75As-NQR/NMR studies on oxygen-deficient iron-based oxypnictide superconductors LaFeAsO1-y (y=0, 0.25, 0.4) and NdFeASO0.6

We report As-75-NQR/NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductors LaFeAsO0.6 (T-c = 28 K) along with the results oil LaFeAsO, LaFeASO(0.75) (T-c = 20 K), and NdFeASO(0.6) (T-c = 53 K). Nuclear spin-lattice relaxation rate 1/T-1 of As-75-NQR at zero field on LaFeASO(0.6) has revealed a T-3 dependence below T-c upon cooling without the coherence peak just below T, evidencing the unconventional Superconducting state with the line-node gap. We have found an intimate relationship between the nuclear quadrupole frequency v(Q) of As-75 and T-c for four samples used in this study. It implies microscopically that the local configuration of Fe Lind As atoms is significantly related to the T, of the Fe-oxypnictide superconductors, namely, the T, can be enhanced up to 50 K when the local configuration of Fe and As atoms is optimal, in which the band structure may be also optimized through the variation of hybridization between As 4p orbitals and Fe 3d orbitals.