P-concentration dependence of the quasiparticle density of states in BaFe₂(As_1-xP_x)₂

³¹P-NMR measurements were performed in the iron pnictides BaFe2(As1-xPx)2 for 0.07 ≤ x ≤ 0.64 in order to investigate evolution of electronic state with isovalent substitution. In general, the spin part of the Knight shift Kspin and (T1T)^-1/2 are proportional to the quasiparticle density of states at the Fermi level N(EF) at high temperature where the Korringa relation holds. We found that Kspin and (T1T)^-1/2 are almost unaffected by the P-substitution, indicating that N(EF) is nearly constant. These results are quantitatively consistent with our band calculation. We claim that the slight changes of N(EF) by P-substitution in BaFe2(As1-xPx)2 is not enough to explain the Tc variation ranging from 30 K to 0 K, and thus suggest that antiferromagnetic correlations related to the quantum critical character are essential for the high Tc superconductivity.