725 research outputs found
Berichte der Bildungsberatungsstelle Mosbach über Regionen geringer Bildungsdichte in Nordbaden und Nordwürttemberg
Far-infrared absorption in parallel quantum wires with weak tunneling
We study collective and single-particle intersubband excitations in a system
of quantum wires coupled via weak tunneling. For an isolated wire with
parabolic confinement, the Kohn's theorem guarantees that the absorption
spectrum represents a single sharp peak centered at the frequency given by the
bare confining potential. We show that the effect of weak tunneling between two
parabolic quantum wires is twofold: (i) additional peaks corresponding to
single-particle excitations appear in the absorption spectrum, and (ii) the
main absorption peak acquires a depolarization shift. We also show that the
interplay between tunneling and weak perpendicular magnetic field drastically
enhances the dispersion of single-particle excitations. The latter leads to a
strong damping of the intersubband plasmon for magnetic fields exceeding a
critical value.Comment: 18 pages + 6 postcript figure
Microwave-induced magnetotransport phenomena in two-dimensional electron systems: Importance of electrodynamic effects
We discuss possible origins of recently discovered microwave induced
photoresistance oscillations in very-high-electron-mobility two-dimensional
electron systems. We show that electrodynamic effects -- the radiative decay,
plasma oscillations, and retardation effects, -- are important under the
experimental conditions, and that their inclusion in the theory is essential
for understanding the discussed and related microwave induced magnetotransport
phenomena.Comment: 5 pages, including 2 figures and 1 tabl
Far-Infrared Excitations below the Kohn Mode: Internal Motion in a Quantum Dot
We have investigated the far-infrared response of quantum dots in modulation
doped GaAs heterostructures. We observe novel modes at frequencies below the
center-of-mass Kohn mode. Comparison with Hartree-RPA calculations show that
these modes arise from the flattened potential in our field-effect confined
quantum dots. They reflect pronounced relative motion of the charge density
with respect to the center-of-mass.Comment: 8 pages, LaTeX with integrated 6 PostScript figure
Structures performance, benefit, cost-study
New technology concepts and structural analysis development needs which could lead to improved life cycle cost for future high-bypass turbofans were studied. The NASA-GE energy efficient engine technology is used as a base to assess the concept benefits. Recommended programs are identified for attaining these generic structural and other beneficial technologies
Gauge invariant grid discretization of Schr\"odinger equation
Using the Wilson formulation of lattice gauge theories, a gauge invariant
grid discretization of a one-particle Hamiltonian in the presence of an
external electromagnetic field is proposed. This Hamiltonian is compared both
with that obtained by a straightforward discretization of the continuous
Hamiltonian by means of balanced difference methods, and with a tight-binding
Hamiltonian. The proposed Hamiltonian and the balanced difference one are used
to compute the energy spectrum of a charged particle in a two-dimensional
parabolic potential in the presence of a perpendicular, constant magnetic
field. With this example we point out how a "naive" discretization gives rise
to an explicit breaking of the gauge invariance and to large errors in the
computed eigenvalues and corresponding probability densities; in particular,
the error on the eigenfunctions may lead to very poor estimates of the mean
values of some relevant physical quantities on the corresponding states. On the
contrary, the proposed discretized Hamiltonian allows a reliable computation of
both the energy spectrum and the probability densities.Comment: 7 pages, 4 figures, discussion about tight-binding Hamiltonians adde
Magnetoplasmon excitations in arrays of circular and noncircular quantum dots
We have investigated the magnetoplasmon excitations in arrays of circular and
noncircular quantum dots within the Thomas-Fermi-Dirac-von Weizs\"acker
approximation. Deviations from the ideal collective excitations of isolated
parabolically confined electrons arise from local perturbations of the
confining potential as well as interdot Coulomb interactions. The latter are
unimportant unless the interdot separations are of the order of the size of the
dots. Local perturbations such as radial anharmonicity and noncircular symmetry
lead to clear signatures of the violation of the generalized Kohn theorem. In
particular, the reduction of the local symmetry from SO(2) to results in
a resonant coupling of different modes and an observable anticrossing behaviour
in the power absorption spectrum. Our results are in good agreement with recent
far-infrared (FIR) transmission experiments.Comment: 25 pages, 6 figures, typeset in RevTe
Influence of shape of quantum dots on their far-infrared absorption
We investigate the effects of the shape of quantum dots on their far-infrared
absorption in an external magnetic field by a model calculation. We focus our
attention on dots with a parabolic confinement potential deviating from the
common circular symmetry, and dots having circular doughnut shape. For a
confinement where the generalized Kohn theorem does not hold we are able to
interprete the results in terms of a mixture of a center-of-mass mode and
collective modes reflecting an excitation of relative motion of the electrons.
The calculations are performed within the time-dependent Hartree approximation
and the results are compared to available experimental results.Comment: RevTeX, 16 pages with 10 postscript figures included. Submitted to
Phys. Rev.
Collective charge-density excitations of non-circular quantum dots in a magnetic field
Recent photoabsorption measurements have revealed a rich fine structure in
the collective charge-density excitation spectrum of few-electron quantum dots
in the presence of magnetic fields. We have performed systematic computational
studies of the far-infrared density response of quantum dots, using
time-dependent density-functional theory in the linear regime and treating the
dots as two-dimensional disks. It turns out that the main characteristics
observed in the experiment can be understood in terms of the electronic shell
structure of the quantum dots. However, new features arise if a breaking of the
circular symmetry of the dots is allowed, leading to an improved description of
the experimental results.Comment: 18 pages, 5 figures, submitted to Phys. Rev.
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