^{31}P and ^{75}As NMR evidence for a residual density of states at zero energy in superconducting BaFe_{2}(As_{0.67}P_{0.33})_{2}

Abstract

31P and 75As NMR measurements were performed in superconducting BaFe2(As0.67P0.33)2 with Tc=30 K. The nuclear-spin-lattice relaxation rate T 1−1 and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and T 1−1 in the superconducting (SC) state decreases without a coherence peak just below Tc, as observed in (Ba1−xKx)Fe2As2. In contrast to other iron arsenide superconductors, the T 1−1∝T behavior is observed below 4 K, indicating the presence of a residual density of states at zero energy. Our results suggest that strikingly different SC gaps appear in BaFe2(As1−xPx)2 despite a comparable Tc value, an analogous phase diagram, and similar Fermi surfaces to (Ba1−xKx)Fe2As2.