in the SHQW is approximately 4 × 10 10 cm − $^{2}$. There is a large uncertainty in calculating and measuring the actual exciton density in a QW with a 2DEG-exciton system. Nevertheless, the observed appearance of high-intensity PL at a point 25 µ m from the edge of the excitation region less than 0 . 1 ns after excitation is a possible manifestation of a long range transport of exciton excitation energy. Further research is needed to understand this exciton excitation energy transport mechanism and its potential application to optoelectronic devices. 

The experimental results obtained from measurement of the exciton dephasing rate show that electron-exciton scattering is twice as efficient as exciton-exciton scattering. It is important to note that the enhanced electron-exciton scattering also leads to efficient relaxation of exciton energy in agreement with previous theoretical results [24]. Efficient exciton energy relaxation involves collective plasmon-phonon interactions in the exciton/two-dimensional electron gas (exciton/2DEG) system which could result in the formation of the second sound branch. Experimental results [13] clearly show that the collective screening response of the exciton/2DEG system to a photo-induced potential results in fundamental changes in exciton characteristics including the possibility of condensation and transport of excitons. The 2DEG density in our experiment is 2 × 10 11 cm − $^{2}$. 

Negatively charged excitons (trions), which consist of two electrons bound to a single hole with a theoretical [26] and observed [27–29] binding energy relative to the exciton of approximately 1 meV have been observed in modulation- doped GaAs QWs. A photoluminescence peak from trions has been observed from the exciton 2DEG system in our structure around 1 meV below the exciton PL peak only when the exciton linewidth has been minimized using slight variations of the applied gate voltage [30]. In the present experiments, broadening of the PL linewidths at V$_{g}$ = − 36 V and the short exciton lifetime of 0 . 5 ns at V$_{g}$ = 0 precluded the resolution of a PL peak from trions. 

In conclusion, calculations of the superfluid density and the K-T transition temperature for the 2DEG-exciton system show that the K-T transition temperature increases with increasing exciton density and support the possible occurrence of a fast transport of exciton excitation energy. 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 D . The superfluid density and the K-T transition temperature in CQWs increase when the phenomenological screening parameters β ( n, n$_{e}$ ) for a single well or θ ( n, n$_{e}$, D ) for CQWs in the presence of 2DEG increase. 

Acknowledgments: O. L. B. would like to acknowledge the support by PSC CUNY grant 621360040, GG would like 

