75As-NQR Study on Iron-Based Oxypnictide Superconductor LaFeAsO0.6

We report 75 As-NQR study on the oxygen-deficient iron-based oxypnictide superconductor LaFeAsO 0.6 ( T c =28 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 LaFeAsO, LaFeAsO 0.75 , LaFeAsO 0.6 , and NdFeAsO 0.6 . It implies microscopically that the local configuration of Fe and As atoms is significantly related to the T c of the Fe-oxypnictide superconductors.