459 research outputs found
Dependences of the Casimir-Polder interaction between an atom and a cavity wall on atomic and material properties
The Casimir-Polder and van der Waals interactions between an atom and a flat
cavity wall are investigated under the influence of real conditions including
the dynamic polarizability of the atom, actual conductivity of the wall
material and nonzero temperature of the wall. The cases of different atoms near
metal and dielectric walls are considered. It is shown that to obtain accurate
results for the atom-wall interaction at short separations, one should use the
complete tabulated optical data for the complex refractive index of the wall
material and the accurate dynamic polarizability of an atom. At relatively
large separations in the case of a metal wall, one may use the plasma model
dielectric function to describe the dielectric properties of wall material. The
obtained results are important for the theoretical interpretation of
experiments on quantum reflection and Bose-Einstein condensation.Comment: 5 pages, 1 figure, iopart.cls is used, to appear in J. Phys. A
(special issue: Proceedings of QFEXT05, Barcelona, Sept. 5-9, 2005
Dependences of the van der Waals atom-wall interaction on atomic and material properties
The 1%-accurate calculations of the van der Waals interaction between an atom
and a cavity wall are performed in the separation region from 3 nm to 150 nm.
The cases of metastable He and Na atoms near the metal,
semiconductor or dielectric walls are considered. Different approximations to
the description of wall material and atomic dynamic polarizability are
carefully compared. The smooth transition to the Casimir-Polder interaction is
verified. It is shown that to obtain accurate results for the atom-wall van der
Waals interaction at shortest separations with an error less than 1% one should
use the complete optical tabulated data for the complex refraction index of the
wall material and the accurate dynamic polarizability of an atom. The obtained
results may be useful for the theoretical interpretation of recent experiments
on quantum reflection and Bose-Einstein condensation of ultracold atoms on or
near surfaces of different nature.Comment: 14 pages, 5 figures, 3 tables, accepted for publication in Phys. Rev.
A theory of Plasma Membrane Calcium Pump stimulation and activity
The ATP-driven Plasma Membrane Calcium pump or Ca(2+)-ATPase (PMCA) is
characterized by a high affinity to calcium and a low transport rate compared
to other transmembrane calcium transport proteins. It plays a crucial role for
calcium extrusion from cells. Calmodulin is an intracellular calcium buffering
protein which is capable in its Ca(2+) liganded form of stimulating the PMCA by
increasing both the affinity to calcium and the maximum calcium transport rate.
We introduce a new model of this stimulation process and derive analytical
expressions for experimental observables in order to determine the model
parameters on the basis of specific experiments. We furthermore develop a model
for the pumping activity. The pumping description resolves the seeming
contradiction of the Ca(2+):ATP stoichiometry of 1:1 during a translocation
step and the observation that the pump binds two calcium ions at the
intracellular site. The combination of the calcium pumping and the stimulation
model correctly describes PMCA function. We find that the processes of
calmodulin-calcium complex attachment to the pump and of stimulation have to be
separated. Other PMCA properties are discussed in the framework of the model.
The presented model can serve as a tool for calcium dynamics simulations and
provides the possibility to characterize different pump isoforms by different
type-specific parameter sets.Comment: 24 pages, 6 figure
Thermal quantum field theory and the Casimir interaction between dielectrics
The Casimir and van der Waals interaction between two dissimilar thick
dielectric plates is reconsidered on the basis of thermal quantum field theory
in Matsubara formulation. We briefly review two main derivations of the
Lifshitz formula in the framework of thermal quantum field theory without use
of the fluctuation-dissipation theorem. A set of special conditions is
formulated under which these derivations remain valid in the presence of
dissipation. The low-temperature behavior of the Casimir and van der Waals
interactions between dissimilar dielectrics is found analytically from the
Lifshitz theory for both an idealized model of dilute dielectrics and for real
dielectrics with finite static dielectric permittivities. The free energy,
pressure and entropy of the Casimir and van der Waals interactions at low
temperatures demonstrate the same universal dependence on the temperature as
was previously discovered for ideal metals. The entropy vanishes when
temperature goes to zero proving the validity of the Nernst heat theorem. This
solves the long-standing problem on the consistency of the Lifshitz theory with
thermodynamics in the case of dielectric plates. The obtained asymptotic
expressions are compared with numerical computations for both dissimilar and
similar real dielectrics and found to be in excellent agreement. The role of
the zero-frequency term in Matsubara sum is investigated in the case of
dielectric plates. It is shown that the inclusion of conductivity in the model
of dielectric response leads to the violation of the Nernst heat theorem. The
applications of this result to the topical problems of noncontact atomic
friction and the Casimir interaction between real metals are discussed.Comment: 39 pages, 4 figures, to appear in Phys. Rev.
