Spin-pairing and penetration depth measurements from nuclear magnetic resonance in NaFe$_{0.975}$Co$_{0.025}$As

TL;DR

This study uses NMR measurements on NaFe$_{0.975}$Co$_{0.025}$As to confirm spin-singlet superconductivity and determine the penetration depth, aligning with BCS theory predictions.

Contribution

It provides the first NMR evidence of spin-singlet pairing and measures the penetration depth in this specific iron-based superconductor.

Findings

Superconductivity is a spin-singlet state consistent with BCS theory.

The temperature dependence of spin susceptibility follows the Yosida function.

The superconducting penetration depth at zero temperature is approximately 5327 Å.

Abstract

We have performed $^{75}$As nuclear magnetic resonance (NMR) Knight shift measurements on single crystals of NaFe$_{0.975}$Co$_{0.025}$As to show that its superconductivity is a spin-paired, singlet state consistent with predictions of the weak-coupling BCS theory. We use a spectator nucleus, $^{23}$Na, uncoupled from the superconducting condensate, to determine the diamagnetic magnetization and to correct for its effect on the $^{75}$As NMR spectra. The resulting temperature dependence of the spin susceptibility follows the Yosida function as predicted by BCS for an isotropic, single-valued energy gap. Additionally, we have analyzed the $^{23}$Na spectra that become significantly broadened by vortices to obtain the superconducting penetration depth as a function of temperature with $\lambda_{ab}(0) = 5,327 \pm$ 78$\,\AA$.