1,374 research outputs found
Electrochemical Reduction of Oxygen and Nitric Oxide at Low Temperature on La1−xSrxCoO3−delta Cathodes
An analytic interface dynamo over a shear layer of finite depth
Parker's analytic Cartesian interface dynamo is generalized to the case of a
shear layer of finite thickness and low resistivity ("tachocline"), bounded by
a perfect conductor ("radiative zone") on the one side, and by a highly
diffusive medium ("convective zone") supporting an -effect on the other
side. In the limit of high diffusivity contrast between the shear layer and the
diffusive medium, thought to be relevant for the Sun, a pair of exact
dispersion relations for the growth rate and frequency of dynamo modes is
analytically derived. Graphic solution of the dispersion relations displays a
somewhat unexpected, non-monotonic behaviour, the mathematical origin of which
is elucidated. The dependence of the results on the parameter values (dynamo
number and shear layer thickness) is investigated. The implications of this
result for the solar dynamo problem are discussed.Comment: 11 pages, 4 figures Geophys. Astrophys. Fluid Dyn., in pres
Black holes and Higgs stability
We study the effect of primordial black holes on the classical rate of
nucleation of AdS regions within the standard electroweak vacuum. We find that
the energy barrier for transitions to the new vacuum, which characterizes the
exponential suppression of the nucleation rate, can be reduced significantly in
the black-hole background. A precise analysis is required in order to determine
whether the the existence of primordial black holes is compatible with the form
of the Higgs potential at high temperature or density in the Standard Model or
its extensions.Comment: 27 pages, 10 figures, conclusions expanded, to appear in JCA
Collective dynamics of molecular motors pulling on fluid membranes
The collective dynamics of weakly coupled processive molecular motors are
considered theoretically. We show, using a discrete lattice model, that the
velocity-force curves strongly depend on the effective dynamic interactions
between motors and differ significantly from a simple mean field prediction.
They become essentially independent of if it is large enough. For strongly
biased motors such as kinesin this occurs if . The study of a
two-state model shows that the existence of internal states can induce
effective interactions.Comment: 5 pages, 5 figure
Transient Nucleation near the Mean-Field Spinodal
Nucleation is considered near the pseudospinodal in a one-dimensional
model with a non-conserved order parameter and long-range
interactions. For a sufficiently large system or a system with slow relaxation
to metastable equilibrium, there is a non-negligible probability of nucleation
occurring before reaching metastable equilibrium. This process is referred to
as transient nucleation. The critical droplet is defined to be the
configuration of maximum likelihood that is dynamically balanced between the
metastable and stable wells. Time-dependent droplet profiles and nucleation
rates are derived, and theoretical results are compared to computer simulation.
The analysis reveals a distribution of nucleation times with a distinct peak
characteristic of a nonstationary nucleation rate. Under the quench conditions
employed, transient critical droplets are more compact than the droplets found
in metastable equilibrium simulations and theoretical predictions.Comment: 7 Pages, 5 Figure
Two-meson cloud contribution to the baryon antidecuplet binding
We study the two-meson virtual cloud contribution to the self-energy of the
SU(3) antidecuplet, to which the Theta+ pentaquark is assumed to belong. This
is motivated by the large branching ratio of the N(1710) decay into two pions
and one nucleon. We derive effective Lagrangians that describe the N(1710)
decay into N-pi-pi with two pions in s or p wave. We obtain increased binding
for all members of the antidecuplet and a contribution to the mass splitting
between states with different strangeness which is at least 20 % of the
empirical one. We also provide predictions for three-body decays of the
pentaquark antidecuplet.Comment: 13 pages, To appear in Phys. Rev.
Strong Interactive Massive Particles from a Strong Coupled Theory
Minimal walking technicolor models can provide a nontrivial solution for
cosmological dark matter, if the lightest technibaryon is doubly charged.
