Anomalous Meissner effect in pnictide superconductors

TL;DR

This paper investigates the anomalous Meissner effect in pnictide superconductors, revealing a linear increase in flux expulsion with magnetic field, suggesting a novel field-driven suppression of magnetic pair-breaking.

Contribution

It reports the first observation of an unusual flux expulsion behavior in pnictide superconductors, contrasting with traditional expectations and proposing a new interpretation involving magnetic pair-breaking suppression.

Findings

Flux expulsion increases linearly with magnetic field up to 5.5 T.

Behavior differs from conventional type-II superconductors.

Possible field-driven suppression of magnetic pair-breaking.

Abstract

The Meissner effect has been studied in Ba(Fe0.926Co0.074)2As2 and Ba0.6K0.4Fe2As2 single crystals and compared to well known, type-II superconductors LuNi2B2C and V3Si. Whereas flux penetration is mostly determined by the bulk pinning (and, perhaps, surface barrier) resulting in a large negative magnetization, the flux expulsion upon cooling in a magnetic field is very small, which could also be due to pinning and/or surface barrier effects. However, in stark contrast with the expected behavior, the amount of the expelled flux increases almost linearly with the applied magnetic field, at least up to our maximum field of 5.5 T, which far exceeds the upper limit for the surface barrier. One interpretation of the observed behavior is that there is a field-driven suppression of magnetic pair-breaking.