Vortex clusters and multiquanta flux lattices in thin films of anisotropic superconductors

TL;DR

This paper investigates equilibrium vortex structures in thin anisotropic superconducting films under tilted magnetic fields, revealing conditions for vortex clustering and multiquanta flux lattice formation due to altered vortex interactions.

Contribution

It introduces a detailed analysis of vortex configurations in anisotropic superconductors, highlighting the formation of vortex clusters and multiquanta flux lattices influenced by magnetic field tilting.

Findings

Tilted vortices can form clusters with sizes depending on tilt angle and film thickness.

Vortex-vortex interaction potential develops a minimum at certain tilt angles, leading to clustering.

Flux lattices with multiple flux quanta per unit cell can form, with phase transitions between different M states.

Abstract

The distinctive features of equilibrium vortex structures in thin films of anisotropic superconductors in tilted magnetic fields are studied for the limits of moderate and strong anisotropy. The energetically favorable shape of isolated vortex lines is found in the framework of two particular models describing these limiting cases: London theory with an anisotropic mass tensor and London-type model for a stack of Josephson--decoupled superconducting layers. The increase of the field tilting is shown to result in qualitative changes in the vortex--vortex interaction potential: the balance between long--range attractive and repulsive forces occurs to be responsible for a formation of a minimum of the interaction potential vs the intervortex distance. This minimum appears to exist only for a certain restricted range of the vortex tilting angles which shrinks with the decrease of the system anisotropy parameter. Tilted vortices with such unusual interaction potential form clusters with the size depending on the field tilting angle and film thickness or/and can arrange into multiquanta flux lattice. The magnetic flux through the unit cells of the corresponding flux line lattices equals to an integer number $M$ of flux quanta. Thus, the increase in the field tilting should be accompanied by the series of the phase transitions between the vortex lattices with different $M$.