On the Decoupling of Layered Superconducting Films in Parallel Magnetic Field

TL;DR

This paper investigates the decoupling behavior of layered superconducting films under parallel magnetic fields using a frustrated XY model, revealing exponential vanishing of the critical field and crossover phenomena in magnetization.

Contribution

It provides a theoretical analysis of vortex lattice decoupling in layered superconductors, including the calculation of critical fields and magnetization crossover effects.

Findings

Parallel and perpendicular vortex lattices become independent at high fields.

The lower-critical field vanishes exponentially near decoupling transition.

Magnetization shows a crossover rather than a sharp transition.

Abstract

The issue of the decoupling of extreme type-II superconducting thin films ($\lambda_L\rightarrow\infty$) with weakly Josephson-coupled layers in magnetic field parallel to the layers is considered via the corresponding frustrated $XY$ model used to describe the mixed phase in the critical regime. For the general case of arbitrary field orientations such that the perpendicular magnetic field component is larger than the decoupling cross-over scale characteristic of layered superconductors, we obtain independent parallel and perpendicular vortex lattices. Specializing to the double-layer case, we compute the parallel lower-critical field with entropic effects included, and find that it vanishes exponentially as temperature approaches the layer decoupling transition in zero-field. The parallel reversible magnetization is also calculated in this case, where we find that it shows a cross-over phenomenon as a function of parallel field in the intermediate regime of the mixed phase in lieu of a true layer-decoupling transition. It is argued that such is the case for any finite number of layers, since the isolated double layer represents the weakest link.