  The spatially resolved electrical transport properties have been studied on
the surface of optimally-doped superconducting Ba(Fe1-xCox)2As2 single crystal
by using a four-probe scanning tunneling microscopy. While some non-uniform
contrast appears near the edge of the cleaved crystal, the scanning electron
microscopy (SEM) reveals mostly uniform contrast. For the regions that showed
uniform SEM contrast, a sharp superconducting transition at TC = 22.1 K has
been observed with a transition width (delta)Tc = 0.2 K. In the non-uniform
contrast region, TC is found to vary between 19.6 and 22.2 K with (delta)Tc
from 0.3 to 3.2 K. The wavelength dispersive x-ray spectroscopy reveals that Co
concentration remains 7.72% in the uniform region, but changes between 7.38%
and 7.62% in the non-uniform region. Thus the variations of superconductivity
are associated with local compositional change.
