Bose-Einstein condensation in the presence of an impurity
  It is shown that Bose-Einstein condensation occurs for an ideal gas in two spatial dimensions in the presence of one impurity which is described quantum mechanically in terms of a point-like vortex and a contact interaction. This model is exactly solvable and embodies as a special case the analogous problem in three spatial dimensions. 
Paola Giacconi, Fabio Maltoni, Roberto Soldati