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 $^{ 57}K$ decreases almost to 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 give 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$_{ \pm}$-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, we found the remarkable decrease of $1/T_1T$ upon cooling for both the Fe and As sites, which originates from the decrease of low-energy spectral weight of spin fluctuations over whole ${ \bm q}$ 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.
Categories: cond-mat.supr-con cond-mat.str-el