Phase Diagram of Quasi 2 Dimensional Electron-Hole Gas in Quantum Wells

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.