  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.

Superconductivity