9,297 research outputs found
Boundary critical behaviour at -axial Lifshitz points: the special transition for the case of a surface plane parallel to the modulation axes
The critical behaviour of -dimensional semi-infinite systems with
-component order parameter is studied at an -axial bulk
Lifshitz point whose wave-vector instability is isotropic in an -dimensional
subspace of . Field-theoretic renormalization group methods are
utilised to examine the special surface transition in the case where the
potential modulation axes, with , are parallel to the surface.
The resulting scaling laws for the surface critical indices are given. The
surface critical exponent , the surface crossover exponent
and related ones are determined to first order in
\epsilon=4+\case{m}{2}-d. Unlike the bulk critical exponents and the surface
critical exponents of the ordinary transition, is -dependent already
at first order in . The \Or(\epsilon) term of is
found to vanish, which implies that the difference of and
the bulk exponent is of order .Comment: 21 pages, one figure included as eps file, uses IOP style file
The target asymmetry in hard vector-meson electroproduction and parton angular momenta
The target asymmetry for electroproduction of vector mesons is investigated
within the handbag approach. While the generalized parton distribution (GPD) H
is taken from a previous analysis of the elctroproduction cross section, we
here construct the GPD E from double distributions and constrain it by the
Pauli form factors of the nucleon, positivity bounds and sum rules. Predictions
for the target asymmetry are given for various vector mesons and discussed how
experimental data on the asymmetry will further constrain E and what we may
learn about the angular momenta the partons carry.Comment: 24 pages, 11 figures, late
Generalized parton distributions in the deuteron
We introduce generalized quark and gluon distributions in the deuteron, which
can be measured in exclusive processes like deeply virtual Compton scattering
and meson electroproduction. We discuss the basic properties of these
distributions, and point out how they probe the interplay of nucleon and parton
degrees of freedom in the deuteron wave function
Logarithmic corrections in the two-dimensional Ising model in a random surface field
In the two-dimensional Ising model weak random surface field is predicted to
be a marginally irrelevant perturbation at the critical point. We study this
question by extensive Monte Carlo simulations for various strength of disorder.
The calculated effective (temperature or size dependent) critical exponents fit
with the field-theoretical results and can be interpreted in terms of the
predicted logarithmic corrections to the pure system's critical behaviour.Comment: 10 pages, 4 figures, extended version with one new sectio
Compton telescope with coded aperture mask: Imaging with the INTEGRAL/IBIS Compton mode
Compton telescopes provide a good sensitivity over a wide field of view in
the difficult energy range running from a few hundred keV to several MeV. Their
angular resolution is, however, poor and strongly energy dependent. We present
a novel experimental design associating a coded mask and a Compton detection
unit to overcome these pitfalls. It maintains the Compton performance while
improving the angular resolution by at least an order of magnitude in the field
of view subtended by the mask. This improvement is obtained only at the expense
of the efficiency that is reduced by a factor of two. In addition, the
background corrections benefit from the coded mask technique, i.e. a
simultaneous measurement of the source and background. This design is
implemented and tested using the IBIS telescope on board the INTEGRAL satellite
to construct images with a 12' resolution over a 29 degrees x 29 degrees field
of view in the energy range from 200 keV to a few MeV. The details of the
analysis method and the resulting telescope performance, particularly in terms
of sensitivity, are presented
Dynamic critical behavior of model A in films: Zero-mode boundary conditions and expansion near four dimensions
The critical dynamics of relaxational stochastic models with nonconserved
-component order parameter and no coupling to other slow
variables ("model A") is investigated in film geometries for the cases of
periodic and free boundary conditions. The Hamiltonian governing
the stationary equilibrium distribution is taken to be O(n) symmetric and to
involve, in the case of free boundary conditions, the boundary terms
associated with the two
confining surface planes , , at and , where
the enhancement variables are presumed to be subcritical or
critical. A field-theoretic RG study of the dynamic critical behavior at
bulk dimensions is presented, with special attention paid to the
cases where the classical theories involve zero modes at . This
applies when either both take the critical value
associated with the special surface transition, or
else periodic boundary conditions are imposed. Owing to the zero modes, the
expansion becomes ill-defined at . Analogously to the
static case, the field theory can be reorganized to obtain a well-defined
small- expansion involving half-integer powers of ,
modulated by powers of . Explicit results for the scaling
functions of -dependent finite-size susceptibilities at temperatures and of layer and surface susceptibilities at the bulk critical
point are given to orders and , respectively. For
the case of periodic boundary conditions, the consistency of the expansions to
with exact large- results is shown.Comment: Latex file with 8 eps files included; text added in conclusions and
abstract, typos correcte
Integration of CasADi and JModelica.org
This paper presents the integration of two open source
softwares: CasADi, which is a framework for efficient
evaluation of expressions and their derivatives, and the
Modelica-based platform JModelica.org. The integration of the tools is based on an XML format for exchange of DAE models. The JModelica.org platform supports export of model in this XML format, whereas CasADi supports import of models expressed in this format. Furthermore, we have carried out comparisons
with ACADO, which is a multiple shooting package for solving optimal control problems.
