41,291 research outputs found
Screening in Ionic Systems: Simulations for the Lebowitz Length
Simulations of the Lebowitz length, , are reported
for t he restricted primitive model hard-core (diameter ) 1:1 electrolyte
for densi ties and .
Finite-size eff ects are elucidated for the charge fluctuations in various
subdomains that serve to evaluate . On extrapolation to the
bulk limit for the low-density expansions (Bekiranov and
Fisher, 1998) are seen to fail badly when (with ). At highe r densities rises above the Debye
length, \xi_{\text{D}} \prop to \sqrt{T/\rho}, by 10-30% (upto ); the variation is portrayed fairly well by generalized
Debye-H\"{u}ckel theory (Lee and Fisher, 19 96). On approaching criticality at
fixed or fixed , remains finite with
but displays a
weak entropy-like singularity.Comment: 4 pages 5 figure
Scaling for Interfacial Tensions near Critical Endpoints
Parametric scaling representations are obtained and studied for the
asymptotic behavior of interfacial tensions in the \textit{full} neighborhood
of a fluid (or Ising-type) critical endpoint, i.e., as a function \textit{both}
of temperature \textit{and} of density/order parameter \textit{or} chemical
potential/ordering field. Accurate \textit{nonclassical critical exponents} and
reliable estimates for the \textit{universal amplitude ratios} are included
naturally on the basis of the ``extended de Gennes-Fisher'' local-functional
theory. Serious defects in previous scaling treatments are rectified and
complete wetting behavior is represented; however, quantitatively small, but
unphysical residual nonanalyticities on the wetting side of the critical
isotherm are smoothed out ``manually.'' Comparisons with the limited available
observations are presented elsewhere but the theory invites new, searching
experiments and simulations, e.g., for the vapor-liquid interfacial tension on
the two sides of the critical endpoint isotherm for which an amplitude ratio
is predicted.Comment: 42 pages, 6 figures, to appear in Physical Review
Vectorial Loading of Processive Motor Proteins: Implementing a Landscape Picture
Individual processive molecular motors, of which conventional kinesin is the
most studied quantitatively, move along polar molecular tracks and, by exerting
a force on a tether, drag cellular cargoes, {\em in
vivo}, or spherical beads, {\em in vitro}, taking up to hundreds of
nanometer-scale steps. From observations of velocities and the dispersion of
displacements with time, under measured forces and controlled fuel supply
(typically ATP), one may hope to obtain insight into the molecular motions
undergone in the individual steps. In the simplest situation, the load force
may be regarded as a scalar resisting force, , acting
parallel to the track: however, experiments, originally by Gittes {\em et al.}
(1996), have imposed perpendicular (or vertical) loads, , while more
recently Block and coworkers (2002, 2003) and Carter and Cross (2005) have
studied {\em assisting} (or reverse) loads, , and also sideways (or
transverse) loads
Universality of Ionic Criticality: Size- and Charge-Asymmetric Electrolytes
Grand canonical simulations designed to resolve critical universality classes
are reported for :1 hard-core electrolyte models with diameter ratios
. For Ising-type behavior prevails.
