Integer and half-integer flux-quantum transitions in a niobium/iron-pnictide loop

TL;DR

This study presents phase-sensitive evidence of sign-changing s-wave pairing in iron-pnictide superconductors by observing flux-quantum transitions in niobium/iron-pnictide loops, supporting theories of unconventional superconductivity.

Contribution

It introduces a novel phase-sensitive technique to detect pairing symmetry and provides the first experimental evidence of sign reversal in the order parameter of NdFeAsO0.88F0.12.

Findings

Observation of integer and half-integer flux-quantum transitions.

Evidence supporting sign-changing s-wave pairing symmetry.

Implications for understanding the mechanism of pnictide superconductivity.

Abstract

The recent discovery of iron-based superconductors challenges the existing paradigm of high-temperature superconductivity. Owing to their unusual multi-orbital band structure, magnetism, and electron correlation, theories propose a unique sign reversed s-wave pairing state, with the order parameter changing sign between the electron and hole Fermi pockets. However, because of the complex Fermi surface topology and material related issues, the predicted sign reversal remains unconfirmed. Here we report a novel phase-sensitive technique for probing unconventional pairing symmetry in the polycrystalline iron-pnictides. Through the observation of both integer and half-integer flux-quantum transitions in composite niobium/iron-pnictide loops, we provide the first phase-sensitive evidence of the sign change of the order parameter in NdFeAsO0.88F0.12, lending strong support for microscopic models predicting unconventional s-wave pairing symmetry. These findings have important implications on the mechanism of pnictide superconductivity, and lay the groundwork for future studies of new physics arising from the exotic order in the FeAs-based superconductors.