Nanoscale superconducting-gap variations and lack of phase separation in optimally doped BaFe1.86Co0.14As2

We present tunneling data from superconducting BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor, clear coherencelike peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, Δ. The average peak-to-peak separation gives a 2Δ of ∼7.4kBTc, which exceeds the BCS weak coupling value for either s- or d-wave superconductivity. The characteristic length scales of the deviations from the average gap value and of anticorrelations observed between the gap magnitude and both the zero-bias conductance and coherence peak strength match well with the average separation between the Co dopant ions in the superconducting FeAs planes.