Strong-Coupling Spin-Singlet Superconductivity with Multiple Full Gaps in Hole-Doped Ba0.6K0.4Fe2As2 Probed by 57Fe-NMR

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We present 57Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba0.6K0.4Fe2As2 ($T_{\text{c}} = 38$ K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate ($1/T_{1}$) demonstrate the development of wave-number ($q$)-dependent spin fluctuations, except at $q = 0$, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The 57Fe-$1/T_{1}$ results are totally consistent with a $s^{\pm}$-wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective $1/T_{1}$ data for Ba0.6K0.4Fe2As2 and LaFeAsO0.7, which seemingly follow a $T^{5}$- and a $T^{3}$-like behaviors below $T_{\text{c}}$, are consistently explained in terms of this model only by changing the size of the superconducting gap.
2009-10-15