THE FLUX PATTERN IN THE HIGH-TC SUPERCONDUCTOR YBA2CU3O7 STUDIED BY Y-89-NMR

The field distribution in the superconducting state of c-axis oriented powders of YBa2Cu3O7 has been probed by Y-89-NMR in a field of B = 7.5 T. For B parallel-to c-axis line broadening is only detected below 83 K, i.e. 10% lower than the zero field transition temperature T(c)(0) congruent-to 92 K. The temperature dependence of the linewidth (DELTA) is well described by a two fluid formula DELTA infinity (1 - t4) with t = T/T(n). The normalization temperature T(n) is associated with the transition from a molten flux state to a flux lattice or flux glass. The ratio factor 2 of the low T linewidths for B parallel-to c and B perpendicular-to c is explained by a misorientation of a few degrees in the perpendicular-to-direction.