Spectroscopy of charged donorsand many-electron effects in semiconductor quantum wells

Far infrared magnetospectroscopic studies of negative donor ions (D^-), and donors in the presence of many excess electrons in high magnetic fields in GaAs/AlGaAs quantum wells are reviewed. Both singlet and triplet transitions of well-center D^--ions were observed and are in good agreement with recent theoretical calculations. For off-well-center D^--ions evidence for a predicted magnetic-field-induced "unbinding" of the second electron was found. In the presence of many excess electrons the D^--singlet and -triplet transitions are blue-shifted substantially and evolve into bound magnetoplasmon excitations. Cusps are observed at integral and fractional Landau-level filling factors (ν) in a plot of normalized blue-shift of the D^--singlet-like bound magnetoplasmon transition vs. ν. For ν < 1, the singlet-like bound magnetoplasmon transition continuously approaches the isolated D^--singlet transition with increasing magnetic field, while the triplet-like transition loses strength, irrespective of the electron density. Exact diagonalization studies of a donor ion with a few electrons in a parabolic lateral confining potential show the importance of electron-electron interactions and localization due to the long-range fluctuating potential in explaining this behavior. High pressure studies in a specially designed diamond anvil cell exhibit a continuous evolution from bound magnetoplasmon transitions to isolated D^--transitions to neutral donor transitions in a single sample as the pressure is increased and the electron density in the wells is decreased.