Electron correlations in quasi-2D structures at high magnetic fields in the presence of embedded donor impurities

Systematic far infrared (FIR) magneto-spectroscopic studies have been carried out on a series of GaAs / AlGaAs multiple quantum well (QW) structures, δ-doped with Si donors in both well and barrier centers. Filling-factor (ν)-dependent energy blueshifts of the many-electron singlet-like transition have been observed in the integer and fractional Quantum Hall regime. For all samples studied the blueshift of this transition decreases with increasing B for ν < 1, while the many-electron triplet-like transition simultaneously loses strength. This high-field behavior of the many-electron system, i.e. appearing to approach a collection of isolated two-electron negatively-charged donor (D^-) ions, is understood in the context of exact diagonalization studies of a donor and confined electrons. Our results show that FIR magneto-optical excitations of the system depend sensitively on the microscopic electron configuration in the vicinity of the positive donor ions due to electron correlations and localization at high magnetic fields.