1,693 research outputs found
Exact Results for the Asymmetric Simple Exclusion Process with a Blockage
We present new results for the current as a function of transmission rate in
the one dimensional totally asymmetric simple exclusion process (TASEP) with a
blockage that lowers the jump rate at one site from one to r < 1. Exact finite
volume results serve to bound the allowed values for the current in the
infinite system. This proves the existence of a gap in allowed density
corresponding to a nonequilibrium ``phase transition'' in the infinite system.
A series expansion in r, derived from the finite systems, is proven to be
asymptotic for all sufficiently large systems. Pade approximants based on this
series, which make specific assumptions about the nature of the singularity at
r = 1, match numerical data for the ``infinite'' system to a part in 10^4.Comment: 18 pages, LaTeX (including figures in LaTeX picture mode
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Cholinergic regulation of mood: from basic and clinical studies to emerging therapeutics.
Mood disorders are highly prevalent and are the leading cause of disability worldwide. The neurobiological mechanisms underlying depression remain poorly understood, although theories regarding dysfunction within various neurotransmitter systems have been postulated. Over 50 years ago, clinical studies suggested that increases in central acetylcholine could lead to depressed mood. Evidence has continued to accumulate suggesting that the cholinergic system has a important role in mood regulation. In particular, the finding that the antimuscarinic agent, scopolamine, exerts fast-onset and sustained antidepressant effects in depressed humans has led to a renewal of interest in the cholinergic system as an important player in the neurochemistry of major depression and bipolar disorder. Here, we synthesize current knowledge regarding the modulation of mood by the central cholinergic system, drawing upon studies from human postmortem brain, neuroimaging, and drug challenge investigations, as well as animal model studies. First, we describe an illustrative series of early discoveries which suggest a role for acetylcholine in the pathophysiology of mood disorders. Then, we discuss more recent studies conducted in humans and/or animals which have identified roles for both acetylcholinergic muscarinic and nicotinic receptors in different mood states, and as targets for novel therapies
Vacuum Geometry of the N=2 Wess-Zumino Model
We give a mathematically rigorous construction of the moduli space and vacuum
geometry of a class of quantum field theories which are N=2 supersymmetric
Wess-Zumino models on a cylinder. These theories have been proven to exist in
the sense of constructive quantum field theory, and they also satisfy the
assumptions used by Vafa and Cecotti in their study of the geometry of ground
states. Since its inception, the Vafa-Cecotti theory of
topological-antitopological fusion, or tt* geometry, has proven to be a
powerful tool for calculations of exact quantum string amplitudes. However, tt*
geometry postulates the existence of certain vector bundles and holomorphic
sections built from the ground states. Our purpose in the present article is to
give a mathematical proof that this postulate is valid within the context of
the two-dimensional N=2 supersymmetric Wess-Zumino models. We also give a
simpler proof in the case of holomorphic quantum mechanics.Comment: 38 page
Large Scale Simulations of Two-Species Annihilation, A+B->0, with Drift
We present results of computer simulations of the diffusion-limited reaction
process A+B->0, on the line, under extreme drift conditions, for lattices of up
to 2^{27} sites, and where the process proceeds to completion (no particles
left). These enormous simulations are made possible by the renormalized
reaction-cell method (RRC). Our results allow us to resolve an existing
controversy about the rate of growth of domain sizes, and about corrections to
scaling of the concentration decay.Comment: 13 pages, RevTeX, Submitted to Physics Letters
Spatial Organization in the Reaction A + B --> inert for Particles with a Drift
We describe the spatial structure of particles in the (one dimensional)
two-species annihilation reaction A + B --> 0, where both species have a
uniform drift in the same direction and like species have a hard core
exclusion. For the case of equal initial concentration, at long times, there
are three relevant length scales: the typical distance between similar
(neighboring) particles, the typical distance between dissimilar (neighboring)
particles, and the typical size of a cluster of one type of particles. These
length scales are found to be generically different than that found for
particles without a drift.Comment: 10 pp of gzipped uuencoded postscrip
When is a bottleneck a bottleneck?
Bottlenecks, i.e. local reductions of capacity, are one of the most relevant
scenarios of traffic systems. The asymmetric simple exclusion process (ASEP)
with a defect is a minimal model for such a bottleneck scenario. One crucial
question is "What is the critical strength of the defect that is required to
create global effects, i.e. traffic jams localized at the defect position".
Intuitively one would expect that already an arbitrarily small bottleneck
strength leads to global effects in the system, e.g. a reduction of the maximal
current. Therefore it came as a surprise when, based on computer simulations,
it was claimed that the reaction of the system depends in non-continuous way on
the defect strength and weak defects do not have a global influence on the
system. Here we reconcile intuition and simulations by showing that indeed the
critical defect strength is zero. We discuss the implications for the analysis
of empirical and numerical data.Comment: 8 pages, to appear in the proceedings of Traffic and Granular Flow
'1
Phase diagram and edge effects in the ASEP with bottlenecks
We investigate the totally asymmetric simple exclusion process (TASEP) in the
presence of a bottleneck, i.e. a sequence of consecutive defect sites with
reduced hopping rate. The influence of such a bottleneck on the phase diagram
is studied by computer simulations and a novel analytical approach. We find a
clear dependence of the current and the properties of the phase diagram not
only on the length of the bottleneck, but also on its position. For bottlenecks
near the boundaries, this motivates the concept of effective boundary rates.
Furthermore the inclusion of a second, smaller bottleneck far from the first
one has no influence on the transport capacity. These results will form the
basis of an effective description of the disordered TASEP and are relevant for
the modelling of protein synthesis or intracellular transport systems where the
motion of molecular motors is hindered by immobile blocking molecules.Comment: accepted by Physica
Localized defects in a cellular automaton model for traffic flow with phase separation
We study the impact of a localized defect in a cellular automaton model for
traffic flow which exhibits metastable states and phase separation. The defect
is implemented by locally limiting the maximal possible flow through an
increase of the deceleration probability. Depending on the magnitude of the
defect three phases can be identified in the system. One of these phases shows
the characteristics of stop-and-go traffic which can not be found in the model
without lattice defect. Thus our results provide evidence that even in a model
with strong phase separation stop-and-go traffic can occur if local defects
exist. From a physical point of view the model describes the competition
between two mechanisms of phase separation.Comment: 14 pages, 7 figure
Self-organized Criticality on Small World Networks
We study the BTW-height model of self-organized criticality on a square
lattice with some long range connections giving to the lattice the character of
small world network. We find that as function of the fraction of long
ranged bonds the power law of the avalanche size and lifetime distribution
changes following a crossover scaling law with crossover exponents 2/3 and 1
for size and lifetime respectively.Comment: 7 figure
Absence of self-organized criticality in a random-neighbor version of the OFC stick-slip model
We report some numerical simulations to investigate the existence of a
self-organized critical (SOC) state in a random-neighbor version of the OFC
model for a range of parameters corresponding to a non-conservative case. In
contrast to a recent work, we do not find any evidence of SOC. We use a more
realistic distribution of energy among sites to perform some analytical
calculations that agree with our numerical conclusions.Comment: 7 pages, 4 figures, submitted to Physica
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