  High-resolution scanning Hall probe microscopy has been used to image vortex
configurations in very large periodic arrays of artificial pinning sites.
Strong matching effects are seen at fields where either one or two vortices can
sit at a site; with three vortices per site, however, no clear matching is
observed. Matching effects have been also been observed at several fractional
multiples of the matching field, including 1/5, 1/4, 1/3, 1/2, and 3/4. These
fractional values are characterized by striking domain structure and grain
boundaries.
