8,925 research outputs found
Revisiting the optical -symmetric dimer
Optics has proved a fertile ground for the experimental simulation of quantum
mechanics. Most recently, optical realizations of -symmetric
quantum mechanics have been shown, both theoretically and experimentally,
opening the door to international efforts aiming at the design of practical
optical devices exploiting this symmetry. Here, we focus on the optical
-symmetric dimer, a two-waveguide coupler were the materials show
symmetric effective gain and loss, and provide a review of the linear and
nonlinear optical realizations from a symmetry based point of view. We go
beyond a simple review of the literature and show that the dimer is just the
smallest of a class of planar -waveguide couplers that are the optical
realization of Lorentz group in 2+1 dimensions. Furthermore, we provide a
formulation to describe light propagation through waveguide couplers described
by non-Hermitian mode coupling matrices based on a non-Hermitian generalization
of Ehrenfest theorem.Comment: 25 pages, 12 figure
Rings and arcs around evolved stars. II. The Carbon Star AFGL 3068 and the Planetary Nebulae NGC 6543, NGC 7009 and NGC 7027
We present a detailed comparative study of the arcs and fragmented ring-like
features in the haloes of the planetary nebulae (PNe) NGC 6543, NGC 7009, and
NGC 7027 and the spiral pattern around the carbon star AFGL 3068 using
high-quality multi-epoch HST images. This comparison allows us to investigate
the connection and possible evolution between the regular patterns surrounding
AGB stars and the irregular concentric patterns around PNe. The radial proper
motion of these features, ~15 km/s, are found to be consistent with the AGB
wind and their linear sizes and inter-lapse times (500-1900 yr) also agree with
those found around AGB stars, suggesting a common origin. We find evidence
using radiative-hydrodynamic simulations that regular patterns produced at the
end of the AGB phase become highly distorted by their interactions with the
expanding PN and the anisotropic illumination and ionization patterns caused by
shadow instabilities. These processes will disrupt the regular (mostly spiral)
patterns around AGB stars, plausibly becoming the arcs and fragmented rings
observed in the haloes of PNe.Comment: 13 pages, 9 figures, accepted for publication in MNRA
Self-Averaging in the Three Dimensional Site Diluted Heisenberg Model at the critical point
We study the self-averaging properties of the three dimensional site diluted
Heisenberg model. The Harris criterion \cite{critharris} states that disorder
is irrelevant since the specific heat critical exponent of the pure model is
negative. According with some analytical approaches \cite{harris}, this implies
that the susceptibility should be self-averaging at the critical temperature
(). We have checked this theoretical prediction for a large range of
dilution (including strong dilution) at critically and we have found that the
introduction of scaling corrections is crucial in order to obtain
self-averageness in this model. Finally we have computed critical exponents and
cumulants which compare very well with those of the pure model supporting the
Universality predicted by the Harris criterion.Comment: 11 pages, 11 figures, 14 tables. New analysis (scaling corrections in
the g2=0 scenario) and new numerical simulations. Title and conclusions
chang
Detection of new eruptions in the Magellanic Clouds LBVs R 40 and R 110
We performed a spectroscopic and photometric analysis to study new eruptions
in two luminous blue variables (LBVs) in the Magellanic Clouds. We detected a
strong new eruption in the LBV R40 that reached in 2016, which is
around mag brighter than the minimum registered in 1985. During this new
eruption, the star changed from an A-type to a late F-type spectrum. Based on
photometric and spectroscopic empirical calibrations and synthetic spectral
modeling, we determine that R\,40 reached ~K
during this new eruption. This object is thereby probably one of the coolest
identified LBVs. We could also identify an enrichment of nitrogen and r- and
s-process elements. We detected a weak eruption in the LBV R 110 with a maximum
of mag in 2011, that is, around mag brighter than in the
quiescent phase. On the other hand, this new eruption is about mag
fainter than the first eruption detected in 1990, but the temperature did not
decrease below 8500 K. Spitzer spectra show indications of cool dust in the
circumstellar environment of both stars, but no hot or warm dust was present,
except by the probable presence of PAHs in R\,110. We also discuss a possible
post-red supergiant nature for both stars
Finite-Difference Equations in Relativistic Quantum Mechanics
Relativistic Quantum Mechanics suffers from structural problems which are
traced back to the lack of a position operator , satisfying
with the ordinary momentum operator
, in the basic symmetry group -- the Poincar\'e group. In this paper
we provide a finite-dimensional extension of the Poincar\'e group containing
only one more (in 1+1D) generator , satisfying the commutation
relation with the ordinary boost generator
. The unitary irreducible representations are calculated and the
carrier space proves to be the set of Shapiro's wave functions. The generalized
equations of motion constitute a simple example of exactly solvable
finite-difference set of equations associated with infinite-order polarization
equations.Comment: 10 LaTeX pages, final version, enlarged (2 more pages
Group Approach to the Quantization of the P\"oschl-Teller dynamics
The quantum dynamics of a particle in the Modified P\"oschl-Teller potential
is derived from the group by applying a Group Approach to
Quantization (GAQ). The explicit form of the Hamiltonian as well as the ladder
operators is found in the enveloping algebra of this basic symmetry group. The
present algorithm provides a physical realization of the non-unitary,
finite-dimensional, irreducible representations of the group. The
non-unitarity manifests itself in that only half of the states are
normalizable, in contrast with the representations of SU(2) where all the
states are physical.Comment: 17 pages, LaTe
Gas exchange behavior of canopy and understory species during the rainy season in a forest area in Eastern brazilian Amazonia.
New insights in particle dynamics from group cohomology
The dynamics of a particle moving in background electromagnetic and
gravitational fields is revisited from a Lie group cohomological perspective.
Physical constants characterising the particle appear as central extension
parameters of a group which is obtained from a centrally extended kinematical
group (Poincare or Galilei) by making local some subgroup. The corresponding
dynamics is generated by a vector field inside the kernel of a presymplectic
form which is derived from the canonical left-invariant one-form on the
extended group. A non-relativistic limit is derived from the geodesic motion
via an Inonu-Wigner contraction. A deeper analysis of the cohomological
structure reveals the possibility of a new force associated with a non-trivial
mixing of gravity and electromagnetism leading to in principle testable
predictions.Comment: 8 pages, LaTeX, no figures. To appear in J. Phys. A (Letter to the
editor
- …