59Co and 75As NMR Investigation of Electron-Doped High Tc Superconductor BaFe1.8Co0.2As2 (Tc = 22 K)

We report an NMR investigation of the superconductivity in BaFe 2 As 2 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 a b -plane, and is consistent with the singlet pairing scenario.