arXiv:1002.2736v1 [cond-mat.mes-hall] 13 Feb 2010 Collective Properties of Excitons in Presence of a Two-Dimensional Electron Gas 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. Key words: A. Quantum wells; A. Semiconductors; D. Electron-electron interactions; D. Phase transitions. PACS numbers: 71.35.Lk, 73.20.Mf, 73.21.Fg The great interest to the dipole excitons in coupled quantum wells (CQWs) with spatially separated electron QW and hole QW interest has been stimulated by the possibility of Bose-Einstein condensation (BEC) and the superfluidity of dipole excitons formed from electron-hole (e-h) pairs. These may result in persistent electrical currents in each QW or coherent optical properties and Josephson junction phenomena [1–7]. The great experimental success was achieved now in this field [8–11]. The coupled QW system is conceptually simple: negative electrons are trapped in a two-dimensional plane, while an equal number of positive holes is trapped in a parallel plane a distance D away. In this system, the electron and hole wavefunctions have very little overlap, so that the excitons can have very long lifetime ( > 100 ns), and therefore they can be treated as metastable particles to which quasiequlibrium statistics apply. Also, when D is large enough, Oleg L. Berman $^{1}$, Godfrey Gumbs$^{2}$, and Patrick A. Folkes3 $^{1}$Physics Department, New York City College of Technology of the City University of New York 300 Jay Street, Brooklyn, NY 11201 2 Department of Physics and Astronomy, Hunter College of the City University of New York 695 Park Avenue, New York, NY 10065 3 Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783-1197 (Dated: November 14, 2018)