75As-NQR/NMR Studies on Oxygen-Deficient Iron-Based Oxypnictide Superconductors LaFeAsO1-y (y = 0, 0.25, 0.4) and NdFeAsO0.6

We report 75 As-NQR/NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductors LaFeAsO 0.6 ( T c = 28 K) along with the results on 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 75 As-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 c , evidencing the unconventional superconducting state with the line–node gap. We have found an intimate relationship between the nuclear quadrupole frequency ν Q of 75 As and T c for four samples used in this study. It implies microscopically that the local configuration of Fe and As atoms is significantly related to the T c of the Fe–oxypnictide superconductors, namely, the T c 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 3 d orbitals.