CasADi, in turn, has been interfaced with ACADO Toolkit, enabling users to define optimal control problems using Modelica and Optimica specifications, and use solve using direct multiple shooting.
In addition, a collocation algorithm targeted at solving large-
scale DAE constrained dynamic optimization problems has been implemented. This implementation explores CasADi’s Python and IPOPT interfaces, which offers a convenient, yet highly efficient environment for development of optimization algorithms. The algorithms are evaluated using industrially relevant benchmark problems
Effects of surfaces on resistor percolation
We study the effects of surfaces on resistor percolation at the instance of a
semi-infinite geometry. Particularly we are interested in the average
resistance between two connected ports located on the surface. Based on general
grounds as symmetries and relevance we introduce a field theoretic Hamiltonian
for semi-infinite random resistor networks. We show that the surface
contributes to the average resistance only in terms of corrections to scaling.
These corrections are governed by surface resistance exponents. We carry out
renormalization group improved perturbation calculations for the special and
the ordinary transition. We calculate the surface resistance exponents
\phi_{\mathcal S \mathnormal} and \phi_{\mathcal S \mathnormal}^\infty for
the special and the ordinary transition, respectively, to one-loop order.Comment: 19 pages, 3 figure
Critical behaviour near multiple junctions and dirty surfaces in the two-dimensional Ising model
We consider m two-dimensional semi-infinite planes of Ising spins joined
together through surface spins and study the critical behaviour near to the
junction. The m=0 limit of the model - according to the replica trick -
corresponds to the semi-infinite Ising model in the presence of a random
surface field (RSFI). Using conformal mapping, second-order perturbation
expansion around the weakly- and strongly-coupled planes limits and
differential renormalization group, we show that the surface critical behaviour
of the RSFI model is described by Ising critical exponents with logarithmic
corrections to scaling, while at multiple junctions (m>2) the transition is
first order. There is a spontaneous junction magnetization at the bulk critical
point.Comment: Old paper, for archiving. 6 pages, 1 figure, IOP macro, eps
Transverse Deformation of Parton Distributions and Transversity Decomposition of Angular Momentum
Impact parameter dependent parton distributions are transversely distorted
when one considers transversely polarized nucleons and/or quarks. This provides
a physical mechanism for the T-odd Sivers effect in semi-inclusive
deep-inelastic scattering. The transverse distortion can also be related to
Ji's sum rule for the angular momentum carried by the quarks. The distortion of
chirally odd impact parameter dependent parton distributions is related to
chirally odd GPDs. This result is used to provide a decomposition of the quark
angular momentum w.r.t. quarks of definite transversity. Chirally odd GPDs can
thus be used to determine the correlation between quark spin and quark angular
momentum in unpolarized nucleons. Based on the transverse distortion, we also
suggest a qualitative connection between chirally odd GPDs and the Boer-Mulders
effect.Comment: 12 pages, 1 figure, version to appear in PR
- …