Lateral projection as a possible explanation of the nontrivial boundary dependence of the Casimir force
We find the lateral projection of the Casimir force for a configuration of a
sphere above a corrugated plate. This force tends to change the sphere position
in the direction of a nearest corrugation maximum. The probability distribution
describing different positions of a sphere above a corrugated plate is
suggested which is fitted well with experimental data demonstrating the
nontrivial boundary dependence of the Casimir force.Comment: 5 pages, 1 figur
Exact Casimir-Polder potential between a particle and an ideal metal cylindrical shell and the proximity force approximation
We derive the exact Casimir-Polder potential for a polarizable microparticle
inside an ideal metal cylindrical shell using the Green function method. The
exact Casimir-Polder potential for a particle outside a shell, obtained
recently by using the Hamiltonian approach, is rederived and confirmed. The
exact quantum field theoretical result is compared with that obtained using the
proximity force approximation and a very good agreement is demonstrated at
separations below 0.1, where is the radius of the cylinder. The
developed methods are applicable in the theory of topological defects.Comment: 8 pages, 4 figures, Accepted for publication in Eur. Phys. J.
First LIGO search for gravitational wave bursts from cosmic (super)strings
We report on a matched-filter search for gravitational wave bursts from
cosmic string cusps using LIGO data from the fourth science run (S4) which took
place in February and March 2005. No gravitational waves were detected in 14.9
days of data from times when all three LIGO detectors were operating. We
interpret the result in terms of a frequentist upper limit on the rate of
gravitational wave bursts and use the limits on the rate to constrain the
parameter space (string tension, reconnection probability, and loop sizes) of
cosmic string models.Comment: 11 pages, 3 figures. Replaced with version submitted to PR
All-sky LIGO Search for Periodic Gravitational Waves in the Early S5 Data
We report on an all-sky search with the LIGO detectors for periodic
gravitational waves in the frequency range 50--1100 Hz and with the frequency's
time derivative in the range -5.0E-9 Hz/s to zero. Data from the first eight
months of the fifth LIGO science run (S5) have been used in this search, which
is based on a semi-coherent method (PowerFlux) of summing strain power.
Observing no evidence of periodic gravitational radiation, we report 95%
confidence-level upper limits on radiation emitted by any unknown isolated
rotating neutron stars within the search range. Strain limits below 1.E-24 are
obtained over a 200-Hz band, and the sensitivity improvement over previous
searches increases the spatial volume sampled by an average factor of about 100
over the entire search band. For a neutron star with nominal equatorial
ellipticity of 1.0E-6, the search is sensitive to distances as great as 500
pc--a range that could encompass many undiscovered neutron stars, albeit only a
tiny fraction of which would likely be rotating fast enough to be accessible to
LIGO. This ellipticity is at the upper range thought to be sustainable by
conventional neutron stars and well below the maximum sustainable by a strange
quark star.Comment: 6 pages, 1 figur
Implications For The Origin Of GRB 051103 From LIGO Observations
We present the results of a LIGO search for gravitational waves (GWs)
associated with GRB 051103, a short-duration hard-spectrum gamma-ray burst
(GRB) whose electromagnetically determined sky position is coincident with the
spiral galaxy M81, which is 3.6 Mpc from Earth. Possible progenitors for
short-hard GRBs include compact object mergers and soft gamma repeater (SGR)
giant flares. A merger progenitor would produce a characteristic GW signal that
should be detectable at the distance of M81, while GW emission from an SGR is
not expected to be detectable at that distance. We found no evidence of a GW
signal associated with GRB 051103. Assuming weakly beamed gamma-ray emission
with a jet semi-angle of 30 deg we exclude a binary neutron star merger in M81
as the progenitor with a confidence of 98%. Neutron star-black hole mergers are
excluded with > 99% confidence. If the event occurred in M81 our findings
support the the hypothesis that GRB 051103 was due to an SGR giant flare,
making it the most distant extragalactic magnetar observed to date.Comment: 8 pages, 3 figures. For a repository of data used in the publication,
go to: https://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=15166 . Also see
the announcement for this paper on ligo.org at:
http://www.ligo.org/science/Publication-GRB051103/index.ph
- …