Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped NaFe$_{0.985}$Co$_{0.015}$As

TL;DR

This study uses inelastic neutron scattering to investigate spin excitations in partially detwinned NaFe$_{0.985}$Co$_{0.015}$As, revealing that the observed double resonance is due to superconducting gap anisotropy rather than static antiferromagnetic order.

Contribution

It demonstrates that the double resonance in spin excitations arises from superconducting gap anisotropy, not from static or fluctuating antiferromagnetic order.

Findings

Double resonance persists at both wave vectors in detwinned samples.

Double resonance is linked to superconducting gap anisotropy.

Results exclude static AF order as the cause of the double resonance.

Abstract

We use inelastic neutron scattering (INS) to study the spin excitations in partially detwinned NaFe$_{0.985}$Co$_{0.015}$As which has coexisting static antiferromagnetic (AF) order and superconductivity ($T_c=15$ K, $T_N=30$ K). In previous INS work on a twinned sample, spin excitations form a dispersive sharp resonance near $E_{r1}=3.25$ meV and a broad dispersionless mode at $E_{r1}=6$ meV at the AF ordering wave vector ${\bf Q}_{\rm AF}={\bf Q}_1=(1,0)$ and its twinned domain ${\bf Q}_2=(0,1)$. For partially detwinned NaFe$_{0.985}$Co$_{0.015}$As with the static AF order mostly occurring at ${\bf Q}_{\rm AF}=(1,0)$, we still find a double resonance at both wave vectors with similar intensity. Since ${\bf Q}_1=(1,0)$ characterizes the explicit breaking of the spin rotational symmetry associated with the AF order, these results indicate that the double resonance cannot be due to the static and fluctuating AF orders, but originate from the superconducting gap anisotropy.