Composite Flux-Line Lattices Stabilized in Superconducting Films by a Regular Array of Artificial Defects

Abstract

Flux creep rate and critical current have been measured in superconducting Pb $/$Ge multilayers with a square lattice of submicron holes. The new composite flux-line lattices consisting of single or multiquanta vortices at holes and single cortices at interstitial positions have been identified. The ``metallic'' behavior of the flux lines at the interstices at temperatures close to the critical temperature leads to a first-order phase transition analogous to the Mott insulator-metal transition for flux lines, introduced by Nelson and Vinokur.