Multiple Superconducting Gaps and Anisotropic Spin Fluctuations in Hole-Doped and Electron-Doped Iron-Pnictides: NMR Studies

We present extensive As-75-NMR data on the electron-doped pnictides PrFeAsO0.89F0.11 (T-c = 45 K), LaFeAsO0.92F0.08 (T-c = 23 K), and the hole-doped Ba0.72K0.28Fe2As2 (T-c = 31.5 K) single crystal. 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 step-wise variation at low temperatures. The Knight shift decreases below T-c and shows a step-wise T variation as well. These results indicate spin-singlet superconductivity with multiple gaps.