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 carri ed out on a series of GaAs / AlGaAs multiple quantum well (QW) structures, delta-doped with Si donors in both well and barrier centers. Filling-factor (nu)-dependent energy blueshifts of the many-electron singlet-like transit ion have been observed in the integer and fractional Quantum Hall regime. F or all samples studied the blueshift of this transition decreases with incr easing B for nu < 1, while the many-electron triplet-like transition simult aneously loses strength. This high-field behavior of the many-electron syst em, i.e. appearing to approach a collection of isolated two-electron negati vely-charged donor (D-) ions, is understood in the context of exact diagona lization studies of a donor and confined electrons. Our results show that F IR magneto-optical excitations of the system depend sensitively on the micr oscopic electron configuration in the vicinity of the positive donor ions d ue to electron correlations and localization at high magnetic fields. (C) 1 998 Elsevier Science B.V. All rights reserved.