615 research outputs found
The sodium-potassium pump controls the intrinsic firing of the cerebellar Purkinje neuron
In vitro, cerebellar Purkinje cells can intrinsically fire action potentials in a repeating trimodal or bimodal pattern. The trimodal pattern consists of tonic spiking, bursting, and quiescence. The bimodal pattern consists of tonic spiking and quiescence. It is unclear how these firing patterns are generated and what determines which firing pattern is selected. We have constructed a realistic biophysical Purkinje cell model that can replicate these patterns. In this model, Na+/K+ pump activity sets the Purkinje cell's operating mode. From rat cerebellar slices we present Purkinje whole cell recordings in the presence of ouabain, which irreversibly blocks the Na+/K+ pump. The model can replicate these recordings. We propose that Na+/K+ pump activity controls the intrinsic firing mode of cerbellar Purkinje cells
Final spins from the merger of precessing binary black holes
The inspiral of binary black holes is governed by gravitational radiation
reaction at binary separations r < 1000 M, yet it is too computationally
expensive to begin numerical-relativity simulations with initial separations r
> 10 M. Fortunately, binary evolution between these separations is well
described by post-Newtonian equations of motion. We examine how this
post-Newtonian evolution affects the distribution of spin orientations at
separations r ~ 10 M where numerical-relativity simulations typically begin.
Although isotropic spin distributions at r ~ 1000 M remain isotropic at r ~ 10
M, distributions that are initially partially aligned with the orbital angular
momentum can be significantly distorted during the post-Newtonian inspiral.
Spin precession tends to align (anti-align) the binary black hole spins with
each other if the spin of the more massive black hole is initially partially
aligned (anti-aligned) with the orbital angular momentum, thus increasing
(decreasing) the average final spin. Spin precession is stronger for
comparable-mass binaries, and could produce significant spin alignment before
merger for both supermassive and stellar-mass black hole binaries. We also
point out that precession induces an intrinsic accuracy limitation (< 0.03 in
the dimensionless spin magnitude, < 20 degrees in the direction) in predicting
the final spin resulting from the merger of widely separated binaries.Comment: 20 pages, 16 figures, new PN terms, submitted to PR
Numerical solution of the Boltzmann equation for the collective modes of trapped Fermi gases
We numerically solve the Boltzmann equation for trapped fermions in the
normal phase using the test-particle method. After discussing a couple of tests
in order to estimate the reliability of the method, we apply it to the
description of collective modes in a spherical harmonic trap. The numerical
results are compared with those obtained previously by taking moments of the
Boltzmann equation. We find that the general shape of the response function is
very similar in both methods, but the relaxation time obtained from the
simulation is significantly longer than that predicted by the method of
moments. It is shown that the result of the method of moments can be corrected
by including fourth-order moments in addition to the usual second-order ones
and that this method agrees very well with our numerical simulations.Comment: 13 pages, 8 figures, accepted for publication in Phys. Rev.
Phase transitions detached from stationary points of the energy landscape
The stationary points of the potential energy function V are studied for the
\phi^4 model on a two-dimensional square lattice with nearest-neighbor
interactions. On the basis of analytical and numerical results, we explore the
relation of stationary points to the occurrence of thermodynamic phase
transitions. We find that the phase transition potential energy of the \phi^4
model does in general not coincide with the potential energy of any of the
stationary points of V. This disproves earlier, allegedly rigorous, claims in
the literature on necessary conditions for the existence of phase transitions.
Moreover, we find evidence that the indices of stationary points scale
extensively with the system size, and therefore the index density can be used
to characterize features of the energy landscape in the infinite-system limit.
