Order parameter symmetry in the high-temperature superconducting cuprates

Novel mechanisms of superconductivity have been proposed to explain the behavior of the high-temperature cuprate superconductors (HTS). A number of these mechanisms require an unconventional symmetry for the superconducting order parameter. Our experiments utilize quasiparticle and Josephson tunneling to probe the symmetry of the superconducting state. Quasiparticle tunneling probes the density of excited states, and so can examine the magnitude but not the phase of the order parameter. Josephson coupling investigates the structure of the order parameter. Both of these techniques are very surface sensitive and probe the order parameter to within a coherence length of the junction interface. There can be selection rules for Josephson coupling if the junctions are made along certain symmetry axes of the crystals. In particular, coupling from a d-wave to s-wave superconductor should be zero if the junction axis is in the c-axis direction, perpendicular to the plane of the d-wave. We have fabricated sandwich Josephson junctions, between a HTS such as {dollar}rm YBasb2Cusb3Osb7{dollar} or {dollar}rm Bisb2Srsb2CaCusb2Osb8,{dollar} and conventional "low-temperature" superconductors such as Pb, to assess the order parameter symmetry along various crystallographic directions. We have also fabricated grain boundary junctions in {dollar}rm YBasb2Cusb3Osb7{dollar} to gain information concerning transport in twist and basal-plane-faced grain boundaries. The high-temperature superconductors are very susceptible to damage from exposure to atmosphere. In our studies we have either sealed off the HTS interfaces from the outside environment, or treated them and characterized them as much as possible. In both cases, we measured zero coupling between c-axis oriented HTS and Pb junctions, and a large coupling between a-axis HTS and Pb. We find this to be consistent with d-wave pairing.