Vortex relaxation and coupling in superconducting heterostructures studied by STM

TL;DR

This study investigates how vortex lattice order in superconducting heterostructures is affected by layer thickness and interface properties, revealing the roles of vortex line bending and Josephson coupling in vortex disorder restoration.

Contribution

It provides direct imaging evidence of vortex lattice reordering mechanisms in superconducting heterostructures with varying interface conditions and layer thicknesses.

Findings

Vortex lattice disorder is influenced by the thickness of the weakly pinning layer.

Clean interfaces delay vortex order restoration until the first bending mode.

Insulating interfaces restore vortex order more rapidly.

Abstract

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.