459 research outputs found
Coherent eta photoproduction on the deuteron in the resonance region
Coherent eta photoproduction on the deuteron is studied in the
resonance region neglecting eta rescattering and two-body processes. For the
elementary reaction on the nucleon, we have considered the dominant
resonance and as background the nucleon pole terms as well as
and meson exchange. We have studied the influence of different
choices for the neutron resonance amplitude, different prescriptions for fixing
the invariant mass of the resonance amplitude and different methods for
deriving the elementary production amplitude for an arbitrary reference frame.Comment: 33 pages revtex including 13 postscript figure
Selfconsistent Approximations in Mori's Theory
The constitutive quantities in Mori's theory, the residual forces, are
expanded in terms of time dependent correlation functions and products of
operators at , where it is assumed that the time derivatives of the
observables are given by products of them. As a first consequence the
Heisenberg dynamics of the observables are obtained as an expansion of the same
type. The dynamic equations for correlation functions result to be
selfconsistent nonlinear equations of the type known from mode-mode coupling
approximations. The approach yields a neccessary condition for the validity of
the presented equations. As a third consequence the static correlations can be
calculated from fluctuation-dissipation theorems, if the observables obey a Lie
algebra. For a simple spin model the convergence of the expansion is studied.
As a further test, dynamic and static correlations are calculated for a
Heisenberg ferromagnet at low temperatures, where the results are compared to
those of a Holstein Primakoff treatment.Comment: 51 pages, Latex, 3 eps figures included, elsart and epsf style files
included, also available at
http://athene.fkp.physik.th-darmstadt.de/public/wolfram.html and
ftp://athene.fkp.physik.th-darmstadt.de/pub/publications/wolfram
Minimal Model for Sand Dunes
We propose a minimal model for aeolian sand dunes. It combines an analytical
description of the turbulent wind velocity field above the dune with a
continuum saltation model that allows for saturation transients in the sand
flux. The model provides a qualitative understanding of important features of
real dunes, such as their longitudinal shape and aspect ratio, the formation of
a slip face, the breaking of scale invariance, and the existence of a minimum
dune size.Comment: 4 pages, 4 figures, replaced with publishd versio
Bifurcations of two coupled classical spin oscillators
Two classical, damped and driven spin oscillators with an isotropic exchange
interaction are considered. They represent a nontrivial physical system whose
equations of motion are shown to allow for an analytic treatment of local
codimension 1 and 2 bifurcations. In addition, numerical results are presented
which exhibit a Feigenbaum route to chaos.Comment: 16 pages, .dvi and postscrip
High-Precision Temperature Control of a Crystal Growth Furnace at 1,500 C
For crystal growth of semiconductor materials a short-term temperature stability of 0.1 C at 1500 C is one of the essential parameters to be addressed for achieving high-quality crystals. Hence, for temperature monitoring and control with high precision in a floating zone furnace two sets of thermo-sensors, type B thermocouples and optical fibre thermometers, have been implemented and successfully operated in the furnace for more than 2000 h. The optical fibre thermometers consist of an optical system made of sapphire (two fibres plus a prism in between for deflection) and transmit the infra-red radiation of the heater to the outside of the hot core of the furnace for pyrometric temperature measurement. A dedicated control algorithm has been set up which controlled the power settings to the individual heaters. Both sensor types showed no degradation after this period and yielded a short-term stability at 1200 C of 0.05 C (optical fibre thermometers), respectively 0.08 C (thermocouples)
Barchan dune corridors: field characterization and investigation of control parameters
The structure of the barchan field located between Tarfaya and Laayoune
(Atlantic Sahara, Morocco) is quantitatively investigated and compared to that
in La Pampa de la Joya (Arequipa, Peru). On the basis of field measurements, we
show how the volume, the velocity and the output sand flux of a dune can be
computed from the value of its body and horn widths. The dune size distribution
is obtained from the analysis of aerial photographs. It shows that these fields
are in a statistically homogeneous state along the wind direction and present a
`corridor' structure in the transverse direction, in which the dunes have a
rather well selected size. Investigating the possible external parameters
controlling these corridors, we demonstrate that none among topography,
granulometry, wind and sand flux is relevant. We finally discuss the dynamical
processes at work in these fields (collisions and wind fluctuations), and
investigate the way they could regulate the size of the dunes. Furthermore we
show that the overall sand flux transported by a dune field is smaller than the
maximum transport that could be reached in the absence of dunes, i.e. in
saltation over the solid ground.Comment: revised version for JGR-ES, 36 pages, 21 figure
Amplitude equations near pattern forming instabilities for strongly driven ferromagnets
A transversally driven isotropic ferromagnet being under the influence of a
static external and an uniaxial internal anisotropy field is studied. We
consider the dissipative Landau-Lifshitz equation as the fundamental equation
of motion and treat it in ~dimensions. The stability of the spatially
homogeneous magnetizations against inhomogeneous perturbations is analyzed.
Subsequently the dynamics above threshold is described via amplitude equations
and the dependence of their coefficients on the physical parameters of the
system is determined explicitly. We find soft- and hard-mode instabilities,
transitions between sub- and supercritical behaviour, various bifurcations of
higher codimension, and present a series of explicit bifurcation diagrams. The
analysis of the codimension-2 point where the soft- and hard-mode instabilities
coincide leads to a system of two coupled Ginzburg-Landau equations.Comment: LATeX, 25 pages, submitted to Z.Phys.B figures available via
[email protected] in /pub/publications/frank/zpb_95
(postscript, plain or gziped
Coherent -photoproduction on He and C in the near-threshold region
Coherent meson photoproduction on He and C is considered in
the near-threshold region. The elementary photoproduction operator
includes contributions from the and resonances as
well as -channel vector meson exchange and the nucleon pole terms. Due to
the suppression of the dominant resonance for spin and isospin
saturated nuclei, the reaction is mainly governed by exchange.
Furthermore, the influence of Fermi motion and of different prescriptions for
the choice of the invariant reaction energy in the elementary
amplitude is studied.Comment: 27 pages revtex including 9 postscript figure
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