649 research outputs found
Rotational levels in quantum dots
Low energy spectra of isotropic quantum dots are calculated in the regime of
low electron densities where Coulomb interaction causes strong correlations.
The earlier developed pocket state method is generalized to allow for
continuous rotations. Detailed predictions are made for dots of shallow
confinements and small particle numbers, including the occurance of spin
blockades in transport.Comment: RevTeX, 10 pages, 2 figure
Theory of momentum resolved tunneling into a short quantum wire
Motivated by recent tunneling experiments in the parallel wire geometry, we
calculate results for momentum resolved tunneling into a short one-dimensional
wire, containing a small number of electrons. We derive some general theorems
about the momentum dependence, and we carry out exact calculations for up to
N=4 electrons in the final state, for a system with screened Coulomb
interactions that models the situation of the experiments. We also investigate
the limit of large using a Luttinger-liquid type analysis. We consider the
low-density regime, where the system is close to the Wigner crystal limit, and
where the energy scale for spin excitations can be much lower than for charge
excitations, and we consider temperatures intermediate between the relevant
spin energies and charge excitations, as well as temperatures below both energy
scales.Comment: 19 pages, 13 figures, clarified text in a few points, added 1 figure,
updated reference
Completing Queries: Rewriting of IncompleteWeb Queries under Schema Constraints
Reactive Web systems, Web services, and Web-based publish/
subscribe systems communicate events as XML messages, and in
many cases require composite event detection: it is not sufficient to react
to single event messages, but events have to be considered in relation to
other events that are received over time.
Emphasizing language design and formal semantics, we describe the
rule-based query language XChangeEQ for detecting composite events.
XChangeEQ is designed to completely cover and integrate the four complementary
querying dimensions: event data, event composition, temporal
relationships, and event accumulation. Semantics are provided as
model and fixpoint theories; while this is an established approach for rule
languages, it has not been applied for event queries before
Spin and Charge Luttinger-Liquid Parameters of the One-Dimensional Electron Gas
Low-energy properties of the homogeneous electron gas in one dimension are
completely described by the group velocities of its charge (plasmon) and spin
collective excitations. Because of the long range of the electron-electron
interaction, the plasmon velocity is dominated by an electrostatic contribution
and can be estimated accurately. In this Letter we report on Quantum Monte
Carlo simulations which demonstrate that the spin velocity is substantially
decreased by interactions in semiconductor quantum wire realizations of the
one-dimensional electron liquid.Comment: 13 pages, figures include
Ladder approximation to spin velocities in quantum wires
The spin sector of charge-spin separated single mode quantum wires is
studied, accounting for realistic microscopic electron-electron interactions.
We utilize the ladder approximation (LA) to the interaction vertex and exploit
thermodynamic relations to obtain spin velocities. Down to not too small
carrier densities our results compare well with existing quantum Monte-Carlo
(QMC) data. Analyzing second order diagrams we identify logarithmically
divergent contributions as crucial which the LA includes but which are missed,
for example, by the self-consistent Hartree-Fock approximation. Contrary to
other approximations the LA yields a non-trivial spin conductance. Its
considerably smaller computational effort compared to numerically exact
methods, such as the QMC method, enables us to study overall dependences on
interaction parameters. We identify the short distance part of the interaction
to govern spin sector properties.Comment: 6 pages, 6 figures, to appear in Physical Review
Zur Gegenwart der Fabrik der Zukunft: ForschungsaktivitÀten im bundesdeutschen Maschinenbau
Gegenstand der folgenden Untersuchung sind die Forschungs- und Entwicklungs-(FuE-)AktivitĂ€ten der Unternehmen in der bundesdeutschen Maschinenbauindustrie. Im Mittelpunkt der Analyse stehen strategische Orientierungen dieser AktivitĂ€ten, ihre organisatorische Ausgestaltung in den Unternehmen und die Bedeutung der Zusammenarbeit mit anderen Unternehmen bzw. wissenschaftlichen Einrichtungen. Die These wird vertreten, daĂ gegenwĂ€rtig grundlegende technologische VerĂ€nderungen die etablierten FuE-Strukturen in Maschinenbauunternehmen einem zunehmenden Anpassungsdruck aussetzen.The following study is concerned with research and development (R & D) activities in the West German machine-building industry. R & D strategies, the organization of R & D within companies and the relevance of cooperation with other companies or scientific institutions are analyzed. It is argued that fundamental technological change puts increasing pressure on machine-building companies to adapt and restructure traditionally established R&D activities.1. Einleitung 2. Interne FuE-Organisation: Inkrementelle Innovationsstrategie 3. Heterogene Branchenstruktur: Vertikale Koordination 4. Ăkonomische Konkurrenzposition: GefĂ€hrdete Spitze 5. Technologische Herausforderungen: Technologischer Paradigmenwechsel 6. Reorganisation der FuE-Struktur: Autonomisierung, Diversifikation und Integration 7. Intraindustrielle Zusammenarbeit: Neue Kooperationsformen 8. "Fabrik der Zukunft": Staatliche Steuerungsleistungen? Literatu
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