We present $^{ 57}$Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba$_{ 0.6}$K$_{ 0.4}$Fe$_2$As$_2$ ($T_c$ = 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 $^{ 57}$Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The $^{ 57}$Fe-$1/T_1$ results are totally consistent with a $s^\pm$-wave model with multiple full gaps, regardless of doping with either electrons or holes.
Categories: cond-mat.supr-con