Z. X. Jiang

Pressure Tuning of Many-Electron Impurity Interactions in Confined Semiconductor Structures

We report studies of the free carrier and donor-bound FIR excitations of a confined electron gas in modulation doped GaAs/AlGaAs quantum wells (QW) as a function of the QW electron density. Applied pressure is used to tune the electron density via the Γ–X well–barrier crossover. As electrons are removed from the QWs, we observe successively the quenching of cyclotron resonance, the evolution of the D— singlet-like magnetoplasmon resonance into the D— singlet transition of isolated donors, and the emergence of the neutral donor D0 1s–2p+ line. Calculations predict a sharp drop in the QW electron density for 2.3 to 3.1 GPa, in accordance with experiment. A rapid decrease with B-field in the blue shift of the magnetoplasmon resonance at 2.2 GPa in one sample shows that pressure has shifted the ν < 1 filling-factor regime to a factor-of-two lower field.

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

Abstract 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 − ) 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.

High-field cyclotron resonance and electron-phonon interaction in modulation-doped multiple quantum well structures

A systematic study of electron cyclotron resonance (CR) in two sets of GaAs/Al0.3Ga0.7As modulation-doped quantum-well samples (well widths between 12 and 24 nm) has been carried out in magnetic fields up to 30 T. Polaron CR is the dominant transition in the region of GaAs optical phonons for the set of lightly doped samples, and the results are in good agreement with calculations that include the interaction with interface optical phonons. The results from the heavily doped set are markedly different. At low magnetic fields (below the GaAs reststrahlen region), all three samples exhibit almost identical CR which shows little effect of the polaron interaction due to screening and Pauli-principle effects. Above the GaAs LO-phonon region (B > similar to 23 T), the three samples behave very differently. For the most lightly doped sample (3 x 10(11) cm(-2)) only one transition minimum is observed, which can be explained as screened polaron CR. A sample of intermediate density (6 x 10(11) cm(-2)) shows two lines above 23 T; the higher frequency branch is indistinguishable from the positions of the single line of the low density sample. For the most heavily, doped sample (1.2 x 10(12) cm(-2)) there is no evidence of high frequency resonance, and the strong, single line observed is indistinguishable from the lower branch observed from sample with intermediate doping density. We suggest that the low frequency branch in our experiment is a magnetoplasmon resonance red-shifted by disorder, and the upper branch is single-particle-like screened polaron CR