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 Delta(PG)/k(B) similar to 560 K, where chi(spin) similar to constant and resisitivty rho proportional to 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.