2,342 research outputs found
Avoided crossings in mesoscopic systems: electron propagation on a non-uniform magnetic cylinder
We consider an electron constrained to move on a surface with revolution
symmetry in the presence of a constant magnetic field parallel to the
surface axis. Depending on and the surface geometry the transverse part of
the spectrum typically exhibits many crossings which change to avoided
crossings if a weak symmetry breaking interaction is introduced. We study the
effect of such perturbations on the quantum propagation. This problem admits a
natural reformulation to which tools from molecular dynamics can be applied. In
turn, this leads to the study of a perturbation theory for the time dependent
Born-Oppenheimer approximation
Institutionelle Erfolgsfaktoren einer Ausdehnung des Ăkologischen Landbaus â Analyse anhand von Regionen mit einem besonders hohen Anteil an ökologisch bewirtschafteter FlĂ€che
Ziel dieser Untersuchung war die Gewinnung von Informationen darĂŒber, inwieweit die Ausdehnung des Ăkologischen Landbaus in Regionen mit einem auffallend hohen Anteil an ökologisch bewirtschafteter FlĂ€che auf den Einfluss institutioneller Faktoren zurĂŒckzufĂŒhren ist, welcher Art diese institutionellen Erfolgsfaktoren sind und ob sie politisch gestĂ€rkt werden können. Forschungsleitend war die Arbeitshypothese, dass auch bei ansonsten gĂŒnstigen Bedingungen das Fehlen oder der ungenĂŒgende Entwicklungsstand institutioneller Voraussetzungen fĂŒr die Ausdehnung des Ăkologischen Landbaus einen entscheidenden Engpass bilden kann.
In einem ersten Arbeitsschritt wurde zunĂ€chst ein umfassendes VerstĂ€ndnis des Ăkologischen Landbaus aus einer institutionellen Sichtweise entwickelt. Ausgehend von einer problemorientierten sowie von einer historisch-kulturalistischen Sicht werden Erfolgsfaktoren (gesellschaftliche Einbettung, Organisation des Wissens, staatliche Förderung, regionale Nachfrage) ausgewĂ€hlt, die die regionale Ausdehnung des Ăkologischen Landbaus theoretisch beeinflussen können, und Erfolgskriterien (FlĂ€chenausdehnung, Einkommen, QualitĂ€t, StabilitĂ€t, Breite gesellschaftlicher AktivitĂ€ten, Akzeptanz und Legitimation) benannt, die der empirischen Beurteilung einer nachhaltigen Entwicklung des Ăkologischen Landbaus dienen.
Den Kern der Studie bilden vier regionale Fallstudien in Regionen mit einem verhĂ€ltnismĂ€Ăig hohem Anteil an ökologisch bewirtschafteter FlĂ€che (SĂŒdbaden, Region Bonn, SĂŒdthĂŒringen, Uckermark). Aufbauend auf eine Beschreibung wird die Bedeutung der Erfolgsfaktoren fĂŒr die Ausweitung des Ăkologischen Landbaus sowie ihre Wirkung auf die o.g. einzelnen Erfolgskriterien analysiert. Hierbei wird erkennbar, dass die Bedingungen, die zu einer FlĂ€chenausdehnung des Ăkologischen Landbaus beitragen, lokal sehr spezifisch sein können und sich heterogene Wirkungen der untersuchten Erfolgsfaktoren abzeichnen. Einzig die verbesserte finanzielle staatliche Förderung begĂŒnstigt in allen Regionen eine Ausweitung, allerdings bevorzugt auf den extensiv bewirtschafteten GrĂŒnlandstandorten.
In den Fallstudien wird ein grundlegender Wandel des gesellschaftlichen Systems Ăkologischer Landbau deutlich. Spezifische StĂ€rken des alten, endogen gewachsenen Ăkologischen Landbaus drohen verloren zu gehen, ohne dass gegenwĂ€rtig adĂ€quate funktional Ă€quivalente Strukturen entwickelt worden sind. Diese Beobachtungen legen insgesamt den Schluss nahe, dass die politisch unterstĂŒtzte Ausdehnung des Ăkologischen Landbaus einen riskanten Wachstumspfad darstellt. Vor diesem Hintergrund wird davor plĂ€diert, sowohl den Fokus als auch die Art der politischen Intervention zu verĂ€ndern, wofĂŒr VorschlĂ€ge gemacht werden
The canonical effect in statistical models for relativistic heavy ion collisions
Enforcing exact conservation laws instead of average ones in statistical
thermal models for relativistic heavy ion reactions gives raise to so called
canonical effect, which can be used to explain some enhancement effects when
going from elementary (e.g. pp) or small (pA) systems towards large AA systems.
