Many-electron effects have been observed in far-infrared magnetospectroscopic studies of GaAs/AlGaAs multiple quantum wells (QWs), $\delta$ doped with Si donors in both well and barrier centers. With increasing excess electron densities in the QWs the negative donor ion $\left(D^{-}\right)$ singlet and triplet transitions are substantially blueshifted, exhibiting cusps at integer and fractional filling factors. At high magnetic fields the many-electron system appears to approach a collection of isolated two-electron $D^{-}$ ions. Exact diagonalization studies of donor and confined electrons show the importance of electron correlations and localization at high magnetic fields in understanding this behavior.

• Received 15 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.3499