Papers by Alexander Dzyubenko
Physica E: Low-dimensional Systems and Nanostructures, 2005
We have initiated experiments on interface fluctuation quantum dots in the GaAs/AlGaAs system wit... more We have initiated experiments on interface fluctuation quantum dots in the GaAs/AlGaAs system with the goal of elucidating the excited states of excitons and the coupling of the center-of-mass and relative motion in the weak lateral confining potential. Magneto-PL and optically detected resonance (ODR) spectroscopy are carried out with a grating spectrometer/ccd system and an optical-access superconducting magnet (10 T).
We show that graphene in a strong magnetic field with partially filled Landau levels sustains cha... more We show that graphene in a strong magnetic field with partially filled Landau levels sustains charged collective excitations, which are bound states of three-particle complexes. Some of these states are optically bright and may be detected in spectroscopy experiments, enhancing the current understanding of electron-electron interactions in graphene. The states can be classified using the geometrical symmetries - non-commutative magnetic translations and generalized rotations - in addition to the dynamical SU(4) symmetry in graphene. From the SU(4) symmetry point of view, such excitations are analogous to bound states of two quarks and one antiquark qqqbar with four flavors. We establish a flavor optical selection rule to identify the bright states for experimental studies.
Spin-singlet and spin-triplet internal transitions of quasi-two-dimensional, negatively charged m... more Spin-singlet and spin-triplet internal transitions of quasi-two-dimensional, negatively charged magneto-excitons (X −) and their evolution with excess electron density have been studied in GaAs/AlGaAs quantum-wells by optically detected resonance (ODR) spectroscopy. In the dilute electron limit, due to magnetic translational invariance, the ODR spectra are dominated by bound-to-continuum bands in contrast to the superficially similar negativelycharged-donor system D − , which exhibits strictly bound-to-bound transitions. With increasing excess electron density in the wells in the magnetic field region corresponding to Landau level filling factors ν < 2 the X −-like transitions
Bulletin of the American Physical Society, 2005
We report optically detected resonance (ODR) experiments and theoretical studies of interface flu... more We report optically detected resonance (ODR) experiments and theoretical studies of interface fluctuation quantum dots (IFQDs) in GaAs/AlGaAs QWs (widths from 2.8 to 14.1 nm) doped in the barriers with donors to allow creation of both excitons and trions. We observe internal transitions of trions and electron cyclotron resonance (CR) in the wider wells. These spectra are similar to those from samples with smooth interfaces except for an additional feature at fields above CR. Based on theoretical predictions, we assign this feature to bound-to-bound triplet transitions, which are strictly forbidden in translationally invariant systems. The ODR signals from the narrow wells show no CR and are interpreted in terms of inhomogeneously broadened internal transitions of charged excitons in the IFQDs. A theoretical model for optical transitions of excitonic complexes in the presence of lateral and magnetic field confinement will be discussed.
Bulletin of the American Physical Society, 2006
Submitted for the MAR06 Meeting of The American Physical Society Neutral and charged excitons in ... more Submitted for the MAR06 Meeting of The American Physical Society Neutral and charged excitons in single GaAs-based interface quantum dots C.J. MEINING, V.R. WHITESIDE, B.D. MCCOMBE, University at Buffalo, The State University of New York, NY 14260, J.G. TISCHLER, A.S. BRACKER, D. GAMMON, Naval Research Laboratory, Washington, D.C. 203755347, A.B. DZYUBENKO, California State University at Bakersfield, CA 93311, M. BYSZEWSKI, M. POTEMSKI, Grenoble High Magnetic Field Laboratory, France — We report circularly polarized photoluminescence (PL) in high magnetic fields (< 28 T) and optically detected resonance (ODR) experiments of interface fluctuation quantum dots (IFQDs) in narrow GaAs/AlGaAs quantum wells (QWs) doped in the barriers with donors to allow creation of both neutral and negatively charged excitons. In the narrowest QW the diamagnetic shift of the trion is smaller than that of the neutral exciton. This is attributed to the larger spatial extent of the trion wavefunction in...
