Weak-coupling superconductivity in NaFe$_0.975$Co$_0.025$As from NMR Knight shift and penetration depth

Based on ARPES and STM work, Co doped NaFeAs has been reported to be a strongly coupled superconductor, similar to cuprates. However, a recent ARPES result, claims otherwise, with $2Δ/k_BT_c$ slightly beyond the BCS limit, 3.53. We have performed $^75$As and $^11$Na nuclear magnetic resonance(NMR) on high-quality, single crystals of NaFe$_0.975$Co$_0.025$As. The temperature dependence of the $^75$As Knight shift, fitted to a Yosida function, suggests moderately weak-coupling superconductivity in a singlet state with $2Δ/k_BT_c = 4.0±0.3$. The temperature dependence of the penetration depth obtained from $^11$Na NMR was found to be consistent with the Knight shift results and $λ_ab(0)$ was found to be $5,327 ± 78 \AA$.