2,247 research outputs found
Anomalous ordering in inhomogeneously strained materials
We study a continuous quasi-two-dimensional order-disorder phase transition
that occurs in a simple model of a material that is inhomogeneously strained
due to the presence of dislocation lines. Performing Monte Carlo simulations of
different system sizes and using finite size scaling, we measure critical
exponents describing the transition of beta=0.18\pm0.02, gamma=1.0\pm0.1, and
alpha=0.10\pm0.02. Comparable exponents have been reported in a variety of
physical systems. These systems undergo a range of different types of phase
transitions, including structural transitions, exciton percolation, and
magnetic ordering. In particular, similar exponents have been found to describe
the development of magnetic order at the onset of the pseudogap transition in
high-temperature superconductors. Their common universal critical exponents
suggest that the essential physics of the transition in all of these physical
systems is the same as in our model. We argue that the nature of the transition
in our model is related to surface transitions, although our model has no free
surface.Comment: 5 pages, 3 figure
A complete devil's staircase in the Falicov-Kimball model
We consider the neutral, one-dimensional Falicov-Kimball model at zero
temperature in the limit of a large electron--ion attractive potential, U. By
calculating the general n-ion interaction terms to leading order in 1/U we
argue that the ground-state of the model exhibits the behavior of a complete
devil's staircase.Comment: 6 pages, RevTeX, 3 Postscript figure
Eigenvalue Separation in Some Random Matrix Models
The eigenvalue density for members of the Gaussian orthogonal and unitary
ensembles follows the Wigner semi-circle law. If the Gaussian entries are all
shifted by a constant amount c/Sqrt(2N), where N is the size of the matrix, in
the large N limit a single eigenvalue will separate from the support of the
Wigner semi-circle provided c > 1. In this study, using an asymptotic analysis
of the secular equation for the eigenvalue condition, we compare this effect to
analogous effects occurring in general variance Wishart matrices and matrices
from the shifted mean chiral ensemble. We undertake an analogous comparative
study of eigenvalue separation properties when the size of the matrices are
fixed and c goes to infinity, and higher rank analogues of this setting. This
is done using exact expressions for eigenvalue probability densities in terms
of generalized hypergeometric functions, and using the interpretation of the
latter as a Green function in the Dyson Brownian motion model. For the shifted
mean Gaussian unitary ensemble and its analogues an alternative approach is to
use exact expressions for the correlation functions in terms of classical
orthogonal polynomials and associated multiple generalizations. By using these
exact expressions to compute and plot the eigenvalue density, illustrations of
the various eigenvalue separation effects are obtained.Comment: 25 pages, 9 figures include
Canalization and Symmetry in Boolean Models for Genetic Regulatory Networks
Canalization of genetic regulatory networks has been argued to be favored by
evolutionary processes due to the stability that it can confer to phenotype
expression. We explore whether a significant amount of canalization and partial
canalization can arise in purely random networks in the absence of evolutionary
pressures. We use a mapping of the Boolean functions in the Kauffman N-K model
for genetic regulatory networks onto a k-dimensional Ising hypercube to show
that the functions can be divided into different classes strictly due to
geometrical constraints. The classes can be counted and their properties
determined using results from group theory and isomer chemistry. We demonstrate
that partially canalized functions completely dominate all possible Boolean
functions, particularly for higher k. This indicates that partial canalization
is extremely common, even in randomly chosen networks, and has implications for
how much information can be obtained in experiments on native state genetic
regulatory networks.Comment: 14 pages, 4 figures; version to appear in J. Phys.
Mean-Field Analysis and Monte Carlo Study of an Interacting Two-Species Catalytic Surface Reaction Model
We study the phase diagram and critical behavior of an interacting one
dimensional two species monomer-monomer catalytic surface reaction model with a
reactive phase as well as two equivalent adsorbing phase where one of the
species saturates the system. A mean field analysis including correlations up
to triplets of sites fails to reproduce the phase diagram found by Monte Carlo
simulations. The three phases coexist at a bicritical point whose critical
behavior is described by the even branching annihilating random walk
universality class. This work confirms the hypothesis that the conservation
modulo 2 of the domain walls under the dynamics at the bicritical point is the
essential feature in producing critical behavior different from directed
percolation. The interfacial fluctuations show the same universal behavior seen
at the bicritical point in a three-species model, supporting the conjecture
that these fluctuations are a new universal characteristic of the model.Comment: 11 pages using RevTeX, plus 4 Postscript figures. Uses psfig.st
Nonlinear evolution of surface morphology in InAs/AlAs superlattices via surface diffusion
Continuum simulations of self-organized lateral compositional modulation
growth in InAs/AlAs short-period superlattices on InP substrate are presented.
Results of the simulations correspond quantitatively to the results of
synchrotron x-ray diffraction experiments. The time evolution of the
compositional modulation during epitaxial growth can be explained only
including a nonlinear dependence of the elastic energy of the growing epitaxial
layer on its thickness. From the fit of the experimental data to the growth
simulations we have determined the parameters of this nonlinear dependence. It
was found that the modulation amplitude don't depend on the values of the
surface diffusion constants of particular elements.Comment: 4 pages, 3 figures, published in Phys. Rev. Lett.
http://link.aps.org/abstract/PRL/v96/e13610
Competition in Social Networks: Emergence of a Scale-free Leadership Structure and Collective Efficiency
Using the minority game as a model for competition dynamics, we investigate
the effects of inter-agent communications on the global evolution of the
dynamics of a society characterized by competition for limited resources. The
agents communicate across a social network with small-world character that
forms the static substrate of a second network, the influence network, which is
dynamically coupled to the evolution of the game. The influence network is a
directed network, defined by the inter-agent communication links on the
substrate along which communicated information is acted upon. We show that the
influence network spontaneously develops hubs with a broad distribution of
in-degrees, defining a robust leadership structure that is scale-free.
Furthermore, in realistic parameter ranges, facilitated by information exchange
on the network, agents can generate a high degree of cooperation making the
collective almost maximally efficient.Comment: 4 pages, 2 postscript figures include
Memory effects in response functions of driven vortex matter
Vortex flow in driven type II superconductors shows strong memory and history
dependent effects. Here, we study a schematic microscopic model of driven
vortices to propose a scenario for a broad set of these kind of phenomena
ranging from ``rejuvenation'' and ``stiffening'' of the system response, to
``memory'' and ``irreversibility'' in I-V characteristics
Control of mobility in molecular organic semiconductors by dendrimer generation
Conjugated dendrimers are of interest as novel materials for light-emitting diodes. They consist of a luminescent chromophore at the core with highly branched conjugated dendron sidegroups. In these materials, light emission occurs from the core and is independent of generation. The dendron branching controls the separation between the chromophores, We present here a family of conjugated dendrimers and investigate the effect of dendron branching on light emission and charge transport. We apply a number of transport measurement techniques to thin films of a conjugated dendrimer in a light-emitting diode configuration to determine the effect of chromophore spacing on charge transport. We find that the mobility is reduced by two orders of magnitude as the size of the molecule doubles with increased branching or dendrimer generation. The degree of branching allows a unique control of mobility by molecular structure. An increase in chromophore separation also results in a reduction of intermolecular interactions, which reduces the red emission tail in film photoluminescence. We find that the steady-state charge transport is well described by a simple device model incorporating the effect of generation, and use the materials to shed light on the interpretation of transient electroluminescence data. We demonstrate the significance of the ability to tune the mobility in bilayer devices, where a more balanced charge transport can be achieved
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