Anisotropy of strong pinning in multi-band superconductors

TL;DR

This paper investigates how anisotropy affects the critical current density in multi-band iron-based superconductors, considering both weak and strong pinning limits, and explains recent experimental observations.

Contribution

It introduces a phenomenological model distinguishing anisotropy factors for penetration depth and coherence length, explaining the anisotropic behavior of critical currents in different superconductors.

Findings

In highly anisotropic materials, the critical current's angular dependence is dominated by coherence-length anisotropy.

Strong pinning can cause an apparent inversion of anisotropy in less anisotropic superconductors.

The ratio of critical current densities along different axes directly measures the coherence length anisotropy.

Abstract

The field-angular dependence and anisotropy of the critical current density in iron-based superconductors is evaluated using a phenomenological approach featuring distinct anisotropy factors for the penetration depth and the coherence length. Both the weak collective pinning limit, and the strong pinning limit relevant for iron-based superconductors at low magnetic fields are considered. It is found that in the more anisotropic materials, such as SmFeAsO and NdFeAsO, the field-angular dependence is completely dominated by the coherence-length (upper-critical field) anisotropy, thereby explaining recent results on the critical current in these materials. In less anisotropic superconductors, strong pinning can lead to an apparent inversion of the anisotropy. Finally, it is shown that, under all circumstances, the ratio of c-axis and ab-plane critical current densities for magnetic field along the ab-plane directly yields the coherence length anisotropy factor {\epsilon}{\xi}.