  Square arrays of sub-micrometer columnar defects in thin
YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ (YBCO) films with spacings down to 300 nm have
been fabricated by a He ion beam projection technique. Pronounced peaks in the
critical current and corresponding minima in the resistance demonstrate the
commensurate arrangement of flux quanta with the artificial pinning landscape,
despite the strong intrinsic pinning in epitaxial YBCO films. Whereas these
vortex matching signatures are exactly at predicted values in field-cooled
experiments, they are displaced in zero-field cooled, magnetic-field ramped
experiments, conserving the equidistance of the matching peaks and minima.
These observations reveal an unconventional critical state in a cuprate
superconductor with an artificial, periodic pinning array. The long-term
stability of such out-of-equilibrium vortex arrangements paves the way for
electronic applications employing fluxons.
