  We have studied the collective properties of two-dimensional (2D) excitons
immersed within a quantum well which contains 2D excitons and a two-dimensional
electron gas (2DEG). We have also analyzed the excitations for a system of 2D
dipole excitons with spatially separated electrons and holes in a pair of
quantum wells (CQWs) when one of the wells contains a 2DEG. Calculations of the
superfluid density and the Kosterlitz-Thouless (K-T) phase transition
temperature for the 2DEG-exciton system in a quantum well have shown that the
K-T transition temperature increases with increasing exciton density and that
it might be possible to have fast long range transport of excitons. The
superfluid density and the K-T transition temperature for dipole excitons in
CQWs in the presence of a 2DEG in one of the wells increases with increasing
inter-well separation.
