Flux Phases and Quantized Pinning Force in Superconductor with a Periodic Lattice of Pinning Centres

Flux phases have been studied in superconducting Pb/Ge multilayers with a square lattice of submicrometre holes. Sharp peaks in the magnetic-field dependence of the critical current have been observed at integer H m and rational H k/l matching fields. At integer fields an artificial "flux line crystal" is formed, consisting of a stable square lattice of single or multiple flux quanta trapped by the holes. The existence of such a crystal leads to the quantization of the pinning force. New rational flux lines patterns at H = H k/l , qualitatively different from those previously seen in superconducting networks, have been identified.