Hans Andreas Nickel

Internal transitions of negatively charged magnetoexcitons and many body effects in a two-dimensional electron gas.

Internal transitions of quasi-two-dimensional, negatively charged magnetoexcitons ( X-) and their evolution with excess electron density have been studied in GaAs/AlGaAs quantum wells. In the dilute electron limit, due to magnetic translational invariance, the optically detected resonance spectra are dominated by bound-to-continuum bands in contrast to the negatively charged donor system D-, which exhibits strictly bound-to-bound transitions. With increasing excess electron density Landau-level filling factors nu<2 the X--like transitions are blueshifted; they are absent for nu>2. The blueshifted transitions are explained in terms of a new type of collective excitation---magnetoplasmons bound to a mobile valence band hole.

Charged Magnetoexcitons in Two Dimensions: Isolated X— and Many‐Electron Effects

The states of charged magnetoexcitons in two-dimensional systems are considered. Exact optical selection rules for intra- and inter-band processes are discussed. The effect of excess electrons on internal transitions of negatively charged excitons X in quantum wells is studied experimentally and theoretically. An experimentally observed blue-shift with excess electron density is explained in terms of collective excitations, magnetoplasmons bound to a valence band hole.

Internal transitions of two-dimensional charged magneto-excitons X−: theory and experiment

Abstract Internal spin-singlet and spin-triplet transitions of charged excitons X− in magnetic fields in quantum wells have been studied experimentally and theoretically. The allowed X− transitions are photoionizing and exhibit a characteristic double-peak structure, which reflects the rich structure of the magnetoexciton continua in higher Landau levels (LLs). We discuss a novel exact selection rule, a hidden manifestation of translational invariance, that governs transitions of charged mobile complexes in a magnetic field.

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.

Hydrostatic pressure dependence of negative-donor-ion singlet and singlet-like bound magnetoplasmon transitions in doped GaAs/AlGaAs quantum wells

Abstract Applied hydrostatic pressure modifies the Coulomb bound states of a quasi-two-dimensional electron gas in quantum wells by increasing the effective mass and by tuning the free electron density. Here, we explore these mechanisms by measuring the effects of pressure on the cyclotron resonance, the D 0 1 s → 2 p + transition, and the D − -singlet and singlet-like transitions in low-and high-density, modulation-doped GaAs quantum wells. For low doping density, detailed calculations employing a pressure-dependent electron mass agree well with the observed magnetic field and pressure dependencies. For high doping density and low fields, the blue-shift of the D − -singlet-like transition at fields below 8 T decreases with applied pressure as anticipated, due to loss of free electrons via the Γ–X crossover. However, near ∼7.5 T , this singlet-like transition exhibits an anomalous branching for pressures above 4 kbar , which indicates the presence of a resonant level and obscures the blue-shift at high fields.

Full-spectrum optically detected resonance (ODR) spectroscopy of GaAs/AlGaAs quantum wells

Abstract Resonant magneto-absorption of far-infrared (FIR) laser radiation by neutral and negatively charged donors and free electrons, and the mechanisms of coupling of the power absorbed to various photoluminescence recombination paths in a Si-doped GaAs/AlGaAs multiple-quantum-well structure was studied by optically detected resonance (ODR) spectroscopy. A sensitive charge-coupled-device detection scheme was used to record complete PL/ODR spectra at high resolution with good signal-to-noise. The rich and complex ODR spectra were analyzed by a line-fitting procedure. Results on the neutral donor 1s–2p + transition show that at low FIR laster intensities, the recombination is modified by processes that do not involve carrier heating. At high FIR laser intensities, carrier heating effects dominate.

Interaction of an electron gas with photoexcited electron–hole pairs in modulation-doped GaAs and CdTe quantum wells

The nature of the correlated electron gas and its response to photo-injected electron–hole pairs in nominally undoped and modulation-doped multiple quantum-well structures was studied by experiment and theory, revealinga new type of optically active excitation, magnetoplasmons bound to a mobile valence hole. These excitations are blue-shifted from the corresponding transition of the isolated charged magnetoexciton X − . The observed blue-shift of X − is larger than that of two-electron negative donor D − , in agreement with theoretical predictions. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 71.35.Cc; 71.35.Ji; 73.21.Fg

Many body effects and internal transitions of confined excitons in GaAs and CdTe quantum wells

Abstract Many body effects contribute significantly to the energy states of electron–hole pairs confined in quantum wells in the presence of excess electrons. We present results of optically detected resonance spectroscopy of the internal transitions of photo-excited electron–hole pairs in the presence of excess electrons for GaAs QWs and CdTe QWs. Compared to the case of isolated negatively charged excitons, excess electrons produce a large blue shift of the internal transitions in modulation-doped GaAs quantum wells (QWs) for filling factor 2 no internal transitions are observed. These measurements demonstrate the strong effects of electron–electron correlations on the internal transitions of charged excitons in these quasi-2D systems and the importance of magnetic translation invariance. In the presence of excess electrons, the observed internal transitions are those of a magnetoplasmon bound to a mobile valence band hole.

Internal Transitions of Negatively Charged Excitons and Many-Electron Effects in GaAs Quantum Wells

States of charged magnetoexcitons in quasi-two-dimensional systems are investigated. The effect of excess electrons on internal transitions of negatively charged excitons X− in GaAs quantum wells is studied experimentally by optically detected resonance (ODR) spectroscopy and theoretically. An experimentally observed blue shift with excess electron density is explained in terms of collective excitations—magnetoplasmon bound to a mobile valence band hole. A possibility to observe photoluminescence of dark X− states in angle-resolved experiments is also discussed.

Magneto-Spectroscopy of Two-Dimensional Systems: Many- and Few-Body Effects

We present a classification of states and exact optical selection rules for charged electron–hole (e-h) complexes in magnetic fields that follow from magnetic translations and axial symmetry. A possibility to observe photoluminescence of dark X− states in angle-resolved experiments is discussed. The effect of excess electrons on internal transitions of negatively charged excitons X− in GaAs quantum wells is studied theoretically and experimentally (by optically detected resonance (ODR) spectroscopy). An experimentally observed blue shift with excess electron density is explained in terms of collective excitations—magnetoplasmons bound to a mobile valence band hole.