Anisotropic spin fluctuations and multiple superconducting gaps in hole-doped Ba 0.72 K 0.28 Fe 2 As 2 : NMR in a single crystal

We report the 75 As-NMR study on a single crystal of the hole-doped iron-pnictide superconductor Ba 0.72 K 0.28 Fe 2 As 2 ( T c =31.5 K). We find that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin lattice relaxation rate 1/ T 1 decreases below T c with no coherence peak and shows a stepwise variation at low temperatures, which is indicative of multiple superconducting gaps, as in the electron-doped Pr(La)FeAsO 1- x F x . Furthermore, no evidence was obtained for a microscopic coexistence of a long-range magnetic order and superconductivity.