... Convention Center Room: 506 Sponsoring Unit: DCMP Chair: Sergio Ulloa, Ohio University Abstra... more ... Convention Center Room: 506 Sponsoring Unit: DCMP Chair: Sergio Ulloa, Ohio University Abstract ID: BAPS.2007.MAR.U43.8. Abstract: U43.00008 : Localized charged magnetoexcitons in 2D systems. 9:24 AM9:36 AM. Preview Abstract. Authors: Diana Cosma Alexander ...
Optically detected resonance spectroscopy has been used to investigate effects of weak random lat... more Optically detected resonance spectroscopy has been used to investigate effects of weak random lateral potential energy fluctuations on internal transitions of charged magneto-excitons (trions) in quasi two-dimensional GaAs/AlGaAs quantum-well (QW) structures. Resonant changes in the ensemble photoluminescence induced by far-infrared radiation were studied as a function of magnetic field for samples having: 1) no growth interrupts (short range well-width fluctuations), and 2) intentional growth interrupts (long range monolayer well-width differences). Only boundto-continuum internal transitions of the negatively charged trion are observed for samples of type 1. In contrast, a feature on the high field (low energy) side of electron cyclotron resonance is seen for samples of type 2 with well widths of 14.1 and 8.4 nm. This feature is attributed to a bound-to-bound transition of the spin-triplet with non-zero oscillator strength resulting from breaking of translational symmetry.
Singlet and triplet states of positively (X{sup +}) and negatively (X{sup -}) charged excitons in... more Singlet and triplet states of positively (X{sup +}) and negatively (X{sup -}) charged excitons in ZnSe-based quantum wells have been studied by means of photoluminescence in pulsed magnetic fields up to 50 T. The binding energy of the X{sup -} singlet state shows a monotonic increase with magnetic field with a tendency to saturation, while that of the X{sup +} slightly decreases. The triplet X{sup +} and X{sup -} states, being unbound at zero magnetic field, noticeably increase their binding energy in high magnetic fields. The experimental evidence for the interaction between the triplet and singlet states of lTions leading to their anticrossing in magnetic fields has been found.
The states of charged magnetoexcitons in two-dimensional systems are considered. Exact optical se... more The states of charged magnetoexcitons in two-dimensional systems are considered. Exact optical selection rules for intraand 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.
Physica E: Low-dimensional Systems and Nanostructures
I theoretically study shake-up processes in photoabsorption of an interacting low-density twodime... more I theoretically study shake-up processes in photoabsorption of an interacting low-density twodimensional electron gas (2DEG) in magnetic fields. Such processes, in which an incident photon creates an electron-hole pair and simultaneously excites one electron to one of the higher Landau levels, were observed experimentally [D.R. Yakovlev et al., Phys. Rev. Lett. 79, 3974 (1997)] and were called combined exciton-cyclotron resonance (ExCR). The recently developed theory of ExCR [A.B. Dzyubenko, Phys. Rev. B 64, 241101 (2001)] allows for a consistent treatment of the Coulomb correlations, establishes the exact ExCR selection rules, and predicts the high field features of ExCR. In this work, I generalize the existing theory of high-field ExCR in the 2DEG to the case when the hole is excited to higher hole Landau levels.
Acta Physica Polonica A
Low temperature measurements of magneto-photoluminescence and optically detected resonance spectr... more Low temperature measurements of magneto-photoluminescence and optically detected resonance spectroscopy in magnetic fields up to 10 T were carried out on GaAs/Al 0.3 Ga 0.7 As quantum well samples grown by molecular beam epitaxy with lateral fluctuation quantum dots produced by growth interruption. Monolayer fluctuations of the quantum well width form lateral quantum dots for which confinement energies are less than Coulomb correlation energies. Five different width quantum wells (2.8-14.1 nm) were grown in a single sample, doped in the barriers with Si donors to allow for photoluminescence of both excitons and trions. We report studies of the optically detected resonance spectra associated with the ensemble photoluminescence of all of the wells including observation of bound-to-continuum internal transitions of trions, both singlet and triplet, and electron cyclotron resonance for the wider wells, which also show a clear bound-to-bound triplet transition. The latter is forbidden by magnetic translational invariance, but can be seen in these samples because this symmetry is broken by the lateral fluctuations, whose characteristic dimensions are greater than the trion orbit size. The two narrowest wells show strong broad optically detected resonance signals associated with inhomogeneously broadened internal transitions of the strongly correlated trions in the lateral dots. The optically detected resonance signals peak well below the calculated positions of electron cyclotron resonance. As expected for localized carriers and excitons, there is no free electron cyclotron resonance. We also present preliminary measurements of optically detected resonance spectra from a single dot.
