  Experiments observe an enhanced superconducting gap over impurities as
compared to the clean-bulk value. In order to shed more light on this
phenomenon, we perform simulations within the framework of Bogoliubov-deGennes
theory applied to the attractive Hubbard model. The simulations qualitatively
reproduce the experimentally observed enhancement effect; it can be traced back
to an increased particle density in the metal close to the impurity site. In
addition, the simulations display significant differences between a thin (2D)
and a very thick (3D) film. In 2D pronounced Friedel oscillations can be
observed, which decay much faster in (3D) and therefore are more difficult to
resolve. Also this feature is in qualitative agreement with the experiment.
