Magnetoluminescence of the two-dimensional electron-hole fluid

Band-gap renormalization due to the presence of a dense electron-hole plasma in GaAs quantum wells is studied by time-resolved luminescence and magnetoluminescence spectroscopy. We show that heterostructures with long carrier lifetimes exhibit saturation of the band-gap renormalization in a magnetic field, in accord with previous results. However, the usual carrier-density-dependent gap shrinkage in a magnetic field is observed in multiple quantum wells with short carrier lifetimes. The strong dependence of the magneto-optical properties on the carrier lifetimes indicates that two different phases can be formed in the plasma at high magnetic field: a highly correlated phase, analogous to the condensed electronic or excitonic state at the equilibrium density, in quantum wells with sufficiently long carrier lifetimes; and a free-carrier gas in heterostructures with short lifetimes. In the former state, the band gap is found to renormalize by an amount equal to twice the exciton binding energy, independent of the actual photogeneration rate. On the other hand, condensation processes are prevented in the short-lived phase, thus resulting in the expected density-dependent band-gap renormalization.