Nanocrack junctions in unidirectionally twinned thin films of YBa2Cu3O7-δ on (001) NdGaO3

We found unidirectional nanocracks in unidirectionally twinned films of YBa2Cu3O7-delta on NdGaO3 which are normal to the twin planes. In 70-250 nm thick films, transmission electron and atomic force microscopy show that these unidirectional nanocracks are found to be 1-2 mu m apart and have 2-4 nm width. Microbridges that cross these nanocracks behave as natural Josephson junctions. They show Shapiro steps under microwave irradiation and suppression of the critical current I-c. in magnetic fields. A very large in-plane anisotropy of the resistivities and I-c was observed and this was correlated with the nanocracks orientation, length, and him thickness. The observed anisotropy of I-c near T-c is shown to be consistent with a d+is wave symmetry of the order parameter in YBa2Cu3O7-delta. (C) 1998 American Institute of Physics. [S0003-6951(98)00151-X].