Meissner response of a bulk superconductor with an embedded sheet of reduced penetration depth

TL;DR

This study models the susceptibility change in a superconductor with embedded sheets of reduced penetration depth, matching experimental observations and estimating local superfluid density enhancements.

Contribution

It introduces a numerical method to predict susceptibility stripes caused by twin planes in superconductors, linking theory with scanning SQUID microscopy data.

Findings

Predicted susceptibility stripes match experimental shapes.

Estimated enhanced superfluid density on twin planes.

Suggested twin planes have higher critical temperature than bulk.

Abstract

We calculate the change in susceptibility resulting from a thin sheet with reduced penetration depth embedded perpendicular to the surface of an isotropic superconductor, in a geometry applicable to scanning Superconducting QUantum Interference Device (SQUID) microscopy, by numerically solving Maxwell's and London's equations using the finite element method. The predicted stripes in susceptibility agree well in shape with the observations of Kalisky et al. of enhanced susceptibility above twin planes in the underdoped pnictide superconductor Ba(Fe1-xCox)2As2 (Ba-122). By comparing the predicted stripe amplitudes with experiment and using the London relation between penetration depth and superfluid density, we estimate the enhanced Cooper pair density on the twin planes, and the barrier force for a vortex to cross a twin plane. Fits to the observed temperature dependence of the stripe amplitude suggest that the twin planes have a higher critical temperature than the bulk, although stripes are not observed above the bulk critical temperature.