Josephson interferometry measurements in high-T(c) grain boundary junctions

We study Josephson junctions formed at grain boundary interfaces in the high-Tc cuprate materials. The inherent properties of these Josephson junctions depend strongly on the pairing symmetry and the electronic structure at the interface and can be used to determine the pairing symmetry as well as the structure of the grain boundary interface. Andreev reflection at the (110) surface in d-wave superconductors leads to the formation of zero-energy quasiparticle surface states. It is predicted that these surface states lead to a suppression of the d-wave order parameter and formation of a subdominant pairing phase with a complex order parameter characterized by broken time-reversal symmetry. A similar phenomenon is predicted to occur in the bulk film near magnetic impurities. Calculations demonstrate that the temperature and magnetic field dependence of the critical current of 45° asymmetric grain boundary Josephson junctions are extremely sensitive to the onset of a complex order parameter. We have measured the critical current behavior of grain boundary junctions of YBCO and BSCCO as well as Ca, Co, Ni, and Pr doped YBCO in an effort to observe the onset a complex order parameter. We see no conclusive evidence for the existence of broken time-reversal symmetry due to a complex order parameter. We do distinct see evidence of second-order Josephson tunneling in a 45° asymmetric BSCCO junction in the form of a critical current peak at zero applied magnetic field. The modulation length of this peak is half that of the others and it exhibits a distinct temperature dependence consistent with a second-order sin(2&phis;) critical current component.