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

Abstract

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 $1/T_{1}T$ upon cooling for both the Fe and As sites, which originates from the decrease of low-energy spectral weight of spin fluctuations over whole $\mathbf{q}$ 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.

4 pages, 4 figures,Accepted for publication in J. Phys. Soc. Jpn., vol.78, No.1 (2009)