Strong-Coupling Spin-Singlet Superconductivity with Multiple Full Gaps in Hole-Doped Ba_0.6K_0.4Fe₂As₂ Probed by ⁵⁷Fe-NMR

Abstract

We present ⁵⁷Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba_0.6K_0.4Fe₂As₂ (T_c=38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1⁄T₁) 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⁄T₁ results are totally consistent with a s^±-wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective 1⁄T₁ data for Ba_0.6K_0.4Fe₂As₂ and LaFeAsO_0.7, which seemingly follow a T⁵- and a T³-like behaviors below T_c, are consistently explained in terms of this model only by changing the size of the superconducting gap.