Technibaryon asymmetry generated in the early Universe is related to baryon
asymmetry and it is possible to create excess of techniparticles with charge
(-2). These excessive techniparticles are all captured by , creating
\emph{techni-O-helium} ``atoms'', as soon as is formed in Big
Bang Nucleosynthesis. The interaction of techni-O-helium with nuclei opens new
paths to the creation of heavy nuclei in Big Bang Nucleosynthesis. Due to the
large mass of technibaryons, the ``atomic'' gas decouples from the
baryonic matter and plays the role of dark matter in large scale structure
formation, while structures in small scales are suppressed. Nuclear
interactions with matter slow down cosmic techni-O-helium in Earth below the
threshold of underground dark matter detectors, thus escaping severe CDMS
constraints. On the other hand, these nuclear interactions are not sufficiently
strong to exclude this form of Strongly Interactive Massive Particles by
constraints from the XQC experiment. Experimental tests of this hypothesis are
possible in search for in balloon-borne experiments (or on the ground)
and for its charged techniparticle constituents in cosmic rays and
accelerators. The ``atoms'' can cause cold nuclear transformations in
matter and might form anomalous isotopes, offering possible ways to exclude (or
prove?) their existence.Comment: 41 pages, 4 figure
Distance-Redshift in Inhomogeneous Friedmann-Lemaitre-Robertson-Walker Cosmology
Distance--redshift relations are given in terms of associated Legendre
functions for partially filled beam observations inspatially flat
Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies. These models are
dynamically pressure-free, flat FLRW on large scales but, due to mass
inhomogeneities, differ in their optical properties. The partially filled beam
area-redshift equation is a Lame equation for arbitrary FLRW and is
shown to simplify to the associated Legendre equation for the spatially flat,
i.e. case. We fit these new analytic Hubble curves to recent
supernovae (SNe) data in an attempt to determine both the mass parameter
and the beam filling parameter . We find that current data are
inadequate to limit . However, we are able to estimate what limits are
possible when the number of observed SNe is increased by factor of 10 or 100,
sample sizes achievable in the near future with the proposed SuperNova
Acceleration Probe satellite.Comment: 9 pages, 3 figure
Chaplygin gas dominated anisotropic brane world cosmological models
We present exact solutions of the gravitational field equations in the
generalized Randall-Sundrum model for an anisotropic brane with Bianchi type I
geometry, with a generalized Chaplygin gas as matter source. The generalized
Chaplygin gas, which interpolates between a high density relativistic era and a
non-relativistic matter phase, is a popular dark energy candidate. For a
Bianchi type I space-time brane filled with a cosmological fluid obeying the
generalized Chaplygin equation of state the general solution of the
gravitational field equations can be expressed in an exact parametric form,
with the comoving volume taken as parameter. In the limiting cases of a stiff
cosmological fluid, with pressure equal to the energy density, and for a
pressureless fluid, the solution of the field equations can be expressed in an
exact analytical form. The evolution of the scalar field associated to the
Chaplygin fluid is also considered and the corresponding potential is obtained.
The behavior of the observationally important parameters like shear, anisotropy
and deceleration parameter is considered in detail.Comment: 13 pages, 6 figures, accepted for publication in PR
Heterogeneous condensation of the Lennard-Jones vapor onto a nanoscale seed particle
The heterogeneous condensation of a Lennard-Jones vapor onto a nanoscale seed
particle is studied using molecular dynamics simulations. Measuring the
nucleation rate and the height of the free energy barrier using the mean first
passage time method shows that the presence of a weakly interacting seed has
little effect on the work of forming very small cluster embryos but accelerates
the rate by lowering the barrier for larger clusters. We suggest that this
results from a competition between the energetic and entropic features of
cluster formation in the bulk and at the heterogeneity. As the interaction is
increased, the free energy of formation is reduced for all cluster sizes. We
also develop a simple phenomenological model of film formation on a small seed
that captures the general features of the nucleation process for small
heterogeneities. A comparison of our simulation results with the model shows
that heterogeneous classical nucleation theory provides a good estimate of the
critical size of the film but significantly over-estimates the size of the
barrier.Comment: 9 pages, 10 figures, In Print J. Chem. Phy
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