Measurement of a Sign-Changing Two-Gap Superconducting Phase in Electron-Doped Ba(Fe1-xCox)2As2 Single Crystals Using Scanning Tunneling Spectroscopy

Scanning tunneling spectroscopic studies of Ba(Fe1-xCox)(2)As-2 (x = 0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish above the superconducting transition T-c. The two-gap nature and the slightly doping-and energy-dependent quasiparticle scattering interferences near the wave vectors (+/-pi, 0) and (0, +/-pi) are consistent with sign-changing s-wave superconductivity. The excess zero-bias conductance and the large gap-to-T-c ratios suggest dominant unitary impurity scattering.