Spatial variation of the observed energy gap in granular superconducting NbN films

We have simultaneously measured the surface topography and the tunneling current-voltage (I-V) characteristics of thin films of NbN as a function of lateral position using a low-temperature scanning tunneling microscope. We have imaged the superconducting energy gap by fitting the I-V's with an energy gap for each spatial position. Such images showed significant variation of the gap, including areas with reduced gap when a magnetic field (up to 6 T) was applied. The features in the gap image were often correlated with features in the surface topographic images.