Spin Fluctuations and Unconventional Superconductivity in the Fe-Based Oxypnictide Superconductor LaFeAsO_<0.7> Probed by ^<57>Fe-NMR(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We report ^<57>Fe-NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO_<0.7> (T_c=28K) 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_1T 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.
社団法人日本物理学会の論文
2009-01-15