Distinguishing $s^{\pm}$ and $s^{++}$ electron pairing symmetries by neutron spin resonance in superconducting NaFe$_{0.935}$Co$_{0.045}$As

TL;DR

This study uses inelastic neutron scattering to identify a sharp resonance in a superconducting iron pnictide, providing evidence that supports the $s^{ u}$-pairing symmetry over the $s^{++}$ model, thus clarifying the pairing mechanism.

Contribution

The paper demonstrates that the neutron spin resonance observed in NaFe$_{0.935}$Co$_{0.045}$As supports the $s^{ u}$-pairing symmetry, distinguishing it from the $s^{++}$-pairing symmetry through experimental data and theoretical comparison.

Findings

A sharp neutron spin resonance at 7 meV was observed below $T_c$.

The resonance energy remains nearly constant as temperature approaches $T_c$.

Results are consistent only with the $s^{ u}$-pairing symmetry, not $s^{++}$.

Abstract

A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechansim for superconductivity. For iron pnictide superconductors, the $s^\pm$-pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies ($E\leq 2\Delta$) below $T_c$. On the other hand, the $s^{++}$-pairing symmetry expects a broad spin excitation enhancement at an energy above $2\Delta$ below $T_c$. Although the resonance has been observed in iron pnictide superconductors at an energy below $2\Delta$ consistent with the $s^\pm$-pairing symmetry, the mode has also be interpreted as arising from the $s^{++}$-pairing symmetry with $E\ge 2\Delta$ due to its broad energy width and the large uncertainty in determining the SC gaps. Here we use inelastic neutron scattering to reveal a sharp resonance at E=7 meV in SC NaFe$_{0.935}$Co$_{0.045}$As ($T_c = 18$ K). On warming towards $T_c$, the mode energy hardly softens while its energy width increases rapidly. By comparing with calculated spin-excitations spectra within the $s^{\pm}$ and $s^{++}$-pairing symmetries, we conclude that the ground-state resonance in NaFe$_{0.935}$Co$_{0.045}$As is only consistent with the $s^{\pm}$-pairing, and is inconsistent with the $s^{++}$-pairing symmetry.