DIRECT IMAGING OF INTEGER AND HALF-INTEGER JOSEPHSON VORTICES IN HIGH-T-C GRAIN-BOUNDARIES

We have used a high-resolution scanning SQUID microscope to directly i mage conventional (h/2e) Josephson vortices trapped in grain boundarie s, and half-integer (h/4e) Josephson vortices trapped at the tricrysta l point, of the high-T-c superconductor YBa2Cu3O7-delta grown on tricr ystal substrates of SrTiO3. Our observation of the half-integer vortex at the tricrystal point is the first direct demonstration of the posi tive paramagnetic Meissner effect in a polycrystalline high-T-c film. In addition, our images provide the first direct measurement of the Jo sephson penetration depth.