Spatially resolved superconductivity on the surface of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals

We have used a cryogenic four-probe scanning tunneling microscope and scanning electron microscope (SEM) to investigate local electrical transport properties on the surface of optimally-doped superconducting Ba($_{Fe1-x}$Co$_{x})_{2}$As$_{2}$ single crystal. For the regions that showed uniform SEM contrast, a superconducting transition has been observed with a very sharp transition width of 0.2 K. On the other hand, superconducting transition temperature was found to vary with transition widths up to 3 K in the non-uniform SEM contrast region. The wavelength dispersive x-ray spectroscopy reveals that dopant (Co) concentration remains a fixed value in the uniform region, but changes gradually in the non-uniform region. We will show that local superconductivity is connected with local chemical-compositional change.