3,601 research outputs found
Exact Occupation Time Distribution in a Non-Markovian Sequence and Its Relation to Spin Glass Models
We compute exactly the distribution of the occupation time in a discrete {\em
non-Markovian} toy sequence which appears in various physical contexts such as
the diffusion processes and Ising spin glass chains. The non-Markovian property
makes the results nontrivial even for this toy sequence. The distribution is
shown to have non-Gaussian tails characterized by a nontrivial large deviation
function which is computed explicitly. An exact mapping of this sequence to an
Ising spin glass chain via a gauge transformation raises an interesting new
question for a generic finite sized spin glass model: at a given temperature,
what is the distribution (over disorder) of the thermally averaged number of
spins that are aligned to their local fields? We show that this distribution
remains nontrivial even at infinite temperature and can be computed explicitly
in few cases such as in the Sherrington-Kirkpatrick model with Gaussian
disorder.Comment: 10 pages Revtex (two-column), 1 eps figure (included
Critical dimensions of the diffusion equation
We study the evolution of a random initial field under pure diffusion in
various space dimensions. From numerical calculations we find that the
persistence properties of the system show sharp transitions at critical
dimensions d1 ~ 26 and d2 ~ 46. We also give refined measurements of the
persistence exponents for low dimensions.Comment: 4 pages, 5 figure
Sign-time distribution for a random walker with a drifting boundary
We present a derivation of the exact sign-time distribution for a random
walker in the presence of a boundary moving with constant velocity.Comment: 5 page
Analytical results for generalized persistence properties of smooth processes
We present a general scheme to calculate within the independent interval
approximation generalized (level-dependent) persistence properties for
processes having a finite density of zero-crossings. Our results are especially
relevant for the diffusion equation evolving from random initial conditions,
one of the simplest coarsening systems. Exact results are obtained in certain
limits, and rely on a new method to deal with constrained multiplicative
processes. An excellent agreement of our analytical predictions with direct
numerical simulations of the diffusion equation is found.Comment: 21 pages, 4 figures, to appear in Journal of Physics
Elucidation of the RamA Regulon in Klebsiella pneumoniae Reveals a Role in LPS Regulation
Klebsiella pneumoniae is a significant human pathogen, in part due to high rates of multidrug resistance. RamA is an intrinsic regulator in K. pneumoniae established to be important for the bacterial response to antimicrobial challenge; however, little is known about its possible wider regulatory role in this organism during infection. In this work, we demonstrate that RamA is a global transcriptional regulator that significantly perturbs the transcriptional landscape of K. pneumoniae, resulting in altered microbe-drug or microbe-host response. This is largely due to the direct regulation of 68 genes associated with a myriad of cellular functions. Importantly, RamA directly binds and activates the lpxC, lpxL-2 and lpxO genes associated with lipid A biosynthesis, thus resulting in modifications within the lipid A moiety of the lipopolysaccharide. RamA-mediated alterations decrease susceptibility to colistin E, polymyxin B and human cationic antimicrobial peptide LL-37. Increased RamA levels reduce K. pneumoniae adhesion and uptake into macrophages, which is supported by in vivo infection studies, that demonstrate increased systemic dissemination of ramA overexpressing K. pneumoniae. These data establish that RamA-mediated regulation directly perturbs microbial surface properties, including lipid A biosynthesis, which facilitate evasion from the innate host response. This highlights RamA as a global regulator that confers pathoadaptive phenotypes with implications for our understanding of the pathogenesis of Enterobacter, Salmonella and Citrobacter spp. that express orthologous RamA proteins
Properties of the chiral spin liquid state in generalized spin ladders
We study zero temperature properties of a system of two coupled quantum spin
chains subject to fields explicitly breaking time reversal symmetry and parity.
Suitable choice of the strength of these fields gives a model soluble by Bethe
Ansatz methods which allows to determine the complete magnetic phase diagram of
the system and the asymptotics of correlation functions from the finite size
spectrum. The chiral properties of the system for both the integrable and the
nonintegrable case are studied using numerical techniques.Comment: 19 pages, 9eps figures, Late
Ultrarelativistic limits of boosted dilaton black holes
We investigate the ultrarelativistic limits of dilaton black holes, black
-branes (strings), multi-centered dilaton black hole solutions and black
-brane (string) solutions when the boost velocity approaches the speed of
light. For dilaton black holes and black -branes (boost is along the
transverse directions), the resulting geometries are gravitational shock wave
solutions generated by a single particle and membrane. For the multi-centered
dilaton black hole solutions and black -brane solutions (boost is along the
transverse directions), the limiting geometries are shock wave solutions
generated by multiple particles and membranes. When the boost is along the
membrane directions, for the black -brane and multi-centered black -brane
solution, the resulting geometries describe general plane-fronted waves
propagating along the membranes. The effect of the dilaton on the limit is
considered.Comment: Revtex, 17 pages, no figure
Landau-De Gennes theory of nematic liquid\ud crystals: the Oseen-Frank limit and beyond
We study global minimizers of a continuum Landau-De Gennes energy functional for nematic liquid crystals, in three-dimensional domains, subject to uniaxial boundary conditions. We analyze the physically relevant limit of small elastic constant and show that global minimizers converge strongly, in W 1,2 , to a global minimizer predicted by the Oseen-Frank theory for uniaxial nematic liquid crystals with constant order parameter. Moreover, the convergence is uniform in the interior of the domain, away from the singularities of the limiting Oseen-Frank global minimizer. We obtain results on the rate of convergence of the eigenvalues and the regularity of the eigenvectors of the Landau-De Gennes global minimizer.\ud
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We also study the interplay between biaxiality and uniaxiality in Landau-De Gennes global energy minimizers and obtain estimates for various related quantities such as the biaxiality parameter and the size of admissible strongly biaxial regions
Dynamics of Shock Probes in Driven Diffusive Systems
We study the dynamics of shock-tracking probe particles in driven diffusive
systems and also in equilibrium systems. In a driven system, they induce a
diverging timescale that marks the crossover between a passive scalar regime at
early times and a diffusive regime at late times; a scaling form characterises
this crossover. Introduction of probes into an equilibrium system gives rise to
a system-wide density gradient, and the presence of even a single probe can be
felt across the entire system.Comment: Accepted in Journal of Statistical Mechanics: Theory and Experimen
The convex hull for a random acceleration process in two dimensions
We compute exactly the mean perimeter and the mean area of the
convex hull of a random acceleration process of duration T in two dimensions.
We use an exact mapping that relates, via Cauchy's formulae, the computation of
the perimeter and the area of the convex hull of an arbitrary two dimensional
stochastic process [x(t); y(t)] to the computation of the extreme value
statistics of the associated one dimensional component process x(t). The latter
can be computed exactly for the one dimensional random acceleration process
even though the process in non-Markovian. Physically, our results are relevant
in describing theaverage shape of a semi-flexible ideal polymer chain in two
dimensions.Comment: 17 pages, 7 figures, accepeted in Journal of Physics A: Mathematical
and Theoretica
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