22,893 research outputs found
The effects of k-dependent self-energy in the electronic structure of correlated materials
It is known from self-energy calculations in the electron gas and sp
materials based on the GW approximation that a typical quasiparticle
renormalization factor (Z factor) is approximately 0.7-0.8. Band narrowing in
electron gas at rs = 4 due to correlation effects, however, is only
approximately 10%, significantly smaller than the Z factor would suggest. The
band narrowing is determined by the frequency-dependent self-energy, giving the
Z factor, and the momentum-dependent or nonlocal self-energy. The results for
the electron gas point to a strong cancellation between the effects of
frequency- and momentum-dependent self-energy. It is often assumed that for
systems with a nar- row band the self-energy is local. In this work we show
that even for narrow-band materials, such as SrVO3, the nonlocal self-energy is
important.Comment: 7 pages, 6 figure
Fisher matrix forecasts for astrophysical tests of the stability of the fine-structure constant
We use Fisher Matrix analysis techniques to forecast the cosmological impact
of astrophysical tests of the stability of the fine-structure constant to be
carried out by the forthcoming ESPRESSO spectrograph at the VLT (due for
commissioning in late 2017), as well by the planned high-resolution
spectrograph (currently in Phase A) for the European Extremely Large Telescope.
Assuming a fiducial model without variations, we show that ESPRESSO
can improve current bounds on the E\"{o}tv\"{o}s parameter---which quantifies
Weak Equivalence Principle violations---by up to two orders of magnitude,
leading to stronger bounds than those expected from the ongoing tests with the
MICROSCOPE satellite, while constraints from the E-ELT should be competitive
with those of the proposed STEP satellite. Should an variation be
detected, these measurements will further constrain cosmological parameters,
being particularly sensitive to the dynamics of dark energy.Comment: Phys. Lett. B (in press
Structural studies of mesoporous ZrO-CeO and ZrO-CeO/SiO mixed oxides for catalytical applications
In this work the synthesis of ZrO-CeO and
ZrO-CeO/SiO were developed, based on the process to form
ordered mesoporous materials such as SBA-15 silica. The triblock copolymer
Pluronic P-123 was used as template, aiming to obtain crystalline single phase
walls and larger specific surface area, for future applications in catalysis.
SAXS and XRD results showed a relationship between ordered pores and the
material crystallization. 90% of CeO leaded to single phase homogeneous
ceria-zirconia solid solution of cubic fluorite structure (Fmm). The
SiO addition improved structural and textural properties as well as the
reduction behavior at lower temperatures, investigated by XANES measurements
under H atmosphere
Topological Defects in Contracting Universes
We study the behaviour and consequences of cosmic string networks in
contracting universes. They approximately behave during the collapse phase as a
radiation fluids. Scaling solutions describing this are derived and tested
against high-resolution numerical simulations. A string network in a
contracting universe, together with the gravitational radiation it generates,
can affect the dynamics of the universe both locally and globally, and be an
important source of radiation, entropy and inhomogeneity. We discuss possible
implications for bouncing and cyclic models.Comment: Shorter version of astro-ph/0206287. To appear in Phys. Rev. Let
Multicolored Temperley-Lieb lattice models. The ground state
Using inversion relation, we calculate the ground state energy for the
lattice integrable models, based on a recently obtained baxterization of non
trivial multicolored generalization of Temperley-Lieb algebras. The simplest
vertex and IRF models are analyzed and found to have a mass gap.Comment: 15 pages 2 figure
Vorton Formation
In this paper we present the first analytic model for vorton formation. We
start by deriving the microscopic string equations of motion in Witten's
superconducting model, and show that in the relevant chiral limit these
coincide with the ones obtained from the supersonic elastic models of Carter
and Peter. We then numerically study a number of solutions of these equations
of motion and thereby suggest criteria for deciding whether a given
superconducting loop configuration can form a vorton. Finally, using a recently
developed model for the evolution of currents in superconducting strings we
conjecture, by comparison with these criteria, that string networks formed at
the GUT phase transition should produce no vortons. On the other hand, a
network formed at the electroweak scale can produce vortons accounting for up
to 6% of the critical density. Some consequences of our results are discussed.Comment: 41 pages; color figures 3-6 not included, but available from authors.
To appear in Phys. Rev.
Bayesian Updating Rules in Continuous Opinion Dynamics Models
In this article, I investigate the use of Bayesian updating rules applied to
modeling social agents in the case of continuos opinions models. Given another
agent statement about the continuous value of a variable , we will see that
interesting dynamics emerge when an agent assigns a likelihood to that value
that is a mixture of a Gaussian and a Uniform distribution. This represents the
idea the other agent might have no idea about what he is talking about. The
effect of updating only the first moments of the distribution will be studied.
and we will see that this generates results similar to those of the Bounded
Confidence models. By also updating the second moment, several different
opinions always survive in the long run. However, depending on the probability
of error and initial uncertainty, those opinions might be clustered around a
central value.Comment: 14 pages, 5 figures, presented at SigmaPhi200
Social Effects in Science: Modelling Agents for a Better Scientific Practice
Science is a fundamental human activity and we trust its results because it
has several error-correcting mechanisms. Its is subject to experimental tests
that are replicated by independent parts. Given the huge amount of information
available, scientists have to rely on the reports of others. This makes it
possible for social effects to influence the scientific community. Here, an
Opinion Dynamics agent model is proposed to describe this situation. The
influence of Nature through experiments is described as an external field that
acts on the experimental agents. We will see that the retirement of old
scientists can be fundamental in the acceptance of a new theory. We will also
investigate the interplay between social influence and observations. This will
allow us to gain insight in the problem of when social effects can have
negligible effects in the conclusions of a scientific community and when we
should worry about them.Comment: 14 pages, 5 figure
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