Abstract
We report 57Fe-NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO0.7 (Tc=28 K) enriched by 57Fe isotope. In the superconducting state, the spin component of 57Fe-Knight shift decreases to almost zero at low temperatures, and the nuclear spin–lattice relaxation rate 57(1⁄T1) exhibits a T3-like dependence without the coherence peak just below Tc, which provide firm evidence of the unconventional superconducting state formed by spin-singlet Cooper pairing. All these events below Tc are consistently argued in terms of the extended s±-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, we found the remarkable decrease in 1⁄T1T upon cooling for both the Fe and As sites, which originates from the decrease in low-energy spectral weight of spin fluctuations over whole q space upon cooling below room temperature. Such behavior has never been observed for other strongly correlated superconductors where an antiferromagnetic interaction plays a vital role in mediating the Cooper pairing.
