17,901 research outputs found
On-off intermittency and amplitude-phase synchronization in Keplerian shear flows
We study the development of coherent structures in local simulations of the
magnetorotational instability in accretion discs in regimes of on-off
intermittency. In a previous paper [Chian et al., Phys. Rev. Lett. 104, 254102
(2010)], we have shown that the laminar and bursty states due to the on-off
spatiotemporal intermittency in a one-dimensional model of nonlinear waves
correspond, respectively, to nonattracting coherent structures with higher and
lower degrees of amplitude-phase synchronization. In this paper we extend these
results to a three-dimensional model of magnetized Keplerian shear flows.
Keeping the kinetic Reynolds number and the magnetic Prandtl number fixed, we
investigate two different intermittent regimes by varying the plasma beta
parameter. The first regime is characterized by turbulent patterns interrupted
by the recurrent emergence of a large-scale coherent structure known as
two-channel flow, where the state of the system can be described by a single
Fourier mode. The second regime is dominated by the turbulence with sporadic
emergence of coherent structures with shapes that are reminiscent of a
perturbed channel flow. By computing the Fourier power and phase spectral
entropies in three-dimensions, we show that the large-scale coherent structures
are characterized by a high degree of amplitude-phase synchronization.Comment: 17 pages, 10 figure
A Stress/Displacement Virtual Element Method for Plane Elasticity Problems
The numerical approximation of 2D elasticity problems is considered, in the
framework of the small strain theory and in connection with the mixed
Hellinger-Reissner variational formulation. A low-order Virtual Element Method
(VEM) with a-priori symmetric stresses is proposed. Several numerical tests are
provided, along with a rigorous stability and convergence analysis
On Galois-Division Multiple Access Systems: Figures of Merit and Performance Evaluation
A new approach to multiple access based on finite field transforms is
investigated. These schemes, termed Galois-Division Multiple Access (GDMA),
offer compact bandwidth requirements. A new digital transform, the Finite Field
Hartley Transform (FFHT) requires to deal with fields of characteristic p, p
\neq 2. A binary-to-p-ary (p \neq 2) mapping based on the opportunistic
secondary channel is introduced. This allows the use of GDMA in conjunction
with available digital systems. The performance of GDMA is also evaluated.Comment: 6 pages, 4 figures. In: XIX Simposio Brasileiro de Telecomunicacoes,
2001, Fortaleza, CE, Brazi
Inter- and intra-layer excitons in MoS/WS and MoSe/WSe heterobilayers
Accurately described excitonic properties of transition metal dichalcogenide
heterobilayers (HBLs) are crucial to comprehend the optical response and the
charge carrier dynamics of them. Excitons in multilayer systems posses inter or
intralayer character whose spectral positions depend on their binding energy
and the band alignment of the constituent single-layers. In this study, we
report the electronic structure and the absorption spectra of MoS/WS
and MoSe/WSe HBLs from first-principles calculations. We explore the
spectral positions, binding energies and the origins of inter and intralayer
excitons and compare our results with experimental observations. The absorption
spectra of the systems are obtained by solving the Bethe-Salpeter equation on
top of a GW calculation which corrects the independent particle
eigenvalues obtained from density functional theory calculations. Our
calculations reveal that the lowest energy exciton in both HBLs possesses
interlayer character which is decisive regarding their possible device
applications. Due to the spatially separated nature of the charge carriers, the
binding energy of inter-layer excitons might be expected to be considerably
smaller than that of intra-layer ones. However, according to our calculations
the binding energy of lowest energy interlayer excitons is only 20\%
lower due to the weaker screening of the Coulomb interaction between layers of
the HBLs. Therefore, it can be deduced that the spectral positions of the
interlayer excitons with respect to intralayer ones are mostly determined by
the band offset of the constituent single-layers. By comparing oscillator
strengths and thermal occupation factors, we show that in luminescence at low
temperature, the interlayer exciton peak becomes dominant, while in absorption
it is almost invisible.Comment: 17 pages, 4 figure
Spectroscopic confirmation of the planetary nebula nature of PM1-242, PM1-318 and PM1-333 and morphological analysis of the nebulae
We present intermediate resolution long-slit spectra and narrow-band Halpha,
[NII] and [OIII] images of PM1-242, PM318 and PM1-333, three IRAS sources
classified as possible planetary nebulae. The spectra show that the three
objects are true planetary nebulae and allow us to study their physical
properties; the images provide a detailed view of their morphology. PM1-242 is
a medium-to-high-excitation (e.g., HeII4686/Hbeta ~0.4; [NII]6584/Halpha ~0.3)
planetary nebula with an elliptical shape containing [NII] enhanced
point-symmetric arcs. An electron temperature [Te([SIII])] of ~10250 K and an
electron density [Ne([SII])] of ~2300 cm-3 are derived for PM1-242. Abundance
calculations suggest a large helium abundance (He/H ~0.29) in PM1-242. PM1-318
is a high-excitation (HeII4686/Hbeta ~1) planetary nebula with a ring-like
inner shell containing two enhanced opposite regions, surrounded by a fainter
round attached shell brighter in the light of [OIII]. PM1-333 is an extended
planetary nebula with a high-excitation (HeII4686/Hbeta up to ~0.9) patchy
circular main body containing two low-excitation knotty arcs. A low Ne([SII])
of ~450 cm-3 and Te([OIII]) of ~15000 K are derived for this nebula. Abundance
calculations suggest that PM1-333 is a type I planetary nebula. The lack of a
sharp shell morphology, low electron density, and high-excitation strongly
suggest that PM1-333 is an evolved planetary nebula. PM1-333 also shows two
low-ionization polar structures whose morphology and emission properties are
reminiscent of collimated outflows. We compare PM1-333 with other evolved
planetary nebulae with collimated outflows and find that outflows among evolved
planetary nebulae exhibit a large variety of properties, in accordance with
these observed in younger planetary nebula.Comment: Accepted in The Astronomical Journal, 23 pages, 6 figure
- …