We review the recently developed method for computation of canonical
statistical thermodynamics, and give an insight when this is needed in analysis
of experimental data.Comment: 4 pages, 3 figures. Talk given in Strangeness in Quark Matter,
Frankfurt am Main 2001. Submitted to J. Phys. G: Nucl. Part. Phy
Chemical equilibration due to heavy Hagedorn states
A scenario of heavy resonances, called massive Hagedorn states, is proposed
which exhibits a fast ( fm/c) chemical equilibration of (strange)
baryons and anti-baryons at the QCD critical temperature . For
relativistic heavy ion collisions this scenario predicts that hadronization is
followed by a brief expansion phase during which the equilibration rate is
higher than the expansion rate, so that baryons and antibaryons reach chemical
equilibrium before chemical freeze-out occurs.Comment: 9 pages, 2 figures. Invited talk given at 8th International
Conference on Strangeness in Quark Matter (SQM2004), Cape Town, South Africa,
15-20 September 200
Probability distributions in statistical ensembles with conserved charges
The probability distributions for charged particle numbers and their
densities are derived in statistical ensembles with conservation laws. It is
shown that if this limit is properly taken then the canonical and grand
canonical ensembles are equivalent. This equivalence is proven on the most
general, probability distribution level.Comment: 5 pages. A little bit shorter version due to some editorial and
language changes. To be published in Phys. Rev.
Particle production in p-p collisions and prediction for LHC energy
We analyze recent data on particle production yields obtained in p-p
collisions at SPS and RHIC energies within the statistical model. We apply the
model formulated in the canonical ensemble and focus on strange particle
production. We introduce different methods to account for strangeness
suppression effects and discuss their phenomenological verification. We show
that at RHIC the midrapidity data on strange and multistrange particle
multiplicity can be successfully described by the canonical statistical model
with and without an extra suppression effects. On the other hand, SPS data
integrated over the full phase-space require an additional strangeness
suppression factor that is beyond the conventional canonical model. This factor
is quantified by the strangeness saturation parameter or strangeness
correlation volume. Extrapolating all relevant thermal parameters from SPS and
RHIC to LHC energy we present predictions of the statistical model for particle
yields in p-p collisions at sqrt(s) = 14TeV. We discuss the role and the
influence of a strangeness correlation volume on particle production in p-p
collisions at LHC.Comment: 6 pages, 6 figures, 4 tables, submitted to Phys. Rev.
On the Asymptotic Dynamics of a Quantum System Composed by Heavy and Light Particles
We consider a non relativistic quantum system consisting of heavy and
light particles in dimension three, where each heavy particle interacts with
the light ones via a two-body potential . No interaction is assumed
among particles of the same kind. Choosing an initial state in a product form
and assuming sufficiently small we characterize the asymptotic
dynamics of the system in the limit of small mass ratio, with an explicit
control of the error. In the case K=1 the result is extended to arbitrary
. The proof relies on a perturbative analysis and exploits a
generalized version of the standard dispersive estimates for the
Schr\"{o}dinger group. Exploiting the asymptotic formula, it is also outlined
an application to the problem of the decoherence effect produced on a heavy
particle by the interaction with the light ones.Comment: 38 page
Electric field-driven coherent spin reorientation of optically generated electron spin packets in InGaAs
Full electric-field control of spin orientations is one of the key tasks in
semiconductor spintronics. We demonstrate that electric field pulses can be
utilized for phase-coherent +/- pi spin rotation of optically generated
electron spin packets in InGaAs epilayers detected by time-resolved Faraday
rotation. Through spin-orbit interaction, the electric-field pulses act as
local magnetic field pulses (LMFP). By the temporal control of the LMFP, we can
turn on and off electron spin precession and thereby rotate the spin direction
into arbitrary orientations in a 2-dimensional plane. Furthermore, we
demonstrate a spin echo-type spin drift experiment and find an unexpected
partial spin rephasing, which is evident by a doubling of the spin dephasing
time.Comment: 10 pages, 4 figure
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