Spin Fluctuations and Unconventional Superconductivity in the Fe-Based Oxypnictide Superconductor LaFeAsO0.7 Probed by 57Fe-NMR

We report 57 Fe-NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO 0.7 ( T c = 28 K) enriched by 57 Fe isotope. In the superconducting state, the spin component of 57 Fe-Knight shift decreases to almost zero at low temperatures, and the nuclear spin–lattice relaxation rate 57 (1/ T 1 ) exhibits a T 3 -like dependence without the coherence peak just below T c , which provide firm evidence of the unconventional superconducting state formed by spin-singlet Cooper pairing. All these events below T c 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/ T 1 T 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 antife...