Scanning SQUID microscopy of sparsely twinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Magnetic images of two sparsely twinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals were taken in low applied magnetic fields (${\mathrm{H}}_{\mathrm{a}}$<100 mG) with a scanning SQUID microscope. The SQUID sensors have 4 or 8 μm pickup loops and a magnetic flux resolution of ${10}^{\mathrm{-}3}$hc/2e under normal operating conditions. The images showed Meissner screening with a large demagnetization factor and isolated trapped vortices. The trapped vortices carried integral (hc/2e) magnetic flux and were much smaller than the spatial resolution of the sensor. At this level of resolution, there was no evidence for unusual magnetic features, such as fractional vortices, associated with the twin boundaries.