Experimental Determination of the Symmetry of the Superconducting Pairing State in Yttrium Barium Copper Oxide.

In this thesis I present measurements of bimetallic Pb-Au-YBCO dc SQUIDs (Superconducting QUantum Interference Devices) and Josephson junctions that provide an important clue to the origin of the pairing mechanism for high temperature superconductivity in the cuprates. By probing the momentum space anisotropy of the phase of the superconducting order parameter, we can experimentally determine the symmetry of the superconducting pairing state in YBCO and distinguish between the leading candidate states that have been proposed. We find that the superconducting order parameter in YBCO changes sign between orthogonal directions in momentum space, unlike the isotropic order parameter in conventional superconductors. The sign change of the order parameter is consistent with a d_{x^2-y^2} pairing state symmetry.