Phase Diagram of Quasi Two-Dimensional Electron-Hole Gas in Single and Coupled Quantum Wells

Abstract

We present the phase diagram of electron-hole plasma in single and coupled quantum wells. In the last case, electrons and holes are spatially separated. In both single and coupled quantum wells there is a Mott transition from electron-hole plasma to exciton gas. Below the Mott transition in a single quantum well there is a crossover to the biexciton gas phase which rules out the Kosterlitz-Thouless transition of excitons. In coupled quantum wells a strong direct repulsion between excitons prevents the biexciton formation. If the separation between electron well and hole well is large enough, the decrease of temperature leads to a transition to a ferromagnetic exciton phase with a spontaneous spin polarization. Further decrease of temperature leads to the Kosterlitz-Thouless transition of the exciton gas.