Angular dependence of the critical current in thin YBa2Cu3O7-δ films with unidirectional nanocracks

We investigated the angular dependence of the critical current I-c in the a-b plane of thin YBa2Cu3O7-delta films on (001) NdGaO3. Films grown on this particular substrate relieve their stress by developing unidirectional cracks of 2-4 nm width. These unidirectional nanocracks (NC's) behave as Josephson junctions. We have measured I-c(theta(NC)), where theta(NC) is the angle to these nanocracks, in a set of microbridges that were patterned on the film at various angles theta(NC) We fitted the results to a model of Andreev reflections in an anisotropic superconductor given by Tanaka and Kashiwaya [Phys. Rev. B 56, 892 (1997)]. We found that an order parameter having a d + is symmetry fits our data best at high temperatures, while no satisfactory fit could be obtained at low temperatures using a pure d-wave, s-wave, or d + is order parameter.