Fermi-edge singularity in heavily doped GaAs multiple quantum wells

The absorption enhancement at the Fermi level of n-type modulation-doped multiple quantum wells is studied with photoluminescence excitation spectroscopy. We show that the limit of the pure Fermi-edge singularity (i.e., correlation between the sea of electrons and single holes), where no remnants of the band-edge exciton (correlation between single electrons and holes) are left, is only reached for the case of heavy doping (n=1.2×${10}^{12}$ ${\mathrm{cm}}^{\mathrm{-}2}$). A direct comparison between experiments and available many-body theories is only valid in this limit. We find both quantitative as well as qualitative deviations from these theories. Previously neglected broadening mechanisms significantly reduce the enhancement effects. We report for the first time the dependence of the enhancement on the density of the photoexcited holes.