Acta Physica Polonica A, 2000
We review recent experimental advances by the Buffalo group in performing far-infrared magnetospe... more We review recent experimental advances by the Buffalo group in performing far-infrared magnetospectroscopy under he tuning of applied high hydrostatic pressure. Experiments are reported for the effects of pressure on Si donors in modulation doped GaAs/AlGaAs quantum wells. We clearly observe pressure-mediated competition between free (i.e., Landau level) and bound electron states-the latter arising from both neutral (D 0) and charged (D-) donor species. With increasing pressure, there is a progression of the observed spectra from being dominated by cyclotron resonance and the Dsinglet (or singlet-like bound magnetoplasmon) transitions, to showing the D0 is →2p+ line. The main reason for this evolution is the decrease in electrons due to the crossover of the Si levels associated with the F (well) and X (barrier) conduction minima. Indeed, for pressures above 30 kbar the Γ(well)-X(barrier) crossover quenches all the transitions. However, we find strong evidence that electrons are independently lost to a trap, which becomes active several kbar below this crossing. A possible candidate for this trap is residual Se impurities in the barriers. We present the results of detailed numerical calculation which are found to agree very well with the measured field dependencies of the cyclotron resonance, D0 and Dtransition energies. In the sample with the highest doping, a new transition is observed for fields and pressures above 7.5 T and 5 kbar. Reasons for this apparent anomaly are discussed.
Optical Properties of Semiconductor Nanostructures, 2000
Cyclotron Resonance (CR) of free electrons and holes and internal transitions of their bound comp... more Cyclotron Resonance (CR) of free electrons and holes and internal transitions of their bound complexes have been studied in GaAs/AlGaAs quantum wells in high magnetic fields by Optically Detected Resonance (ODR) spectroscopy. Two nearly degenerate 1s→2p+ internal exciton transitions (IETs), as well as two well-separated 1s→2p- IETs resulting from the two distinct heavy-hole magneto-excitons were observed. Measurements of both electron and hole CR and these IETs in the same sample evidence the predicted consequences of the symmetry of the magneto-exciton Hamiltonian. Extensions of these studies to samples that showed neutral (heavy-hole hhX) as well as negatively charged (X-) exciton photoluminescence-lines have permitted the first observation of internal transitions of X-. Both singlet and triplet transitions of X- were observed, and new physics associated with translational invariance for this interesting three-particle complex was revealed. The observation of electron and hole CR and several IETs in a single sample exemplifies the promise of the ODR technique for the study of semiconductor nanostructures.
Physica E: Low-dimensional Systems and Nanostructures, 2000
Applied hydrostatic pressure modiÿes the Coulomb bound states of a quasi-two-dimensional electron... more Applied hydrostatic pressure modiÿes the Coulomb bound states of a quasi-two-dimensional electron gas in quantum wells by increasing the e ective mass and by tuning the free electron density. Here, we explore these mechanisms by measuring the e ects of pressure on the cyclotron resonance, the D 0 1s → 2p + 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 ÿeld and pressure dependencies. For high doping density and low ÿelds, the blue-shift of the D −-singlet-like transition at ÿelds below 8 T decreases with applied pressure as anticipated, due to loss of free electrons via the-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 blueshift at high ÿelds.
Optical Properties of 2D Systems with Interacting Electrons, 2003
ABSTRACT States of charged magnetoexcitons in quasi-two-dimensional systems are investigated. The... more ABSTRACT 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. Key wordscharged magnetoexciton–optically detected resonance–quantum well–internal transitions
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Papers by Alexander Dzyubenko