12,099 research outputs found
Synchronization Model for Stock Market Asymmetry
The waiting time needed for a stock market index to undergo a given
percentage change in its value is found to have an up-down asymmetry, which,
surprisingly, is not observed for the individual stocks composing that index.
To explain this, we introduce a market model consisting of randomly fluctuating
stocks that occasionally synchronize their short term draw-downs. These
synchronous events are parameterized by a ``fear factor'', that reflects the
occurrence of dramatic external events which affect the financial market.Comment: 4 pages, 4 figure
Competition between Diffusion and Fragmentation: An Important Evolutionary Process of Nature
We investigate systems of nature where the common physical processes
diffusion and fragmentation compete. We derive a rate equation for the size
distribution of fragments. The equation leads to a third order differential
equation which we solve exactly in terms of Bessel functions. The stationary
state is a universal Bessel distribution described by one parameter, which fits
perfectly experimental data from two very different system of nature, namely,
the distribution of ice crystal sizes from the Greenland ice sheet and the
length distribution of alpha-helices in proteins.Comment: 4 pages, 3 figures, (minor changes
Critical behavior of a one-dimensional monomer-dimer reaction model with lateral interactions
A monomer-dimer reaction lattice model with lateral repulsion among the same
species is studied using a mean-field analysis and Monte Carlo simulations. For
weak repulsions, the model exhibits a first-order irreversible phase transition
between two absorbing states saturated by each different species. Increasing
the repulsion, a reactive stationary state appears in addition to the saturated
states. The irreversible phase transitions from the reactive phase to any of
the saturated states are continuous and belong to the directed percolation
universality class. However, a different critical behavior is found at the
point where the directed percolation phase boundaries meet. The values of the
critical exponents calculated at the bicritical point are in good agreement
with the exponents corresponding to the parity-conserving universality class.
Since the adsorption-reaction processes does not lead to a non-trivial local
parity-conserving dynamics, this result confirms that the twofold symmetry
between absorbing states plays a relevant role in determining the universality
class. The value of the exponent , which characterizes the
fluctuations of an interface at the bicritical point, supports the
Bassler-Brown's conjecture which states that this is a new exponent in the
parity-conserving universality class.Comment: 19 pages, 22 figures, to be published in Phys. Rev
Reentrant phase diagram of branching annihilating random walks with one and two offsprings
We investigate the phase diagram of branching annihilating random walks with
one and two offsprings in one dimension. A walker can hop to a nearest neighbor
site or branch with one or two offsprings with relative ratio. Two walkers
annihilate immediately when they meet. In general, this model exhibits a
continuous phase transition from an active state into the absorbing state
(vacuum) at a finite hopping probability. We map out the phase diagram by Monte
Carlo simulations which shows a reentrant phase transition from vacuum to an
active state and finally into vacuum again as the relative rate of the
two-offspring branching process increases. This reentrant property apparently
contradicts the conventional wisdom that increasing the number of offsprings
will tend to make the system more active. We show that the reentrant property
is due to the static reflection symmetry of two-offspring branching processes
and the conventional wisdom is recovered when the dynamic reflection symmetry
is introduced instead of the static one.Comment: 14 pages, Revtex, 4 figures (one PS figure file upon request)
(submitted to Phy. Rev. E
Branching annihilating random walks with parity conservation on a square lattice
Using Monte Carlo simulations we have studied the transition from an "active"
steady state to an absorbing "inactive" state for two versions of the branching
annihilating random walks with parity conservation on a square lattice. In the
first model the randomly walking particles annihilate when they meet and the
branching process creates two additional particles; in the second case we
distinguish particles and antiparticles created and annihilated in pairs. Quite
distinct critical behavior is found in the two cases, raising the question of
what determines universality in this kind of systems.Comment: 4 pages, 4 EPS figures include
Dynamic behavior of driven interfaces in models with two absorbing states
We study the dynamics of an interface (active domain) between different
absorbing regions in models with two absorbing states in one dimension;
probabilistic cellular automata models and interacting monomer-dimer models.
