inhomogeneities on all length scales down to 1 nm with a probability distribution p(a) for the characteristic length scale a of the inhomogeneities.