⁵⁹Co and ⁷⁵As NMR investigation of electron-doped high T_c superconductor BaFe_1.8Co_0.2As ₂ (T_c = 22 K)

We report an NMR investigation of the superconductivity in BaFe ₂As₂ induced by Co doping (T_c = 22 K). We demonstrate that Co atoms form an alloy with Fe atoms and donate carriers without creating localized moments. Our finding strongly suggests that the underlying physics of iron-pnictide superconductors is quite different from the widely accepted physical picture of high T_c cuprates as doped Mott insulators. We also show a crossover of electronic properties into a low temperature pseudo-gap phase with a pseudo-gap Δ_PG/k _B ∼ 560 K, where χ_spin ∼ constant and resisitivty ρ ∝ T. The NMR Knight shift below T_c decreases for both along the c-axis and ab-plane, and is consistent with the singlet pairing scenario.