61,837 research outputs found
Isotopic dependence of the fragments' internal temperatures observed in multifragment emission
The internal temperatures of fragments produced by an excited nuclear source
are investigated using the microcanonical version of the Statistical
Multifragmentation Model, with discrete energy. We focus on the fragments'
properties at the breakup stage, before they have time to deexcite by particle
emission. Since the adopted model provides the excitation energy distribution
of these primordial fragments, it allows one to calculate the temperatures of
different isotope families and infer on the sensitivity to their isospin
composition. It is found that, due to the functional form of the nuclear
density of states and the excitation energy distribution of the fragments,
proton rich isotopes are hotter than neutron rich ones. This property has been
taken to be an indication of earlier emission of the former from a source that
cools down as it expands and emits fragments. Although this scenario is
incompatible with the prompt breakup of a thermally equilibrated source, our
results reveal that the latter framework also provides the same qualitative
features just mentioned. Therefore they suggest that this property cannot be
taken as evidence for non-equilibrium emission. We also found that this
sensitivity to the isotopic composition of the fragments depends on the isospin
composition of the source, and that it is weakened as the excitation energy of
the source increases.Comment: 5 pages, 3 figure
Ergodic and Thermodynamic Games
Let and be continuous maps defined on compact sets.
Let where is -invariant and is -invariant, be
pay-off functions for a game (in the usual sense of game theory) between
players that have the set of invariant measures for (player 1) and
(player 2) as possible strategies. Our goal here is to establish the notion of
Nash equilibrium point for the game defined by this pay-offs and strategies.
The main tools came from ergodic optimization (as we are optimizing over the
set of invariant measures) and thermodynamic formalism (when we add to the
integrals above the entropy of measures in order to define a second case to be
explored). Both cases are ergodic versions of non-cooperative games. We show
the existence of Nash equilibrium points with two independent arguments. One of
the arguments works for the case with entropy, and uses only tools of
thermodynamical formalism, while the other, that works in the case without
entropy but can be adapted to deal with both cases, uses the Kakutani fixed
point. We also present examples and briefly discuss uniqueness (or lack of
uniqueness). In the end we present a different example where players are
allowed to collaborate. This final example show connections between cooperative
games and ergodic transport
The relation between velocity dispersions and chemical abundances in RAVE giants
We developed a Bayesian framework to determine in a robust way the relation
between velocity dispersions and chemical abundances in a sample of stars. Our
modelling takes into account the uncertainties in the chemical and kinematic
properties. We make use of RAVE DR5 radial velocities and abundances together
with Gaia DR1 proper motions and parallaxes (when possible, otherwise UCAC4
data is used). We found that, in general, the velocity dispersions increase
with decreasing [Fe/H] and increasing [Mg/Fe]. A possible decrease in velocity
dispersion for stars with high [Mg/Fe] is a property of a negligible fraction
of stars and hardly a robust result. At low [Fe/H] and high [Mg/Fe] the sample
is incomplete, affected by biases, and likely not representative of the
underlying stellar population.Comment: 2 pages, to appear in Proceedings of the IAU Symposium 330,
"Astrometry and Astrophysics in the Gaia Sky", held in April 2017, Nice,
Franc
A generalization of convergence actions
Let a group act properly discontinuously and cocompactly on a locally
compact space . A Hausdorff compact space that contains as an open
subspace has the perspectivity property if the action
extends to an action , by homeomorphisms, such that for
every compact and every element of the unique uniform
structure compatible with the topology of , the set has
finitely many non -small sets. We describe a correspondence between the
compact spaces with the perspectivity property with respect to (and the
fixed action of on it) and the compact spaces with the perspectivity
property with respect to (and the left multiplication on itself). This
generalizes a similar result for convergence group actions.Comment: 57 page
Many-particle correlations and Coulomb effects on temperatures from fragment momentum fluctuations
We investigate correlations in the fragment momentum distribution due to the
propagation of fragments under the influence of their mutual Coulomb field,
after the breakup of an excited nuclear source.The magnitude of the effects on
the nuclear temperatures obtained from such distributions is estimated with the
help of a simple approach in which a charged fragment interacts with a
homogeneous charged sphere. The resuslts are used to correct the temperatures
obtained from the asymptotic momentum distributions of fragments produced by a
Monte-Carlo simulation in which the system's configuration at breakup is
provided by the canonical version of the Statistical Multifragmentation Model.
