Probing microscopic variations of superconductivity on the surface of Ba(Fe_1−xCo_x)₂As₂ single crystals

The spatially resolved electrical transport properties have been studied on the surface of optimally doped superconducting Ba(Fe_1−xCo_x)₂As₂ 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 T_C = 22.1 K has been observed with a transition width ΔT_C = 0.2 K. In the nonuniform contrast region, T_C is found to vary between 19.6 and 22.2 K with ΔT_C 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.

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