5,340 research outputs found
Non-Markovian Dynamics of Charge Carriers in Quantum Dots
We have investigated the dynamics of bound particles in multilevel
current-carrying quantum dots. We look specifically in the regime of resonant
tunnelling transport, where several channels are available for transport.
Through a non-Markovian formalism under the Born approximation, we investigate
the real-time evolution of the confined particles including transport-induced
decoherence and relaxation. In the case of a coherent superposition between
states with different particle number, we find that a Fock-space coherence may
be preserved even in the presence of tunneling into and out of the dot.
Real-time results are presented for various asymmetries of tunneling rates into
different orbitals.Comment: 9 pages, 3 figures, International Workshop on Physics-Based
Mathematical Models for Low-Dimensional Semiconductor Nanostructures. BIRS,
November 18-23, 200
Reflection and Transmission at the Apparent Horizon during Gravitational Collapse
We examine the wave-functionals describing the collapse of a self-gravitating
dust ball in an exact quantization of the gravity-dust system. We show that
ingoing (collapsing) dust shell modes outside the apparent horizon must
necessarily be accompanied by outgoing modes inside the apparent horizon, whose
amplitude is suppressed by the square root of the Boltzmann factor at the
Hawking temperature. Likewise, ingoing modes in the interior must be
accompanied by outgoing modes in the exterior, again with an amplitude
suppressed by the same factor. A suitable superposition of the two solutions is
necessary to conserve the dust probability flux across the apparent horizon,
thus each region contains both ingoing and outgoing dust modes. If one
restricts oneself to considering only the modes outside the apparent horizon
then one should think of the apparent horizon as a partial reflector, the
probability for a shell to reflect being given by the Boltzmann factor at the
Hawking temperature determined by the mass contained within it. However, if one
considers the entire wave function, the outgoing wave in the exterior is seen
to be the transmission through the horizon of the interior outgoing wave that
accompanies the collapsing shells. This transmission could allow information
from the interior to be transferred to the exterior.Comment: 19 pages, no figures. To appear in Phys. Rev.
2-Verma modules and the Khovanov-Rozansky link homologies
We explain how Queffelec-Sartori's construction of the HOMFLY-PT link polynomial can be interpreted in terms of parabolic Verma modules for . Lifting the construction to the world of categorification, we use parabolic 2-Verma modules to give a higher representation theory construction of Khovanov-Rozansky's triply graded link homology
The partial cavity on a 2D foil revisited
The partial cavity on a 2D NACA0015 foil at 6 degrees angle-of-attack is studied numerically. Assuming the fluid to be a continuum of variable density, we solve the RANS equations, complemented with turbulence and cavitation models. Some important details of the mathematical model are pointed out first. We study then carefully what occurs in the numerical simulations in and near the cavity from the inception phase to the stage well before serious unsteadiness (cavity shedding) starts. By making the computations on grids of different densities we get an impression of numerical uncertainties. This is important for the interpretation and the subsequent comparison with what experimental investigations have learned us about the physics of these almost steady partial cavities on foils. The results show that close to inception a cavity exists while the boundary layer is nonseparating. The liquid-vapour interface turns out not to be a material surface, neither at the front end nor at the tail of the cavity. It also appears that the widely accepted re-entrant jet model as conceived from free-streamline theory is not a good description of the flow at the tail. The confrontation of the numerical results with information from experiments indicates that there is agreement and corroboration in several respects, but also intriguing discrepancies are found which require further elucidation.http://deepblue.lib.umich.edu/bitstream/2027.42/84243/1/CAV2009-final43.pd
General spherically symmetric elastic stars in Relativity
The relativistic theory of elasticity is reviewed within the spherically
symmetric context with a view towards the modeling of star interiors possessing
elastic properties such as theones expected in neutron stars. Emphasis is
placed on generality in the main sections of the paper, and the results are
then applied to specific examples. Along the way, a few general results for
spacetimes admitting isometries are deduced, and their consequences are fully
exploited in the case of spherical symmetry relating them next to the the case
in which the material content of the spacetime is some elastic material. This
paper extends and generalizes the pioneering work by Magli and Kijowski [1],
Magli [2] and [3], and complements, in a sense, that by Karlovini and
Samuelsson in their interesting series of papers [4], [5] and [6].Comment: 23 page
Near-Infrared Time-Series Photometry in the Field of Cygnus OB2 Association I - Rotational Scenario For Candidate Members
In the last decades, the early pre main sequence stellar rotational evolution
picture has been constrained by studies targeting different young regions at a
variety of ages. Observational studies suggest a mass-rotation dependence, and
for some mass ranges a connection between rotation and the presence of a
circumstellar disk. Not still fully explored, though, is the role of
environmental conditions on the rotational regulation.
We investigate the rotational properties of candidate members of the young
massive association Cygnus OB2. The Stetson variability index, Lomb-Scargle
periodogram, Saunders statistics, string/rope length method, and visual
verification of folded light curves were applied to select 1224 periodic
variable stars. Completeness and contamination of the periodic sample was
derived from Monte Carlo simulations, out of which 894 periods were considered
reliable. Our study was considered reasonably complete for periods from 2 to 30
days.
The general rotational scenario seen in other young regions is confirmed by
Cygnus OB2 period distributions, with disked stars rotating on average slower
than non-disked stars. A mass-rotation dependence was also verified, but as in
NGC 6530, lower mass stars are rotating on average slower than higher mass
stars, with an excess of slow rotators among the lower mass population. The
effect of the environment on the rotational properties of the association was
investigated by re-analysing the results while taking into account the incident
UV radiation arising from O stars in the association. Results compatible with
the disk-locking scenario were verified for stars with low UV incidence, but no
statistical significant relation between rotation and disk presence was
verified for stars with high UV incidence suggesting that massive stars can
have an important role on regulating the rotation of nearby low mass stars.Comment: Submitted on December 23, 201
When Politicians Talk About Politics: Identifying Political Tweets of Brazilian Congressmen
Since June 2013, when Brazil faced the largest and most significant mass
protests in a generation, a political crisis is in course. In midst of this
crisis, Brazilian politicians use social media to communicate with the
electorate in order to retain or to grow their political capital. The problem
is that many controversial topics are in course and deputies may prefer to
avoid such themes in their messages. To characterize this behavior, we propose
a method to accurately identify political and non-political tweets
independently of the deputy who posted it and of the time it was posted.
Moreover, we collected tweets of all congressmen who were active on Twitter and
worked in the Brazilian parliament from October 2013 to October 2017. To
evaluate our method, we used word clouds and a topic model to identify the main
political and non-political latent topics in parliamentarian tweets. Both
results indicate that our proposal is able to accurately distinguish political
from non-political tweets. Moreover, our analyses revealed a striking fact:
more than half of the messages posted by Brazilian deputies are non-political.Comment: 4 pages, 7 figures, 2 table
Diretrizes para o projeto gráfico da Agropedia brasilis.
Este trabalho estabelece as diretrizes para a construção de um projeto gráfico adequado para a Agropedia brasilis, embasado nas funcionalidades demandadas pelos empregados da Embrapa, em aspectos de usabilidade de sites e nas regras institucionais para a presença digital da empresa
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