  We report on the experimental observation by scanning tunneling microscopy at
low temperature of ring-like features that appear around Co metal clusters
deposited on a clean (110) oriented surface of cleaved p-type InAs crystals.
These features are visible in spectroscopic images within a certain range of
negative tunneling bias voltages due to the presence of a negative differential
conductance in the current-voltage dependence. A theoretical model is
introduced, which takes into account non-equilibrium effects in the small
tunneling junction area. In the framework of this model the appearance of the
ring-like features is explained in terms of interference effects between
electrons tunneling directly and indirectly (via a Co island) between the tip
and the InAs surface.
