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.

• Received 3 October 1995

DOI:https://doi.org/10.1103/PhysRevLett.74.3269