Many-electron effects on quasi-two-dimensional shallow-donor impurity states in high magnetic fields

Far-infrared magneto-optical spectra of shallow donors confined in GaAs quantum wells in the presence of a quasi-two-dimensional electron gas show a characteristic magneto-optical transition whose frequency increases with increasing density of excess electrons, and whose slope exhibits discontinuities versus magnetic field at integer Landau level filling factors. These results demonstrate the importance of electron-electron interactions and possibly dynamical screening in high magnetic fields.