5,069 research outputs found
Surface critical behavior of driven diffusive systems with open boundaries
Using field theoretic renormalization group methods we study the critical
behavior of a driven diffusive system near a boundary perpendicular to the
driving force. The boundary acts as a particle reservoir which is necessary to
maintain the critical particle density in the bulk. The scaling behavior of
correlation and response functions is governed by a new exponent eta_1 which is
related to the anomalous scaling dimension of the chemical potential of the
boundary. The new exponent and a universal amplitude ratio for the density
profile are calculated at first order in epsilon = 5-d. Some of our results are
checked by computer simulations.Comment: 10 pages ReVTeX, 6 figures include
Constraining the Nature of X-ray Cavities in Clusters and Galaxies
We present results from an extensive survey of 64 cavities in the X-ray halos
of clusters, groups and normal elliptical galaxies. We show that the evolution
of the size of the cavities as they rise in the X-ray atmosphere is
inconsistent with the standard model of adiabatic expansion of purely
hydrodynamic models. We also note that the majority of the observed bubbles
should have already been shredded apart by Rayleigh-Taylor and
Richtmyer-Meshkov instabilities if they were of purely hydrodynamic nature.
Instead we find that the data agrees much better with a model where the
cavities are magnetically dominated and inflated by a current-dominated
magneto-hydrodynamic jet model, recently developed by Li et al. (2006) and
Nakamura et al. (2006). We conduct complex Monte-Carlo simulations of the
cavity detection process including incompleteness effects to reproduce the
cavity sample's characteristics. We find that the current-dominated model
agrees within 1sigma, whereas the other models can be excluded at >5sigma
confidence. To bring hydrodynamic models into better agreement, cavities would
have to be continuously inflated. However, these assessments are dependent on
our correct understanding of the detectability of cavities in X-ray
atmospheres, and will await confirmation when automated cavity detection tools
become available in the future. Our results have considerable impact on the
energy budget associated with active galactic nucleus feedback.Comment: 21 pages, 12 figures, emulateapj, accepted for publication in ApJ,
responded to referee's comments and added a new model, conclusions unchange
Critical Casimir effect in films for generic non-symmetry-breaking boundary conditions
Systems described by an O(n) symmetrical Hamiltonian are considered
in a -dimensional film geometry at their bulk critical points. A detailed
renormalization-group (RG) study of the critical Casimir forces induced between
the film's boundary planes by thermal fluctuations is presented for the case
where the O(n) symmetry remains unbroken by the surfaces. The boundary planes
are assumed to cause short-ranged disturbances of the interactions that can be
modelled by standard surface contributions corresponding
to subcritical or critical enhancement of the surface interactions. This
translates into mesoscopic boundary conditions of the generic
symmetry-preserving Robin type .
RG-improved perturbation theory and Abel-Plana techniques are used to compute
the -dependent part of the reduced excess free energy per
film area to two-loop order. When , it takes the scaling
form as
, where are scaling fields associated with the
surface-enhancement variables , while is a standard
surface crossover exponent. The scaling function
and its analogue for the Casimir force
are determined via expansion in and extrapolated to
dimensions. In the special case , the expansion
becomes fractional. Consistency with the known fractional expansions of D(0,0)
and to order is achieved by appropriate
reorganisation of RG-improved perturbation theory. For appropriate choices of
and , the Casimir forces can have either sign. Furthermore,
crossovers from attraction to repulsion and vice versa may occur as
increases.Comment: Latex source file, 40 pages, 9 figure
Microscopic Non-Universality versus Macroscopic Universality in Algorithms for Critical Dynamics
We study relaxation processes in spin systems near criticality after a quench
from a high-temperature initial state. Special attention is paid to the stage
where universal behavior, with increasing order parameter emerges from an early
non-universal period. We compare various algorithms, lattice types, and
updating schemes and find in each case the same universal behavior at
macroscopic times, despite of surprising differences during the early
non-universal stages.Comment: 9 pages, 3 figures, RevTeX, submitted to Phys. Rev. Let
Surface critical behavior of bcc binary alloys
The surface critical behavior of bcc binary alloys undergoing a continuous
B2-A2 order-disorder transition is investigated in the mean-field (MF)
approximation. Our main aim is to provide clear evidence for the fact that
surfaces which break the two-sublattice symmetry generically display the
critical behavior of the NORMAL transition, whereas symmetry-preserving
surfaces exhibit ORDINARY surface critical behavior. To this end we analyze the
lattice MF equations for both types of surfaces in terms of nonlinear
symplectic maps and derive a Ginzburg-Landau model for the symmetry-breaking
(100) surface. The crucial feature of the continuum model is the emergence of
an EFFECTIVE ORDERING (``staggered'') SURFACE FIELD, which depends on
temperature and the other lattice model parameters, and which explains the
appearance of NORMAL critical behavior for symmetry-breaking surfaces.Comment: 16 pages, REVTeX 3.0, 13 EPSF figures, submitted to Phys. Rev.
X-Ray Scattering at FeCo(001) Surfaces and the Crossover between Ordinary and Normal Transitions
In a recent experiment by Krimmel et al. [PRL 78, 3880 (1997)], the critical
behavior of FeCo near a (001) surface was studied by x-ray scattering. Here the
experimental data are reanalyzed, taking into account recent theoretical
results on order-parameter profiles in the crossover regime between ordinary
and normal transitions. Excellent agreement between theoretical expectations
and the experimental results is found.Comment: 9 pages, Latex, 1 PostScript figure, to be published in Phys.Rev.
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