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- $T_c$ superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ grown on tricrystal substrates of SrTi${\mathrm{O}}_3$. 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- $T_c$ film. In addition, our images provide the first direct measurement of the Josephson penetration depth.