Atomically resolved images and spectroscopy of superconducting Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals

We report atomically resolved images and spectroscopy of electron-doped superconducting Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals, with x = 0.12, 0.08 and 0.06 corresponding to T$_{c}$ = 20 K, 24 K and 14 K, respectively. Scanning tunneling microscopic images of the crystals reveal somewhat disordered atomic arrangements of the Ba, As and Fe/Co layers, with averaged lattice constant $\sim $ 0.40 nm for Ba and As layers, and $\sim $ 0.28 nm for the Fe/Co layers. In contrast, spectroscopic studies indicate relatively homogeneous tunneling conductance at constant bias voltages, which differ from the findings on hole-doped (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$. Two types of spectra are observed, corresponding to two energy gaps $\sim $ 5 meV and $\sim $ 10 meV for Ba(Fe$_{0.88}$Co$_{0.12})_{2}$As$_{2}$, and the spatial distribution of the larger gap exhibits quasi super-modulations. The smaller gap may be attributed to superconductivity and the larger gap may be related to remnant spin density waves. Finally, doping dependent spectral evolution of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ will be discussed. Acknowledgement: NSF Grant DMR-0907251.