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23Na and 75As NMR Studies on NaFeAs0.8P0.2 Single Crystals

We report our $^{23}$Na and $^{75}$As NMR studies on isovalent phosphorus-doped NaFeAs$_{0.8}$P$_{0.2}$ ($T_c=$ 33 K) single crystals. Our data suggest a dramatic enhancement of the electron density of states on the Fermi surface, and a large residual electron density of state below $T_c$. However, evidence of antiferromagnetic spin fluctuations is shown by the spin-lattice relaxation rate. The penetration depth, revealed from $^{23}$Na NMR linewidth, follows the universal Uemura relation in iron pnictides. These observations suggest that the correlation effects are still important for the superconductivity in the phosphorus doped sample, although the Fermi surface is probably significantly changed upon doping.