Excitons in coupled double quantum wells

The possibility of macroscopic quantum coherence in two-dimensional exciton systems presents an interesting subject of investigation. For low to moderate densities, excitons in coupled double quantum wells (CDQWs) under an applied electric field behave as bosons interacting via mutual dipole repulsion. Such excitons exist as separately confined electron-hole pairs and exhibit radiative recombination lifetimes which are long compared to thermal relaxation times. The inherent heterojunction interface disorder due to the random nature of epitaxial growth imposes a random potential on the two dimensional exciton population. Experimental and numerical evidence of superfluid transitions exist in similar systems of repulsive bosons in a 2-D random potential. A unique feature of this system, not found in other experimental systems, is the possibility of independent control over the temperature, number density and disorder.; Excitons in CDQWs are analyzed theoretically and studied experimentally. The results of numerical calculations of electron and hole states in CDQWs, including exciton binding energy, radius, radiative lifetime and absorption coefficient are presented. The experimental methods, procedures and results are described. An introduction to the theory of phase transitions is included, and the possibility of a superfluid transition in an exciton gas is discussed. A path integral quantum Monte Carlo algorithm for interacting bosons on a two dimensional lattice is developed.; Data from low temperature photoluminescence (PL) experiments reveal anomalous behaviour in the average exciton energy (first moment of the PL spectrum) as temperature and number density are varied. The data revealed a blue shift in the average exciton energy for decreasing temperatures at constant number densities. Increasing the number density at constant temperatures above 5 K, also resulted in a blue shift of the average exciton energy. However, at temperatures below 4 K, a red shift in the peak PL signal and average exciton energy was observed at the highest number density. The results are compared to other recent experiments in similar CDQW exciton systems which claim to observe exciton condensation.; The anomalous behavior of average exciton energy versus temperature and number density is not attributable to band gap renormalization or errors in temperature measurement. The blue shift in the average exciton energy is consistent with in creased exciton localization at lower temperatures due to potential energy disorder, and with real space exciton exclusion at higher densities. The red shift in the average exciton energy corresponds to enhanced mobility or de-localization due to a possible superfluid transition.