Probing unconventional superconducting symmetries using Josephson interferometry

We study Josephson junctions formed at grain boundary interfaces in the high-T_c cuprate materials. The supercurrents in these junctions depend strongly on the phase anisotropy of the order parameter and on the electronic structure at the interface and can be used to determine the pairing symmetry as well as the physical structure of the grain boundary interface. Andreev reflection at the (1 1 0) 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, possibly allowing the formation of a subdominant pairing phase with a complex order parameter that is characterized by broken time-reversal symmetry. A similar phenomenon is predicted to occur in the bulk material near magnetic impurities. Calculations demonstrate that the temperature and magnetic field dependence of the critical current of 45-degree 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 fabricated from pure and magnetically doped cuprate films in an effort to observe the onset of these complex order parameters. Profound departure from the usual sinusoidal current-phase relation have also been predicted for the cuprate materials. We show evidence for a second-order Josephson component in the critical current diffraction patterns of BSCCO junctions.