SCANNING SQUID MICROSCOPE TESTS OF THE SYMMETRY OF THE HIGH-T-C GAP

Direct observations of the half-integer flux quantum effect in rings, disks, and blanket films of the high-Tc cuprate superconductors YBa2Cu 3O7-delta (YBCO), GdBa2Cu3O7-delta (GdBCO), Tl2Ba2CuO6+delta (Tl2201), and Bi2Sr2CaCu2O8+delta (BSCCO) with three tricrystal substrate geome tries show that their gaps have sign changes consistent with d-wave sy mmetry. The blanket geometry gives the Josephson penetration depth dir ectly, and provides a means for symmetry tests without patterning. Tl2 201 is a tetragonal, single-layer, cuprate without chains. When combin ed with photoemission results, our BSCCO results imply that the gap fo llows d(x2-y2) symmetry closely. Asymmetric 45 degree bicrystal grain boundary samples show spontaneous magnetization, as expected for d-wav e superconductors with interface roughness.