These models exhibit a continuous transition from an active phase into an
absorbing phase, which belongs to the directed Ising (DI) universality class.
In the active phase, the interface spreads ballistically into the absorbing
regions and the interface width diverges linearly in time. Approaching the
critical point, the spreading velocity of the interface vanishes algebraically
with a DI critical exponent. Introducing a symmetry-breaking field that
prefers one absorbing state over the other drives the interface to move
asymmetrically toward the unpreferred absorbing region. In Monte Carlo
simulations, we find that the spreading velocity of this driven interface shows
a discontinuous jump at criticality. We explain that this unusual behavior is
due to a finite relaxation time in the absorbing phase. The crossover behavior
from the symmetric case (DI class) to the asymmetric case (directed percolation
class) is also studied. We find the scaling dimension of the symmetry-breaking
field .Comment: 5 pages, 5 figures, Revte
Chiral phase transitions and quantum critical points of the D3/D7(D5) system with mutually perpendicular E and B fields at finite temperature and density
We study chiral symmetry restoration with increasing temperature and density
in gauge theories subject to mutually perpendicular electric and magnetic
fields using holography. We determine the chiral symmetry breaking phase
structure of the D3/D7 and D3/D5 systems in the temperature-density-electric
field directions. A magnetic field may break the chiral symmetry and an
additional electric field induces Ohm and Hall currents as well as restoring
the chiral symmetry. At zero temperature the D3/D5 system displays a line of
holographic BKT phase transitions in the density-electric field plane, while
the D3/D7 system shows a mean-field phase transition. At intermediate
temperatures, the transitions in the density-electric field plane are of first
order at low density, transforming to second order at critical points as
density rises. At high temperature the transition is only ever first order.Comment: 15 pages, 7 figures, v2: Added a referenc
Optimal concentrations in transport systems
Many biological and man-made systems rely on transport systems for the distribution of material, for example matter and energy. Material transfer in these systems is determined by the flow rate and the concentration of material. While the most concentrated solutions offer the greatest potential in terms of material transfer, impedance typically increases with concentration, thus making them the most difficult to transport. We develop a general framework for describing systems for which impedance increases with concentration, and consider material flow in four different natural systems: blood flow in vertebrates, sugar transport in vascular plants and two modes of nectar drinking in birds and insects. The model provides a simple method for determining the optimum concentration c[subscript opt] in these systems. The model further suggests that the impedance at the optimum concentration μ[subscript opt] may be expressed in terms of the impedance of the pure (c = 0) carrier medium μ[subscript 0] as μ[subscript opt]∼2[superscript α]μ[subscript 0], where the power α is prescribed by the specific flow constraints, for example constant pressure for blood flow (α = 1) or constant work rate for certain nectar-drinking insects (α = 6). Comparing the model predictions with experimental data from more than 100 animal and plant species, we find that the simple model rationalizes the observed concentrations and impedances. The model provides a universal framework for studying flows impeded by concentration, and yields insight into optimization in engineered systems, such as traffic flow.National Science Foundation (U.S.) (Grant 1021779)National Science Foundation (U.S.) (Grant DMS-0907955)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-0820484
Holographic DC conductivities from the open string metric
We study the DC conductivities of various holographic models using the open
string metric (OSM), which is an effective metric geometrizing density and
electromagnetic field effect. We propose a new way to compute the nonlinear
conductivity using OSM. As far as the final conductivity formula is concerned,
it is equivalent to the Karch-O'Bannon's real-action method. However, it yields
a geometrical insight and technical simplifications. Especially, a real-action
condition is interpreted as a regular geometry condition of OSM. As
applications of the OSM method, we study several holographic models on the
quantum Hall effect and strange metal. By comparing a Lifshitz background and
the Light-Cone AdS, we show how an extra parameter can change the temperature
scaling behavior of conductivity. Finally we discuss how OSM can be used to
study other transport coefficients, such as diffusion constant, and effective
temperature induced by the effective world volume horizon.Comment: 33 page
- …