Direct imaging of integer and half-integer Josephson vortices in high-Tc 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 ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ 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.