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

We present Fe-57-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide Superconductor Ba0.6K0.4Fe2As2 (T-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 Fe-57-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The Fe-57-1/T-1 results are totally consistent with a s(+/-)-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 LaFeASO(0.7), which seemingly follow a T-5- and a T-3-like behaviors below T-c, are consistently explained in terms of this model only by changing the size of the superconducting gap.