In a separate calculation, the dynamics of this many-particle charged system is
followed in a molecular dynamics calculation until the fragments are far away
from the breakup volume. The results suggest that, although the magnitude of
the corrections is similar in both models, many-particle correlations present
in the second approach are non-negligible and should be taken into account in
order to minimize ambiguities in such studies.Comment: 7 pages, 4 figure
Low-cost educational robotics applied to physics teaching in Brazil
In this paper we propose strategies and methodologies of teaching topics in
high school physics, through a show of Educational Robotics. The Exhibition was
part of a set of actions promoted by a brazilian government program of
incentive for teaching activities (PIBID) and whose primary focus is the
training of teachers, improvement of teaching in public schools, dissemination
of science and formation of new scientists and researchers. By means of
workshops, banners and prototyping of robotics, we are able to create a
connection between the study areas and their surrounding, making learning
meaningful and accessible for the students involved and contributing to their
cognitive development.Comment: 5 pages and 10 figure
Finite size effects in isobaric ratios
The properties of isobaric ratios, between nuclei produced in the same
reaction, are investigated using the canonical and grand-canonical statistical
ensembles. Although the grand-canonical for- mulae furnish a means to correlate
the ratios with the liquid drop parameters, finite size effects make it
difficult to obtain their actual values from fitting nuclear collision data.Comment: 4 pages, 2 figure
The possible role of van Hove singularities in the high of superconducting HS
We observe that HS has a bcc structure and, with nearest neighbour
hopping only, a strong singularity occurs at zero energy. This singularity is
accompanied with a highly nested Fermi surface, which is {\it not} conducive to
a stable superconducting instability. Introduction of next-nearest-neighbour
hopping removes the singularity, but a `robust' peak remains in the electron
density of states. Solution of the BCS equations shows an enhanced
superconducting due to this peak. Furthermore, nesting is no longer
present, so other instabilities will not compete effectively with
superconductivity. We find high critical temperatures are possible, even with
very modest coupling strengths. We also examine a limit of the equations
(in an Appendix) where an analytical solution is possible over the entire range
of coupling strengths, and therefore the BCS-BEC crossover is fully covered.Comment: 5 pages 2 figure
A systematic study of the superconducting critical temperature in two and three dimensional tight-binding models: a possible scenario for superconducting HS ?
Ever since BCS theory was first formulated it was recognized that a large
electronic density of states at the Fermi level was beneficial to enhancing
. The A15 compounds and the high temperature cuprate materials both have
had an enormous amount of effort devoted to studying the possibility that such
peaks play an important role in the high critical temperatures existing in
these compounds. Here we provide a systematic study of the effect of these
peaks on the superconducting transition temperature for a variety of
tight-binding models of simple structures, both in two and three dimensions. In
three dimensions large enhancements in can occur, due to van Hove
singularities that result in divergences in the density of states. Furthermore,
even in more realistic structures, where the van Hove singularity disappears,
large enhancements in continue due to the presence of `robust' peaks in
the densities of states. Such a peak, recently identified in the bcc structure
of HS, is likely the result of such a van Hove singularity. In certain
regimes, anomalies in the isotope coefficient are also expected.Comment: 15 pages, 15 figures, changes reflect more emphasis on implications
for the 200K superconductor, H3S. A couple of additional changes have been
made to comply with the published versio
Open quantum random walks: ergodicity, hitting times, gambler's ruin and potential theory
In this work we study certain aspects of Open Quantum Random Walks (OQRWs), a
class of quantum channels described by S. Attal et al. \cite{attal}. As a first
objective we consider processes which are nonhomogeneous in time, i.e., at each
time step, a possibly distinct evolution kernel. Inspired by a spectral
technique described by L. Saloff-Coste and J. Z\'u\~niga \cite{saloff}, we
define a notion of ergodicity for finite nonhomogeneous quantum Markov chains
and describe a criterion for ergodicity of such objects in terms of singular
values. As a second objective, and based on a quantum trajectory approach, we
study a notion of hitting time for OQRWs and we see that many constructions are
variations of well-known classical probability results, with the density matrix
degree of freedom on each site giving rise to systems which are seen to be
nonclassical. In this way we are able to examine open quantum versions of the
gambler's ruin, birth-and-death chain and a basic theorem on potential theory.Comment: Revised version. arXiv admin note: substantial text overlap with
arXiv:1504.05398, arXiv:1506.0832
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