Spontaneous symmetry breaking in vortex systems with two repulsive lengthscales.

J S Moodera,J.-B Laloë,W M Desoky,S J Bending,M V Milos̆Ević,P J Curran,A Chaves

doi:10.1038/srep15569

Abstract

Scanning Hall probe microscopy (SHPM) has been used to study vortex structures in thin epitaxial films of the superconductor MgB2. Unusual vortex patterns observed in MgB2 single crystals have previously been attributed to a competition between short-range repulsive and long-range attractive vortex-vortex interactions in this two band superconductor; the type 1.5 superconductivity scenario. Our films have much higher levels of disorder than bulk single crystals and therefore both superconducting condensates are expected to be pushed deep into the type 2 regime with purely repulsive vortex interactions. We observe broken symmetry vortex patterns at low fields in all samples after field-cooling from above Tc. These are consistent with those seen in systems with competing repulsions on disparate length scales, and remarkably similar structures are reproduced in dirty two band Ginzburg-Landau calculations, where the simulation parameters have been defined by experimental observations. This suggests that in our dirty MgB2 films, the symmetry of the vortex structures is broken by the presence of vortex repulsions with two different lengthscales, originating from the two distinct superconducting condensates. This represents an entirely new mechanism for spontaneous symmetry breaking in systems of superconducting vortices, with important implications for pinning phenomena and high current density applications.

Highlights

Scanning Hall probe microscopy (SHPM) has been used to study vortex structures in thin epitaxial films of the superconductor MgB2
Observations of vortex structures in MgB2 have been limited to ultra-pure single crystals which are assumed to be in the clean superconducting limit ( ξ) but it is insightful to systematically tune these parameters via the controlled growth of MgB2 thin films, whose level of disorder is a function of film thickness
An initial visual inspection of the sequence of fields in the 160 nm film, (a) to (d), shows symmetry breaking at all fields where the predominant vortex patterns can be classed as an evolution from: dimers, coexistence of dimers and chains, chains with a predominant stripe direction, and a labyrinth-like structure at the highest field

Summary

Introduction

Scanning Hall probe microscopy (SHPM) has been used to study vortex structures in thin epitaxial films of the superconductor MgB2. These are consistent with those seen in systems with competing repulsions on disparate length scales, and remarkably similar structures are reproduced in dirty two band Ginzburg-Landau calculations, where the simulation parameters have been defined by experimental observations This suggests that in our dirty MgB2 films, the symmetry of the vortex structures is broken by the presence of vortex repulsions with two different lengthscales, originating from the two distinct superconducting condensates. Bitter decoration and scanning probe measurements on single crystals of the two-band superconductor MgB2 have revealed unconventional vortex patterns[16,17,18,19], consistent with those seen in systems of particles interacting with non-monotonic potentials[20] This material appears to be a model system for the investigation of vortex structures interacting over two independent length scales. Observations of vortex structures in MgB2 have been limited to ultra-pure single crystals which are assumed to be in the clean superconducting limit ( ξ) but it is insightful to systematically tune these parameters via the controlled growth of MgB2 thin films, whose level of disorder is a function of film thickness

Methods

Results

Discussion

Conclusion