Direct measurement of potential steps at grain boundaries in the presence of current flow

We have used a new technique to measure simultaneously the surface topography and surface potential of current-carrying polycrystalline ${\mathrm{Au}}_{60}$ ${\mathrm{Pd}}_{40}$ thin films using a scanning tunneling microscope. The variations of the gradients of the surface potential from a macroscopically constant value which are associated with scattering from grain boundaries in these films are observed. We find that the local potential changes abruptly at the boundaries between the grains.