Unbiased estimates of are within 1% of previous (biased)
estimates but the critical densities are 5 % lower. Ising character is
also established for the 2:1 and 3:1 equisized models, along with critical
amplitudes and improved estimates. For , however, strong
finite-size effects reduce the confidence level although classical and O criticality are excluded.Comment: 4 pages, 3 figure
Universality class of criticality in the restricted primitive model electrolyte
The 1:1 equisized hard-sphere electrolyte or restricted primitive model has
been simulated via grand-canonical fine-discretization Monte Carlo. Newly
devised unbiased finite-size extrapolation methods using temperature-density,
(T, rho), loci of inflections, Q = ^2/ maxima, canonical and C_V
criticality, yield estimates of (T_c, rho_c) to +- (0.04, 3)%. Extrapolated
exponents and Q-ratio are (gamma, nu, Q_c) = [1.24(3), 0.63(3); 0.624(2)] which
support Ising (n = 1) behavior with (1.23_9, 0.630_3; 0.623_6), but exclude
classical, XY (n = 2), SAW (n = 0), and n = 1 criticality with potentials
phi(r)>Phi/r^{4.9} when r \to \infty
Probability distribution of the order parameter in the directed percolation universality class
The probability distributions of the order parameter for two models in the
directed percolation universality class were evaluated. Monte Carlo simulations
have been performed for the one-dimensional generalized contact process and the
Domany-Kinzel cellular automaton. In both cases, the density of active sites
was chosen as the order parameter. The criticality of those models was obtained
by solely using the corresponding probability distribution function. It has
been shown that the present method, which has been successfully employed in
treating equilibrium systems, is indeed also useful in the study of
nonequilibrium phase transitions.Comment: 6 pages, 4 figure
Impairments in motor coordination without major changes in cerebellar plasticity in the Tc1 mouse model of Down syndrome
Down syndrome (DS) is a genetic disorder arising from the presence of a third copy of human chromosome
21 (Hsa21). Recently, O’Doherty et al. [An aneuploid mouse strain carrying human chromosome 21 with Down
syndrome phenotypes. Science 309 (2005) 2033–2037] generated a trans-species aneuploid mouse line (Tc1)
that carries an almost complete Hsa21. The Tc1 mouse is the most complete animal model for DS currently
available. Tc1 mice show many features that relate to human DS, including alterations in memory, synaptic
plasticity, cerebellar neuronal number, heart development and mandible size. Because motor deficits are
one of the most frequently occurring features of DS, we have undertaken a detailed analysis of motor behaviour
in cerebellum-dependent learning tasks that require high motor coordination and balance. In addition,
basic electrophysiological properties of cerebellar circuitry and synaptic plasticity have been investigated.
Our results reveal that, compared with controls, Tc1 mice exhibit a higher spontaneous locomotor activity,
a reduced ability to habituate to their environments, a different gait and major deficits on several measures
of motor coordination and balance in the rota rod and static rod tests. Moreover, cerebellar long-term
depression is essentially normal in Tc1 mice, with only a slight difference in time course. Our observations
provide further evidence that support the validity of the Tc1 mouse as a model for DS, which will help us to
provide insights into the causal factors responsible for motor deficits observed in persons with DS
Thermodynamic Casimir effects involving interacting field theories with zero modes
Systems with an O(n) symmetrical Hamiltonian are considered in a
-dimensional slab geometry of macroscopic lateral extension and finite
thickness that undergo a continuous bulk phase transition in the limit
. The effective forces induced by thermal fluctuations at and above
the bulk critical temperature (thermodynamic Casimir effect) are
investigated below the upper critical dimension by means of
field-theoretic renormalization group methods for the case of periodic and
special-special boundary conditions, where the latter correspond to the
critical enhancement of the surface interactions on both boundary planes. As
shown previously [\textit{Europhys. Lett.} \textbf{75}, 241 (2006)], the zero
modes that are present in Landau theory at make conventional
RG-improved perturbation theory in dimensions ill-defined. The
revised expansion introduced there is utilized to compute the scaling functions
of the excess free energy and the Casimir force for temperatures
T\geqT_{c,\infty} as functions of , where
is the bulk correlation length. Scaling functions of the
-dependent residual free energy per area are obtained whose
limits are in conformity with previous results for the Casimir amplitudes
to and display a more reasonable
small- behavior inasmuch as they approach the critical value
monotonically as .Comment: 23 pages, 10 figure
Critical adsorption and critical Casimir forces for geometrically structured confinements
We study the behavior of fluids, confined by geometrically structured
substrates, upon approaching a critical point at T = Tc in their bulk phase
diagram. As generic substrate structures periodic arrays of wedges and ridges
are considered. Based on general renormalization group arguments we calculate,
within mean field approximation, the universal scaling functions for order
parameter profiles of a fluid close to a single structured substrate and
discuss the decay of its spatial variation into the bulk. We compare the excess
adsorption at corrugated substrates with the one at planar walls. The
confinement of a critical fluid by two walls generates effective critical
Casimir forces between them. We calculate corresponding universal scaling
functions for the normal critical Casimir force between a flat and a
geometrically structured substrate as well as the lateral critical Casimir
force between two identically patterned substrates.Comment: 25 pages, 21 figure
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