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Physics of chemoreception
Statistical fluctuations limit the precision with which a microorganism can, in a given time T, determine the concentration of a chemoattractant in the surrounding medium. The best a cell can do is to monitor continually the state of occupation of receptors distributed over its surface. For nearly optimum performance only a small fraction of the surface need be specifically adsorbing. The probability that a molecule that has collided with the cell will find a receptor is Ns/(Ns + pi a), if N receptors, each with a binding site of radius s, are evenly distributed over a cell of radius a. There is ample room for many indenpendent systems of specific receptors. The adsorption rate for molecules of moderate size cannot be significantly enhanced by motion of the cell or by stirring of the medium by the cell. The least fractional error attainable in the determination of a concentration c is approximately (TcaD) - 1/2, where D is diffusion constant of the attractant. The number of specific receptors needed to attain such precision is about a/s. Data on bacteriophage absorption, bacterial chemotaxis, and chemotaxis in a cellular slime mold are evaluated. The chemotactic sensitivity of Escherichia coli approaches that of the cell of optimum design.Molecular and Cellular Biolog
Electromagnetic Transition in Waveguide with Application to Lasers
The electromagnetic transition of two-level atomic systems in a waveguide is
calculated. Compared with the result in free space, the spontaneous emission
rate decrease because the phase space is smaller, and meanwhile, some resonance
appears in some cases. Moreover, the influence of non-uniform electromagnetic
field in a waveguide on absorption and stimulated emission is considered.
Applying the results to lasers, a method to enhance the laser power is
proposed.Comment: 4 pages, 2 figure
Metachronal wave and hydrodynamic interaction for deterministic switching rowers
We employ a model system, called rowers, as a generic physical framework to
define the problem of the coordinated motion of cilia (the metachronal wave) as
a far from equilibrium process. Rowers are active (two-state) oscillators
interacting solely through forces of hydrodynamic origin. In this work, we
consider the case of fully deterministic dynamics, find analytical solutions of
the equation of motion in the long wavelength (continuum) limit, and
investigate numerically the short wavelength limit. We prove the existence of
metachronal waves below a characteristic wavelength. Such waves are unstable
and become stable only if the sign of the coupling is reversed. We also find
that with normal hydrodynamic interaction the metachronal pattern has the form
of stable trains of traveling wave packets sustained by the onset of
anti-coordinated beating of consecutive rowers.Comment: 11 pages, 7 figure
The nuclear Schiff moment and time invariance violation in atoms
Parity and time invariance violating (P,T-odd) nuclear forces produce P,T-odd
nuclear moments. In turn, these moments can induce electric dipole moments
(EDMs) in atoms through the mixing of electron wavefunctions of opposite
parity. The nuclear EDM is screened by atomic electrons. The EDM of an atom
with closed electron subshells is induced by the nuclear Schiff moment.
Previously the interaction with the Schiff moment has been defined for a
point-like nucleus. No problems arise with the calculation of the electron
matrix element of this interaction as long as the electrons are considered to
be non-relativistic. However, a more realistic model obviously involves a
nucleus of finite-size and relativistic electrons. In this work we have
calculated the finite nuclear-size and relativistic corrections to the Schiff
moment. The relativistic corrections originate from the electron wavefunctions
and are incorporated into a ``nuclear'' moment, which we term the local dipole
moment. For mercury these corrections amount to about 25%. We have found that
the natural generalization of the electrostatic potential of the Schiff moment
for a finite-size nucleus corresponds to an electric field distribution which,
inside the nucleus, is well approximated as constant and directed along the
nuclear spin, and outside the nucleus is zero. Also in this work the plutonium
atomic EDM is estimated.Comment: 16 pages, 1 figure, minor misprints correcte
Cooperative spontaneous emission in nonuniform media
The subject of this paper is modification of cooperative spontaneous emission
by a nonuniform medium, with nonuniform distributions of electromagnetic field.
A brief analyzis is presented and it is postulated, that if spontaneous
emission from an atom is strongly suppressed, cooperative emission with another
atom may be a preferred emission channel and counteract the suppression.Comment: The final publication is available at http://www.epj.or
Rotational Cooling of Polar Molecules by Stark-tuned Cavity Resonance
A general scheme for rotational cooling of diatomic heteronuclear molecules
is proposed. It uses a superconducting microwave cavity to enhance the
spontaneous decay via Purcell effect. Rotational cooling can be induced by
sequentially tuning each rotational transition to cavity resonance, starting
from the highest transition level to the lowest using an electric field.
Electrostatic multipoles can be used to provide large confinement volume with
essentially homogeneous background electric field.Comment: 10 pages, 6 figure
The quantum vacuum at the foundations of classical electrodynamics
In the classical theory of electromagnetism, the permittivity and the
permeability of free space are constants whose magnitudes do not seem to
possess any deeper physical meaning. By replacing the free space of classical
physics with the quantum notion of the vacuum, we speculate that the values of
the aforementioned constants could arise from the polarization and
magnetization of virtual pairs in vacuum. A classical dispersion model with
parameters determined by quantum and particle physics is employed to estimate
their values. We find the correct orders of magnitude. Additionally, our simple
assumptions yield an independent estimate for the number of charged elementary
particles based on the known values of the permittivity and the permeability,
and for the volume of a virtual pair. Such interpretation would provide an
intriguing connection between the celebrated theory of classical
electromagnetism and the quantum theory in the weak field limit.Comment: Accepted in Applied Physics B: Special Issue for the 50 years of the
laser. Comments are welcome
Theory of Melting and the Optical Properties of Gold/DNA Nanocomposites
We describe a simple model for the melting and optical properties of a
DNA/gold nanoparticle aggregate. The optical properties at fixed wavelength
change dramatically at the melting transition, which is found to be higher and
narrower in temperature for larger particles, and much sharper than that of an
isolated DNA link. All these features are in agreement with available
experiments. The aggregate is modeled as a cluster of gold nanoparticles on a
periodic lattice connected by DNA bonds, and the extinction coefficient is
computed using the discrete dipole approximation. Melting takes place as an
increasing number of these bonds break with increasing temperature. The melting
temperature corresponds approximately to the bond percolation threshold.Comment: 5 pages, 4 figure. To be published in Phys. Rev.
Low Reynolds number hydrodynamics of asymmetric, oscillating dumbbell pairs
Active dumbbell suspensions constitute one of the simplest model system for
collective swimming at low Reynolds number. Generalizing recent work, we derive
and analyze stroke-averaged equations of motion that capture the effective
hydrodynamic far-field interaction between two oscillating, asymmetric
dumbbells in three space dimensions. Time-averaged equations of motion, as
those presented in this paper, not only yield a considerable speed-up in
numerical simulations, they may also serve as a starting point when deriving
continuum equations for the macroscopic dynamics of multi-swimmer suspensions.
The specific model discussed here appears to be particularly useful in this
context, since it allows one to investigate how the collective macroscopic
behavior is affected by changes in the microscopic symmetry of individual
swimmers.Comment: 10 pages, to appear in EPJ Special Topic
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