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 image conventional (h/2e) Josephson vortices trapped in grain boundaries, and half-integer (h/4e) Josephson vortices trapped at the tricrystal point, of the high- Tc superconductor YBa2Cu3O7-δ grown on tricrystal substrates of SrTiO3. Our observation of the half-integer vortex at the tricrystal point is the first direct demonstration of the positive paramagnetic Meissner effect in a polycrystalline high- Tc film. In addition, our images provide the first direct measurement of the Josephson penetration depth. © 1996 American Physical Society.