Physics – Condensed Matter – Superconductivity
Scientific paper
2008-09-30
J. Phys. Soc. Jpn. 78, 013701/1-4 (2009)
Physics
Condensed Matter
Superconductivity
4 pages, 4 figures,Accepted for publication in J. Phys. Soc. Jpn., vol.78, No.1 (2009)
Scientific paper
10.1143/JPSJ.78.013701
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.
Eisaki Hiroshi
Iyo Akira
Kitaoka Yoshio
Kito Hideo
Miyazawa Keisuke
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