Inelastic neutron and x-ray scattering as probes of the sign structure of the Fe-pnictide superconducting gap

TL;DR

This paper proposes using inelastic neutron or x-ray scattering to determine whether the superconducting gap in Fe-pnictides changes sign or not, addressing debates on the pairing symmetry in these materials.

Contribution

It introduces a method to distinguish between sign-changing and sign-preserving superconducting gaps using phonon spectral weight measurements.

Findings

Spectral weight of phonons at momentum Q can reveal gap sign structure.

The method can differentiate between $ ext{sign}( ext{gap})$ scenarios.

Provides a new experimental approach to study pairing symmetry.

Abstract

Neutron spin-flip scattering observations of a resonance in the superconducting state is often taken as evidence of an unconventional superconducting state in which the gap changes sign $\Delta(k+Q)=-\Delta(k)$ for momentum transfers $Q$ which play an important role in the pairing. Recently questions regarding this identification for the Fe-pnictide superconductors have been raised and it has been suggested that $\Delta(k+Q)=\Delta(k)$. Here we propose that inelastic neutron or x-ray scattering measurements of the spectral weight of a phonon of momentum $Q$ can distinguish between these two pairing scenarios.