Electronic Structure on (001) Surface of Co-doped BaFe$_{2}$As$_{2}$ Studied with Scanning Tunneling Spectroscopy*

Co-doping makes the pnictide compound BaFe$_{2}$As$_{2}$ superconducting. We cleave the single crystals of this compound in UHV and study their surfaces with a low temperature STM. In this talk, we present the scanning tunneling spectrums obtained on the (001) surface of the optimally doped BaFe$_{2}$As$_{2}$ single crystals (Tc = 23K) and compare these spectrums with the ones obtained on the surface of the parent compound. We have found that the major feature of the spectrums on the superconducting compounds is the opening of a superconducting gap of about 6 meV for the optimally doped one. We have also observed other detailed spectrum features. We will discuss the relation between spectrum features and the local environment and also present some theoretical fit to the superconducting energy gap spectrum.