$^{75}$As and $^{23}$Na NMR study in optimally Co doped NaFe$_{0.975}$Co$_{0.025}$As

The normal and superconducting state of NaFe$_{0.975}$Co$_{0.025}$As, single crystals with exceptionally narrow $^{75}$As and $^{23}$Na NMR spectra, were investigated in external magnetic fields from 6.4 T to 24 T. The Knight shift ($^{75}K$) shows an almost linear decrease in the normal state on cooling and a sharp transition to spin-singlet superconductivity below $T_c$. A temperature independent $^{75}K$ at low temperature, below 0.4 $T_c$, indicates that there are no gap nodes. The penetration depth, $\lambda_{ab}$, was found to be 456 $\pm$ 7 nm at zero temperature, after convoluting the normal state spectrum with the vortex field distribution expected from Ginzburg-Landau theory. The spin lattice relaxation rate, $1/T_1$, shows a T$^{3}$ behavior in the superconducting state at low field which becomes T$^{1.5}$ at higher fields. Additionally, the average of $1/T_1$ over the vortex unit cell at T = 4 K is linear in H$^2$.