7,122 research outputs found
Min-oscillations in Escherichia coli induced by interactions of membrane-bound proteins
During division it is of primary importance for a cell to correctly determine
the site of cleavage. The bacterium Escherichia coli divides in the center,
producing two daughter cells of equal size. Selection of the center as the
correct division site is in part achieved by the Min-proteins. They oscillate
between the two cell poles and thereby prevent division at these locations.
Here, a phenomenological description for these oscillations is presented, where
lateral interactions between proteins on the cell membrane play a key role.
Solutions to the dynamic equations are compared to experimental findings. In
particular, the temporal period of the oscillations is measured as a function
of the cell length and found to be compatible with the theoretical prediction.Comment: 17 pages, 5 figures. Submitted to Physical Biolog
Quantum mechanics on manifolds and topological effects
A unique classification of the topological effects associated to quantum
mechanics on manifolds is obtained on the basis of the invariance under
diffeomorphisms and the realization of the Lie-Rinehart relations between the
generators of the diffeomorphism group and the algebra of infinitely
differentiable functions on the manifold. This leads to a unique
("Lie-Rinehart") C* algebra as observable algebra; its regular representations
are shown to be locally Schroedinger and in one to one correspondence with the
unitary representations of the fundamental group of the manifold. Therefore, in
the absence of spin degrees of freedom and external fields, the first homotopy
group of the manifold appears as the only source of topological effects.Comment: A few comments have been added to the Introduction, together with
related references; a few words have been changed in the Abstract and a Note
added to the Titl
D'yakonov-Perel' spin relaxation for degenerate electrons in the electron-hole liquid
http://arxiv.org/PS_cache/arxiv/pdf/1010/1010.5544v1.pdfWe present an analytical study of the D'yakonov-Perel' spin relaxation time for degenerate electrons in a photo-excited electron-hole liquid in intrinsic semiconductors exhibiting a spin-split band structure. The D'yakonov-Perel' spin relaxation of electrons in these materials is controlled by electron-hole scattering, with small corrections from electron-electron scattering and virtually none from electron-impurity scattering. We derive simple expressions (one-dimensional and two- dimensional integrals respectively) for the effective electron-hole and electron-electron scattering rates which enter the spin relaxation time calculation. The electron-hole scattering rate is found to be comparable to the scattering rates from impurities in the electron liquid - a common model for n-type doped semiconductors. As the density of electron-hole pairs decreases (within the degenerate regime), a strong enhancement of the scattering rates and a corresponding slowing down of spin relaxation is predicted due to exchange and correlation effects in the electron-hole liquid. In the opposite limit of high density, the original D'yakonov-Perel' model fails due to decreasing scattering rates and is eventually superseded by free precession of individual quasiparticle spins.This work was supported by the National Science
Foundation under grant number DMR-0705460 and the Department of Energy under grant number DE-FG02-05ER46203. The work of IVT was supported by Spanish MEC (FIS2007-65702-C02-01), \Grupos Consolidados UPV/EHU del Gobierno Vasco" (IT-319-07), and the European Union through e-I3 ETSF project (Contract
No. 211956)
Influence of the Particles Creation on the Flat and Negative Curved FLRW Universes
We present a dynamical analysis of the (classical) spatially flat and
negative curved Friedmann-Lameitre-Robertson-Walker (FLRW) universes evolving,
(by assumption) close to the thermodynamic equilibrium, in presence of a
particles creation process, described by means of a realiable phenomenological
approach, based on the application to the comoving volume (i. e. spatial volume
of unit comoving coordinates) of the theory for open thermodynamic systems. In
particular we show how, since the particles creation phenomenon induces a
negative pressure term, then the choice of a well-grounded ansatz for the time
variation of the particles number, leads to a deep modification of the very
early standard FLRW dynamics. More precisely for the considered FLRW models, we
find (in addition to the limiting case of their standard behaviours) solutions
