9,127 research outputs found
Linear Invariant Systems Theory for Signal Enhancement
This paper discusses a linear time invariant (LTI) systems approach to signal enhancement via projective subspace techniques. It provides closed form expressions for the frequency response of data adaptive finite impulse response eigenfilters. An illustrative example using speech enhancement is also presented.Este artigo apresenta a aplicação da teoria de sistemas lineares invariantes no tempo (LTI) na análise de técnicas de sub-espaço. A resposta em frequência dos filtros resultantes da decomposição em valores singulares é obtida aplicando as propriedades dos sistemas LTI
Phase diagrams of Janus fluids with up-down constrained orientations
A class of binary mixtures of Janus fluids formed by colloidal spheres with
the hydrophobic hemispheres constrained to point either up or down are studied
by means of Gibbs ensemble Monte Carlo simulations and simple analytical
approximations. These fluids can be experimentally realized by the application
of an external static electrical field. The gas-liquid and demixing phase
transitions in five specific models with different patch-patch affinities are
analyzed. It is found that a gas-liquid transition is present in all the
models, even if only one of the four possible patch-patch interactions is
attractive. Moreover, provided the attraction between like particles is
stronger than between unlike particles, the system demixes into two subsystems
with different composition at sufficiently low temperatures and high densities.Comment: 10 pages, 6 figure
Complete breakdown of the Debye model of rotational relaxation near the isotropic-nematic phase boundary: Effects of intermolecular correlations in orientational dynamics
The Debye-Stokes-Einstein (DSE) model of rotational diffusion predicts that
the rotational correlation times vary as , where
is the rank of the orientational correlation function (given in terms of the
Legendre polynomial of rank ). One often finds significant deviation from
this prediction, in either direction. In supercooled molecular liquids where
the ratio falls considerably below three (the Debye limit),
one usually invokes a jump diffusion model to explain the approach of the ratio
to unity. Here we show in a computer simulation study of a
standard model system for thermotropic liquid crystals that this ratio becomes
much less than unity as the isotropic-nematic phase boundary is approached from
the isotropic side. Simultaneously, the ratio (where is
the shear viscosity of the liquid) becomes {\it much larger} than hydrodynamic
value near the I-N transition. We have also analyzed the break down of the
Debye model of rotational diffusion in ratios of higher order rotational
correlation times. We show that the break down of the DSE model is due to the
growth of orientational pair correlation and provide a mode coupling theory
analysis to explain the results.Comment: Submitted to Physical Review
Quasi-Topological Quantum Field Theories and Lattice Gauge Theories
We consider a two parameter family of gauge theories on a lattice
discretization of a 3-manifold and its relation to topological field
theories. Familiar models such as the spin-gauge model are curves on a
parameter space . We show that there is a region of
where the partition function and the expectation value of the
Wilson loop for a curve $\gamma$ can be exactly computed. Depending on the
point of $\Gamma_0$, the model behaves as topological or quasi-topological. The
partition function is, up to a scaling factor, a topological number of $M$. The
Wilson loop on the other hand, does not depend on the topology of $\gamma$.
However, for a subset of $\Gamma_0$, depends on the size of
and follows a discrete version of an area law. At the zero temperature
limit, the spin-gauge model approaches the topological and the
quasi-topological regions depending on the sign of the coupling constant.Comment: 19 pages, 13 figure
Self-Pulsating Semiconductor Lasers: Theory and Experiment
We report detailed measurements of the pump-current dependency of the
self-pulsating frequency of semiconductor CD lasers. A distinct kink in this
dependence is found and explained using rate-equation model. The kink denotes a
transition between a region where the self-pulsations are weakly sustained
relaxation oscillations and a region where Q-switching takes place. Simulations
show that spontaneous emission noise plays a crucial role for the cross-over.Comment: Revtex, 16 pages, 7 figure
Nonlinear oscillator with parametric colored noise: some analytical results
The asymptotic behavior of a nonlinear oscillator subject to a multiplicative
Ornstein-Uhlenbeck noise is investigated. When the dynamics is expressed in
terms of energy-angle coordinates, it is observed that the angle is a fast
variable as compared to the energy. Thus, an effective stochastic dynamics for
the energy can be derived if the angular variable is averaged out. However, the
standard elimination procedure, performed earlier for a Gaussian white noise,
fails when the noise is colored because of correlations between the noise and
the fast angular variable. We develop here a specific averaging scheme that
retains these correlations. This allows us to calculate the probability
distribution function (P.D.F.) of the system and to derive the behavior of
physical observables in the long time limit
Self-isospectrality, mirror symmetry, and exotic nonlinear supersymmetry
We study supersymmetry of a self-isospectral one-gap Poschl-Teller system in
the light of a mirror symmetry that is based on spatial and shift reflections.
