17,333 research outputs found
A solution to the problems of cusps and rotation curves in dark matter halos in the cosmological standard model
We discuss various aspects of the inner structure formation in virialized
dark matter (DM) halos that form as primordial density inhomogeneities evolve
in the cosmological standard model. The main focus is on the study of central
cusps/cores and of the profiles of DM halo rotation curves, problems that
reveal disagreements among the theory, numerical simulations, and observations.
A method that was developed by the authors to describe equilibrium DM systems
is presented, which allows investigating these complex nonlinear structures
analytically and relating density distribution profiles within a halo both to
the parameters of the initial small-scale inhomogeneity field and to the
nonlinear relaxation characteristics of gravitationally compressed matter. It
is shown that cosmological random motions of matter `heat up' the DM particles
in collapsing halos, suppressing cusp-like density profiles within developing
halos, facilitating the formation of DM cores in galaxies, and providing an
explanation for the difference between observed and simulated galactic rotation
curves. The analytic conclusions obtained within this approach can be confirmed
by the N-body model simulation once improved spatial resolution is achieved for
central halo regions.Comment: 44 pages, 16 figures, 1 tabl
Vortex wandering in a forest of splayed columnar defects
We investigate the scaling properties of single flux lines in a random
pinning landscape consisting of splayed columnar defects. Such correlated
defects can be injected into Type II superconductors by inducing nuclear
fission or via direct heavy ion irradiation. The result is often very efficient
pinning of the vortices which gives, e.g., a strongly enhanced critical
current. The wandering exponent \zeta and the free energy exponent \omega of a
single flux line in such a disordered environment are obtained analytically
from scaling arguments combined with extreme-value statistics. In contrast to
the case of point disorder, where these exponents are universal, we find a
dependence of the exponents on details in the probability distribution of the
low lying energies of the columnar defects. The analytical results show
excellent agreement with numerical transfer matrix calculations in two and
three dimensions.Comment: 11 pages, 9 figure
"" Singularities and Finite Density ABJM Theory at Strong Coupling
We study non-analytic behavior in the static charge susceptibility in finite
density states of the ABJM theory using its holographic dual. Emphasis is
placed on a particular state characterized by vanishing entropy density at zero
temperature, and Fermi surface-like singularities in various fermionic
correlation functions. The susceptibility exhibits branch points in the complex
momentum plane, with a real part quantitatively very similar to the location of
the Fermi surface singularities.Comment: 29 pages, 5 figures; v2: Minor additions to overview and discussion,
some references added, version to appear in JHE
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
Crossover Phenomena in the One-Dimensional SU(4) Spin-Orbit Model under Magnetic Fields
We study the one-dimensional SU(4) exchange model under magnetic fields,
which is the simplest effective Hamiltonian in order to investigate the quantum
fluctuations concerned with the orbital degrees of freedom in coupled
spin-orbit systems. The Bethe ansatz approaches and numerical calculations
using the density matrix renormalization group method are employed. The main
concern of the paper is how the system changes from the SU(4) to the SU(2)
symmetric limit as the magnetic field is increased. For this model the
conformal field theory predicts an usual behavior: there is a jump of the
critical exponents just before the SU(2) limit. For a finite-size system,
however, the orbital-orbital correlation functions approach continuously to the
SU(2) limit after interesting crossover phenomena. The crossover takes place in
the magnetization range of 1/3 1/2 for the system with 72 sites studied
in this paper.Comment: 8 pages, 6 Postscript figures, REVTeX, submitted to Phys. Rev.
Persistence and survival in equilibrium step fluctuations
Results of analytic and numerical investigations of first-passage properties
of equilibrium fluctuations of monatomic steps on a vicinal surface are
reviewed. Both temporal and spatial persistence and survival probabilities, as
well as the probability of persistent large deviations are considered. Results
of experiments in which dynamical scanning tunneling microscopy is used to
evaluate these first-passage properties for steps with different microscopic
mechanisms of mass transport are also presented and interpreted in terms of
theoretical predictions for appropriate models. Effects of discrete sampling,
finite system size and finite observation time, which are important in
understanding the results of experiments and simulations, are discussed.Comment: 30 pages, 12 figures, review paper for a special issue of JSTA
Slip avalanches in a fiber bundle model
We study slip avalanches in disordered materials under an increasing external
load in the framework of a fiber bundle model. Over-stressed fibers of the
model do not break, instead they relax in a stick-slip event which may trigger
an entire slip avalanche. Slip avalanches are characterized by the number
slipping fibers, by the slip length, and by the load increment, which triggers
the avalanche. Our calculations revealed that all three quantities are
characterized by power law distributions with universal exponents. We show by
analytical calculations and computer simulations that varying the amount of
disorder of slip thresholds and the number of allowed slips of fibers, the
system exhibits a disorder induced phase transition from a phase where only
small avalanches are formed to another one where a macroscopic slip appears.Comment: 6 pages, 6 figure
Hard hexagon partition function for complex fugacity
We study the analyticity of the partition function of the hard hexagon model
in the complex fugacity plane by computing zeros and transfer matrix
eigenvalues for large finite size systems. We find that the partition function
per site computed by Baxter in the thermodynamic limit for positive real values
of the fugacity is not sufficient to describe the analyticity in the full
complex fugacity plane. We also obtain a new algebraic equation for the low
density partition function per site.Comment: 49 pages, IoP styles files, lots of figures (png mostly) so using
PDFLaTeX. Some minor changes added to version 2 in response to referee
report
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