We conclude that the finite-system stationary points provide one possible
mechanism of how a phase transition can arise, but not the only one.Comment: 5 pages, 3 figure
Multicanonical molecular dynamics by variable-temperature thermostats and variable-pressure barostats
Sampling from flat energy or density distributions has proven useful in equilibrating complex systems
with large energy barriers. Several thermostats and barostats are presented to sample these
flat distributions by molecular dynamics. These methods use a variable temperature or pressure
that is updated on the fly in the thermodynamic controller. These methods are illustrated on a
Lennard-Jones system and a structure-based model of proteins
Multiconfigurational time-dependent Hartree-Fock calculations for photoionization of one-dimensional Helium
The multiconfigurational time-dependent Hartree-Fock equations are discussed
and solved for a one-dimensional model of the Helium atom. Results for the
ground state energy and two-particle density as well as the absorption spectrum
are presented and compared to direct solutions of the time-dependent
Schroedinger equation.Comment: 10 pages, 3 figures, 1 tabl
Comparison of Boltzmann Equations with Quantum Dynamics for Scalar Fields
Boltzmann equations are often used to study the thermal evolution of particle
reaction networks. Prominent examples are the computation of the baryon
asymmetry of the universe and the evolution of the quark-gluon plasma after
relativistic heavy ion collisions. However, Boltzmann equations are only a
classical approximation of the quantum thermalization process which is
described by the so-called Kadanoff-Baym equations. This raises the question
how reliable Boltzmann equations are as approximations to the full
Kadanoff-Baym equations. Therefore, we present in this paper a detailed
comparison between the Kadanoff-Baym and Boltzmann equations in the framework
of a scalar Phi^4 quantum field theory in 3+1 space-time dimensions. The
obtained numerical solutions reveal significant discrepancies in the results
predicted by both types of equations. Apart from quantitative discrepancies, on
a qualitative level the universality respected by the Kadanoff-Baym equations
is severely restricted in the case of Boltzmann equations. Furthermore, the
Kadanoff-Baym equations strongly separate the time scales between kinetic and
chemical equilibration. This separation of time scales is absent for the
Boltzmann equation.Comment: text and figures revised, references added, results unchanged, 21
pages, 10 figures, published in Phys. Rev. D73 (2006) 12500
Transport and optical response of molecular junctions driven by surface plasmon-polaritons
We consider a biased molecular junction subjected to external time-dependent
electromagnetic field. The field for two typical junction geometries (bowtie
antennas and metal nanospheres) is calculated within finite-difference
time-domain technique. Time-dependent transport and optical response of the
junctions is calculated within non-equilibrium Green's function approach
expressed in a form convenient for description of multi-level systems. We
present numerical results for a two-level (HOMO-LUMO) model, and discuss
influence of localized surface plasmon polariton modes on transport.Comment: 9 pages, 6 figure
Mechanisms of two-color laser-induced field-free molecular orientation
Two mechanisms of two-color (\omega + 2\omega) laser-induced field-free
molecular orientation, based on the hyperpolarizability and ionization
depletion, are explored and compared. The CO molecule is used as a
computational example. While the hyperpolarizability mechanism generates small
amounts of orientation at intensities below the ionization threshold,
ionization depletion quickly becomes the dominant mechanism as soon as ionizing
intensities are reached. Only the ionization mechanism leads to substantial
orientation (e.g. on the order of || > 0.1). For intensities typical
of laser-induced molecular alignment and orientation experiments, the two
mechanism lead to robust, characteristic timings of the field-free orientation
wave-packet revivals relative to the the alignment revivals and the revival
time. The revival timings can be used to detect the active orientation
mechanism experimentally
Disclination Asymmetry in Two-Dimensional Nematic Liquid Crystals with Unequal Frank Constants
The behavior of a thin film of nematic liquid crystal with unequal Frank
constants is discussed. Distinct Frank constants are found to imply unequal
core energies for and disclinations. Even so, a topological
constraint is shown to ensure that the bulk densities of the two types of
disclinations are the same. For a system with free boundary conditions, such as
a liquid membrane, unequal core energies simply renormalize the Gaussian
rigidity and line tension.Comment: RevTex forma
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