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We report $^{57}$Fe-NMR studies on the oxygen-deficient iron (Fe)-basedoxypnictide 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 temperaturesand the nuclear spin-lattice relaxation rate $^{57}(1/T_{1})$ exhibits a$T^{3}$-like dependence without the coherence peak just below $T_{c}$, whichgive firm evidence of the unconventional superconducting state formed byspin-singlet Cooper pairing. All these events below $T_c$ are consistentlyargued in terms of the extended s$_{\pm}$-wave pairing with a sign reversal ofthe order parameter among Fermi surfaces. In the normal state, we found theremarkable 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 spinfluctuations over whole ${\bm q}$ space upon cooling below room temperature.Such behavior has never been observed for other strongly correlatedsuperconductors where an antiferromagnetic interaction plays a vital role inmediating the Cooper pairing.

Spin Fluctuations and Unconventional Superconductivity in the Fe-based Oxypnictide Superconductor LaFeAsO_0.7 probed by 57Fe-NMR