382 research outputs found
Cosmological models with linearly varying deceleration parameter
We propose a new law for the deceleration parameter that varies linearly with
time and covers Berman's law where it is constant. Our law not only allows one
to generalize many exact solutions that were obtained assuming constant
deceleration parameter, but also gives a better fit with data (from SNIa, BAO
and CMB), particularly concerning the late time behavior of the universe.
According to our law only the spatially closed and flat universes are allowed;
in both cases the cosmological fluid we obtain exhibits quintom like behavior
and the universe ends with a big-rip. This is a result consistent with recent
cosmological observations.Comment: 12 pages, 7 figures; some typo corrections; to appear in
International Journal of Theoretical Physic
Low-Loss All-Optical Zeno Switch in a Microdisk Cavity Using EIT
We present theoretical results of a low-loss all-optical switch based on
electromagnetically induced transparency and the classical Zeno effect in a
microdisk resonator. We show that a control beam can modify the atomic
absorption of the evanescent field which suppresses the cavity field buildup
and alters the path of a weak signal beam. We predict more than 35 dB of
switching contrast with less than 0.1 dB loss using just 2 micro-Watts of
control-beam power for signal beams with less than single photon intensities
inside the cavity.Comment: Updated with new references, corrected Eq 2a, and added introductory
text. 7 pages, 5 figures, 3 table
Atom gratings produced by large angle atom beam splitters
An asymptotic theory of atom scattering by large amplitude periodic
potentials is developed in the Raman-Nath approximation. The atom grating
profile arising after scattering is evaluated in the Fresnel zone for
triangular, sinusoidal, magneto-optical, and bichromatic field potentials. It
is shown that, owing to the scattering in these potentials, two
\QTR{em}{groups} of momentum states are produced rather than two distinct
momentum components. The corresponding spatial density profile is calculated
and found to differ significantly from a pure sinusoid.Comment: 16 pages, 7 figure
Wavelets techniques for pointwise anti-Holderian irregularity
In this paper, we introduce a notion of weak pointwise Holder regularity,
starting from the de nition of the pointwise anti-Holder irregularity. Using
this concept, a weak spectrum of singularities can be de ned as for the usual
pointwise Holder regularity. We build a class of wavelet series satisfying the
multifractal formalism and thus show the optimality of the upper bound. We also
show that the weak spectrum of singularities is disconnected from the casual
one (denoted here strong spectrum of singularities) by exhibiting a
multifractal function made of Davenport series whose weak spectrum di ers from
the strong one
T-Odd Correlations in pi->e nu_e gamma and pi->mu nu_mu gamma Decays
The transverse lepton polarization asymmetry in pi_l2gamma decays may probe
T-violating interactions beyond the Standard Model. Dalitz plot distributions
of the expected effects are presented and compared to the contribution from the
Standard Model final state interactions. We give an example of a
phenomenologically viable model, where a considerable contribution to the
transverse lepton polarization asymmetry arises.Comment: 19 pages, 5 figures. To be published in Phys.Rev.D. Fixed sign in FSI
contribution figure, fixed formulas in K-bar{K} mixing analysis, added some
minor comment
Kaluza-Klein Type Robertson Walker Cosmological Model With Dynamical Cosmological Term
In this paper we have analyzed the Kaluza-Klein type Robertson Walker (RW)
cosmological models by considering three different forms of variable :
, and
. It is found that, the connecting free parameters of the
models with cosmic matter and vacuum energy density parameters are equivalent,
in the context of higher dimensional space time. The expression for the look
back time, luminosity distance and angular diameter distance are also derived.
This work has thus generalized to higher dimensions the well-known results in
four dimensional space time. It is found that there may be significant
difference in principle at least, from the analogous situation in four
dimensional space time.Comment: 16 pages, no figur
Persistence of a particle in the Matheron-de Marsily velocity field
We show that the longitudinal position of a particle in a
-dimensional layered random velocity field (the Matheron-de Marsily
model) can be identified as a fractional Brownian motion (fBm) characterized by
a variable Hurst exponent for . The
fBm becomes marginal at . Moreover, using the known first-passage
properties of fBm we prove analytically that the disorder averaged persistence
(the probability of no zero crossing of the process upto time ) has a
power law decay for large with an exponent for and
for (with logarithmic correction at ), results that
were earlier derived by Redner based on heuristic arguments and supported by
numerical simulations (S. Redner, Phys. Rev. E {\bf 56}, 4967 (1997)).Comment: 4 pages Revtex, 1 .eps figure included, to appear in PRE Rapid
Communicatio
Non-Vacuum Bianchi Types I and V in f(R) Gravity
In a recent paper \cite{1}, we have studied the vacuum solutions of Bianchi
types I and V spacetimes in the framework of metric f(R) gravity. Here we
extend this work to perfect fluid solutions. For this purpose, we take stiff
matter to find energy density and pressure of the universe. In particular, we
find two exact solutions in each case which correspond to two models of the
universe. The first solution gives a singular model while the second solution
provides a non-singular model. The physical behavior of these models has been
discussed using some physical quantities. Also, the function of the Ricci
scalar is evaluated.Comment: 15 pages, accepted for publication in Gen. Realtiv. Gravi
Quantum interference in the fluorescence of a molecular system
It has been observed experimentally [H.R. Xia, C.Y. Ye, and S.Y. Zhu, Phys.
Rev. Lett. {\bf 77}, 1032 (1996)] that quantum interference between two
molecular transitions can lead to a suppression or enhancement of spontaneous
emission. This is manifested in the fluorescent intensity as a function of the
detuning of the driving field from the two-photon resonance condition. Here we
present a theory which explains the observed variation of the number of peaks
with the mutual polarization of the molecular transition dipole moments. Using
master equation techniques we calculate analytically as well as numerically the
steady-state fluorescence, and find that the number of peaks depends on the
excitation process. If the molecule is driven to the upper levels by a
two-photon process, the fluorescent intensity consists of two peaks regardless
of the mutual polarization of the transition dipole moments. If the excitation
process is composed of both a two-step one-photon process and a one-step,
two-photon process, then there are two peaks on transitions with parallel
dipole moments and three peaks on transitions with antiparallel dipole moments.
This latter case is in excellent agreement with the experiment.Comment: 11 pages, including 8 figure
Dynamical aspects of quantum entanglement for weakly coupled kicked tops
We investigate how the dynamical production of quantum entanglement for
weakly coupled, composite quantum systems is influenced by the chaotic dynamics
of the corresponding classical system, using coupled kicked tops. The linear
entropy for the subsystem (a kicked top) is employed as a measure of
entanglement. A perturbative formula for the entanglement production rate is
derived. The formula contains a correlation function that can be evaluated only
from the information of uncoupled tops. Using this expression and the
assumption that the correlation function decays exponentially which is
plausible for chaotic tops, it is shown that {\it the increment of the strength
of chaos does not enhance the production rate of entanglement} when the
coupling is weak enough and the subsystems (kicked tops) are strongly chaotic.
The result is confirmed by numerical experiments. The perturbative approach is
also applied to a weakly chaotic region, where tori and chaotic sea coexist in
the corresponding classical phase space, to reexamine a recent numerical study
that suggests an intimate relationship between the linear stability of the
corresponding classical trajectory and the entanglement production rate.Comment: 16 pages, 11 figures, submitted to Phys. Rev.
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