Vortex relaxation and coupling in superconducting heterostructures studied by STM

In a sandwich consisting of two superconducting films, one weakly pinning and one strongly pinning, the vortex positions in both films are determined by the strongly pinning material and the vortex lattice is disordered in both films. We used (strongly pinning) NbN and (weakly pinning) MoGe and studied, by directly imaging the vortex core positions with a scanning tunnelling microscope, how this disorder is restored with increasing thickness of MoGe layer or when the interface is made insulating. For clean interfaces we find that the first reordering of the vortex lattice is found at a layer thickness wich is compatible with the first bending mode of the vortex lines. Making the interface insulating we find that order is restored quickly. We argue that this is can be understood from the competition between the Josephson force working on the vortex segments on the one hand, and the elastic restoring forces inside the weakly pinning layer on the other hand.