Measurement of a sign-changing two-gap superconducting phase in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals using scanning tunneling spectroscopy (STS)

We present STS studies of the iron pnictide superconductors Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ (x=0.06, 0.08, 0.12). Studies on single crystals, cleaved at room temperature in a pure argon atmosphere, reveal direct spectroscopic evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish immediately above the superconducting transition, T$_{C}$. Fourier transformation of the tunneling conductance demonstrates slight doping- and energy-dependent quasiparticle scattering interferences (QPI) near the nesting wave-vectors ($\pm \pi $,0)/(0,$\pm \pi )$ and also near ($\pm $2$\pi $,0)/(0,$\pm $2$\pi )$. The dominant QPI near ($\pm \pi $,0)/(0,$\pm \pi )$ and the two-gap nature are consistent with sign-changing s-wave superconductivity. The excess zero-bias conductance and the large gap-to-T$_{C}$ ratios suggest significant unitary impurity scattering. Further studies of the magnetic field dependence will be discussed. This work was supported by NSF Grant DMR-0907251.