corresponding to an early universe characterized respectively by an "eternal"
inflationary-like birth and a spatial curvature dominated singularity. In both
these cases the so-called horizon problem finds a natural solution.Comment: 14 pages, no figures, appeared in Class. Quantum Grav., 18, 193, 200
Dynamic critical behavior of the Chayes-Machta-Swendsen-Wang algorithm
We study the dynamic critical behavior of the Chayes-Machta dynamics for the
Fortuin-Kasteleyn random-cluster model, which generalizes the Swendsen-Wang
dynamics for the q-state Potts model to noninteger q, in two and three spatial
dimensions, by Monte Carlo simulation. We show that the Li-Sokal bound z \ge
\alpha/\nu is close to but probably not sharp in d=2, and is far from sharp in
d=3, for all q. The conjecture z \ge \beta/\nu is false (for some values of q)
in both d=2 and d=3.Comment: Revtex4, 4 pages including 4 figure
Photon Production From The Scattering of Axions Out of a Solenoidal Magnetic Field
We calculate the total cross section for the production of photons from the
scattering of axions by a strong inhomogeneous magnetic field in the form of a
2D delta-function, a cylindrical step function and a 2D Gaussian distribution,
which can be approximately produced by a solenoidal current. The theoretical
result is used to estimate the axion-photon conversion probability which could
be expected in a reasonable experimental situation. The calculated conversion
probabilities for QCD inspired axions are bigger by a factor of 2.67 (for the
cylindrical step function case) than those derived by applying the celebrated
1D calculation of the (inverse) coherent Primakoff effect. We also consider
scattering at a resonance , which corresponds to the
scattering from a delta-function and gives the most enhanced results. Finally,
we analyze the results of this work in the astrophysical extension to suggest a
way in which they may be directed to a solution to some basic solar physics
problems and, in particular, the coronal heating problem.Comment: 19 pages, 1 figure, added analysis of our results in the
astrophysical extensio
Probing the nature of possible open cluster remnants with the Southern Prope r Motion Program
We discuss the nature of eleven Possible Open Cluster Remnants (POCRs)
by using absolute proper motions from the Southern Proper Motion (SPM)
Program 3 (Girard et al. [CITE], AJ, 127, 3060) combined with near infrared photometry from 2MASS.
The analysis is done by considering the distribution of stars
in the Color–Magnitude and the Vector Point diagrams. We successfully probed
the capabilities of the SPM catalog to detect a physical group by looking at the open
cluster Blanco 1. However,
within the uncertainties of the SPM3 catalog and basing
on 2MASS photometry we conclude that only one – ESO 282SC26 – out of eleven objects turns out to be a
probable physical group. We suggest it is an open cluster 1.3 Gyr old and located 1.4 kpc
from the Sun
Lagrangian phase transitions in nonequilibrium thermodynamic systems
In previous papers we have introduced a natural nonequilibrium free energy by
considering the functional describing the large fluctuations of stationary
nonequilibrium states. While in equilibrium this functional is always convex,
in nonequilibrium this is not necessarily the case. We show that in
nonequilibrium a new type of singularities can appear that are interpreted as
phase transitions. In particular, this phenomenon occurs for the
one-dimensional boundary driven weakly asymmetric exclusion process when the
drift due to the external field is opposite to the one due to the external
reservoirs, and strong enough.Comment: 10 pages, 2 figure
Coulomb interaction effects in spin-polarized transport
We study the effect of the electron-electron interaction on the transport of
spin polarized currents in metals and doped semiconductors in the diffusive
regime. In addition to well-known screening effects, we identify two additional
effects, which depend on many-body correlations and exchange and reduce the
spin diffusion constant. The first is the "spin Coulomb drag" - an intrinsic
friction mechanism which operates whenever the average velocities of up-spin
and down-spin electrons differ. The second arises from the decrease in the
longitudinal spin stiffness of an interacting electron gas relative to a
noninteracting one. Both effects are studied in detail for both degenerate and
non-degenerate carriers in metals and semiconductors, and various limiting
cases are worked out analytically. The behavior of the spin diffusion constant
at and below a ferromagnetic transition temperature is also discussed.Comment: 9 figure
- …