The revealed exotic, partially broken nonlinear supersymmetry admits seven
alternatives for a grading operator. One of its local, first order supercharges
may be identified as a Hamiltonian of an associated one-gap, non-periodic
Bogoliubov-de Gennes system. The latter possesses a nonlinear supersymmetric
structure, in which any of the three non-local generators of a Clifford algebra
may be chosen as the grading operator. We find that the supersymmetry
generators for the both systems are the Darboux-dressed integrals of a free
spin-1/2 particle in the Schrodinger picture, or of a free massive Dirac
particle. Nonlocal Foldy- Wouthuysen transformations are shown to be involved
in the supersymmetric structure.Comment: 20 pages, comment added. Published versio
Kaluza-Klein electrically charged black branes in M-theory
We present a class of Kaluza-Klein electrically charged black p-brane
solutions of ten-dimensional, type IIA superstring theory. Uplifting to eleven
dimensions these solutions are studied in the context of M-theory. They can be
interpreted either as a p+1 extended object trapped around the eleventh
dimension along which momentum is flowing or as a boost of the following
backgrounds: the Schwarzschild black (p+1)-brane or the product of the
(10-p)-dimensional Euclidean Schwarzschild manifold with the (p+1)-dimensional
Minkowski spacetime.Comment: 16 pages, uses latex and epsf macro, figures include
Heat exchange between two interacting nanoparticles beyond the fluctuation-dissipation regime
We show that the observed non-monotonic behavior of the thermal conductance
between two nanoparticles when they are brought into contact is originated by
an intricate phase space dynamics. Here it is assumed that this dynamics
results from the thermally activated jumping through a rough energy landscape.
A hierarchy of relaxation times plays the key role in the description of this
complex phase space behaviour. Our theory enables us to analyze the heat
transfer just before and at the moment of contact.Comment: 4 pages, 1 figure, approved for publication in Physical Review
Letter
Chandra ACIS Survey of M33 (ChASeM33): X-ray Imaging Spectroscopy of M33SNR21, the Brightest X-ray Supernova Remnant in M33
We present and interpret new X-ray data for M33SNR21, the brightest X-ray
supernova remnant (SNR) in M33. The SNR is in seen projection against (and
appears to be interacting with) the bright HII region NGC592. Data for this
source were obtained as part of the Chandra ACIS Survey of M33 (ChASeM33) Very
Large Project. The nearly on-axis Chandra data resolve the SNR into a ~5"
diameter (20 pc at our assumed M33 distance of 817+/-58 kpc) slightly
elliptical shell. The shell is brighter in the east, which suggests that it is
encountering higher density material in that direction. The optical emission is
coextensive with the X-ray shell in the north, but extends well beyond the
X-ray rim in the southwest. Modeling the X-ray spectrum with an absorbed sedov
model yields a shock temperature of 0.46(+0.01,-0.02) keV, an ionization
timescale of n_e t = cm s, and
half-solar abundances (0.45 (+0.12, -0.09)). Assuming Sedov dynamics gives an
average preshock H density of 1.7 +/- 0.3 cm. The dynamical age estimate
is 6500 +/- 600 yr, while the best fit value and derived gives
8200 +/- 1700 yr; the weighted mean of the age estimates is 7600 +/- 600 yr. We
estimate an X-ray luminosity (0.25-4.5 keV) of (1.2 +/- 0.2) times
ergs s (absorbed), and (1.7 +/- 0.3) times ergs s
(unabsorbed), in good agreement with the recent XMM-Newton determination. No
significant excess hard emission was detected; the luminosity ergs s (2-8 keV) for any hard point source.Comment: 27 pages, 6 figures (3 color). ApJ (in press
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