Tricrystal Determination of the Symmetry of Cuprate Superconductors

Abstract

Epitaxial thin films of YBa_2Cu_3O_{7 - \delta} (YBCO) grown on substrates designed, for an assumed d-wave symmetry, to have an odd number of sign changes to the component of the order parameter normal to the grain boundaries, show flux quantization in half-integer multiples of \Phi_0 = h/2e at the tricrystal point. We have used a scanning SQUID microscope to directly image the magnetic fields associated with these half-integer flux quanta. Experiments with three different substrate geometries show that this effect results from the symmetry of the superconducting gap, and prove that the gap has sign changes consistent with d-wave symmetry. This effect occurs in rings, disks, and blanket coverage samples. The experiments with blanket samples provide a direct measurement of the Josephson penetration depth, and give a means for making symmetry tests that do not require lithographic patterning. Half-integer flux quantization also occurs for the single-layer, tetragonal cuprate superconductor Tl(2201), indicating that these effects are not due to the unusual properties of YBCO.