286 research outputs found
Oxidative cleavage of carotenoids catalyzed by enzyme models and beta-carotene 15,15´-monooxygenase
The enzyme that catalyzes the central cleavage of β-carotene is an iron monooxygenase. The protein was isolated from chicken intestinal mucosa and overexpressed in two different cell lines. Inductively coupled plasma (ICP) emission analysis revealed that the hydrophobic 60.3-kDa enzyme contains one iron/mole protein. The substrate specificity was investigated, and the reaction mechanism elucidated incubating α-carotene in the presence of highly enriched 17O2 and H218O. A supramolecular enzyme model was synthesized, binding carotenoids Ka > 106 mol-1, which mimics the regiospecific enzymatic cleavage of carotenoid
Reducing decoherence of the confined exciton state in a quantum dot by pulse-sequence control
We study the phonon-induced dephasing of the exciton state in a quantum dot
excited by a sequence of ultra-short pulses. We show that the multiple-pulse
control leads to a considerable improvement of the coherence of the optically
excited state. For a fixed control time window, the optimized pulsed control
often leads to a higher degree of coherence than the control by a smooth single
Gaussian pulse. The reduction of dephasing is considerable already for 2-3
pulses.Comment: Final version (moderate changes
Experimental imaging and atomistic modeling of electron and hole quasiparticle wave functions in InAs/GaAs quantum dots
We present experimental magnetotunneling results and atomistic
pseudopotential calculations of quasiparticle electron and hole wave functions
of self-assembled InAs/GaAs quantum dots. The combination of a predictive
theory along with the experimental results allows us to gain direct insight
into the quantum states. We monitor the effects of (i) correlations, (ii)
atomistic symmetry and (iii) piezoelectricity on the confined carriers and (iv)
observe a peculiar charging sequence of holes that violates the Aufbau
principle.Comment: Submitted to Physical Review B. A version of this paper with figures
can be found at http://www.sst.nrel.gov/nano_pub/mts_preprint.pd
A Large Blue Shift of the Biexciton State in Tellurium Doped CdSe Colloidal Quantum Dots
The exciton-exciton interaction energy of Tellurium doped CdSe colloidal
quantum dots is experimentally investigated. The dots exhibit a strong Coulomb
repulsion between the two excitons, which results in a huge measured biexciton
blue shift of up to 300 meV. Such a strong Coulomb repulsion implies a very
narrow hole wave function localized around the defect, which is manifested by a
large Stokes shift. Moreover, we show that the biexciton blue shift increases
linearly with the Stokes shift. This result is highly relevant for the use of
colloidal QDs as optical gain media, where a large biexciton blue shift is
required to obtain gain in the single exciton regime.Comment: 9 pages, 4 figure
Full configuration interaction approach to the few-electron problem in artificial atoms
We present a new high-performance configuration interaction code optimally
designed for the calculation of the lowest energy eigenstates of a few
electrons in semiconductor quantum dots (also called artificial atoms) in the
strong interaction regime. The implementation relies on a single-particle
representation, but it is independent of the choice of the single-particle
basis and, therefore, of the details of the device and configuration of
external fields. Assuming no truncation of the Fock space of Slater
determinants generated from the chosen single-particle basis, the code may
tackle regimes where Coulomb interaction very effectively mixes many
determinants. Typical strongly correlated systems lead to very large
diagonalization problems; in our implementation, the secular equation is
reduced to its minimal rank by exploiting the symmetry of the effective-mass
interacting Hamiltonian, including square total spin. The resulting Hamiltonian
is diagonalized via parallel implementation of the Lanczos algorithm. The code
gives access to both wave functions and energies of first excited states.
Excellent code scalability in a parallel environment is demonstrated; accuracy
is tested for the case of up to eight electrons confined in a two-dimensional
harmonic trap as the density is progressively diluted and correlation becomes
dominant. Comparison with previous Quantum Monte Carlo simulations in the
Wigner regime demonstrates power and flexibility of the method.Comment: RevTeX 4.0, 18 pages, 6 tables, 9 postscript b/w figures. Final
version with new material. Section 6 on the excitation spectrum has been
added. Some material has been moved to two appendices, which appear in the
EPAPS web depository in the published versio
From Bioorganic Chemistry to Catalysis
Research projects of the Department of Chemistry, University of Basel are reviewed ranging from the synthesis of complex natural products to the development of metalorganic catalysts and organocatalyst
Electron microscopic and optical investigations of the indium distribution GaAs capped InxGa1-xAs islands
Results from a structural and optical analysis of buried InxGa1-xAs islands carried out after the process of GaAs overgrowth are presented. It is found that during the growth process, the indium concentration profile changes and the thickness of the wetting layer emanating from a Stranski-Krastanow growth mode grows significantly. Quantum dots are formed due to strong gradients in the indium concentration, which is demonstrated by photoluminescence and excitation spectroscopy of the buried InxGa1-xAs islands. (C) 1997 American Institute of Physics
Stability of quantum-dot excited-state laser emission under simultaneous ground-state perturbation
The impact of ground state amplification on the laser emission of In(Ga)As
quantum dot excited state lasers is studied in time-resolved experiments. We
find that a depopulation of the quantum dot ground state is followed by a drop
in excited state lasing intensity. The magnitude of the drop is strongly
dependent on the wavelength of the depletion pulse and the applied injection
current. Numerical simulations based on laser rate equations reproduce the
experimental results and explain the wavelength dependence by the different
dynamics in lasing and non-lasing sub-ensembles within the inhomogeneously
broadened quantum dots. At high injection levels, the observed response even
upon perturbation of the lasing sub-ensemble is small and followed by a fast
recovery, thus supporting the capacity of fast modulation in dual-state
devices
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