Probing microscopic variations of superconductivity on the surface of Ba ( Fe_1-x Co_x )₂ As₂ single crystals

The spatially resolved electrical transport properties have been studied on the surface of optimally doped superconducting Ba ( Fe_1-x Co _x )₂ As₂ single crystal by using a four-probe scanning tunneling microscopy. While some nonuniform 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 Δ TC =0.2 K. In the nonuniform contrast region, TC is found to vary between 19.6 and 22.2 K with Δ 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 nonuniform region. Thus the variations of superconductivity are associated with local compositional change.