Magnetic field - dependent Labusch parameter in LiFeAs superconductor from Campbell penetration depth

TL;DR

This study measures the magnetic field-dependent Labusch parameter in LiFeAs superconductors using Campbell penetration depth, revealing insights into vortex pinning, critical current density, and vortex behavior similar to cuprates.

Contribution

It introduces a method to extract true critical current density from Campbell penetration depth and analyzes the magnetic field dependence of the Labusch parameter in LiFeAs.

Findings

Non-parabolic effective pinning potential inferred from field-dependent Labusch parameter.

Field dependence of pinning potential shows evolution from strong to collective pinning.

Estimated critical current density at 2K is approximately 2×10^6 A/cm².

Abstract

A true critical current density, $j_{c}$, as opposite to commonly measured relaxed persistent (Bean) current, $j_{B}$, was extracted from the Campbell penetration depth, $\lambda_{C}(T,H)$ measured in single crystals of LiFeAs. The effective pinning potential is non-parabolic, which follows from the magnetic field - dependent Labusch parameter $\alpha$. At the equilibrium (upon field - cooling), $\alpha(H)$ is non-monotonic, but it is monotonic at a finite gradient of the vortex density. This behavior leads to a faster magnetic relaxation at the lower fields and provides a natural \emph{dynamic} explanation for the fishtail (second peak) effect. We also find the evidence for strong pinning at the lower fields. The inferred field dependence of the pinning potential is consistent with the evolution from strong pinning, through collective pinning and, eventually, to a disordered vortex lattice. The values of $j_{c}(2\text{K}) \simeq 2\times10^{6}$ A/cm$^{2}$ provide an upper estimate of the current carrying capability of LiFeAs. Overall, vortex behavior of almost isotropic, fully-gapped LiFeAs is very similar to highly anisotropic d-wave cuprate superconductors, the similarity that requires further studies in order to understand unconventional superconductivity in cuprates and pnictides.