75As-NQR/NMR studies on oxygen-deficient iron-based oxypnictide superconductors LaFeAsO1-y (y=0, 0.25, 0.4) and NdFeAsO0.6

We report 75As-NQR/NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductors LaFeAsO0.6 (Tc=28 K) along with the results on LaFeAsO, LaFeAsO0.75 (Tc=20 K), and NdFeAsO0.6 (Tc=53 K). Nuclear spin-lattice relaxation rate 1/T1 of 75As-NQR at zero field on LaFeAsO0.6 has revealed a T3 dependence below Tc upon cooling without the coherence peak just below Tc, evidencing the unconventional superconducting state with the line-node gap. We have found an intimate relationship between the nuclear quadrupole frequency vQ of 75As and Tc for four samples used in this study. It implies microscopically that the local configuration of Fe and As atoms is significantly related to the Tc of the Fe-oxypnictide superconductors, namely, the Tc 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 4 p orbitals and Fe 3d orbitals. (author)