Spin pairing and penetration depth measurements from nuclear magnetic resonance in NaFe<inf>0.975</inf>Co<inf>0.025</inf>As

We have performed 75As nuclear magnetic resonance (NMR) Knight shift measurements on single crystals of NaFe0.975Co0.025As 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, 23Na, uncoupled from the superconducting condensate, to determine the diamagnetic magnetization and to correct for its effect on the 75As 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 23Na spectra that become significantly broadened by vortices to obtain the superconducting penetration depth as a function of temperature with λab(0)=5327±78 Å. © 2013 American Physical Society.