552 research outputs found
The in-plane paraconductivity in La_{2-x}Sr_xCuO_4 thin film superconductors at high reduced-temperatures: Independence of the normal-state pseudogap
The in-plane resistivity has been measured in (LSxCO)
superconducting thin films of underdoped (), optimally-doped
() and overdoped () compositions. These films were grown
on (100)SrTiO substrates, and have about 150 nm thickness. The in-plane
conductivity induced by superconducting fluctuations above the superconducting
transition (the so-called in-plane paraconductivity, ) was
extracted from these data in the reduced-temperature range
10^{-2}\lsim\epsilon\equiv\ln(T/\Tc)\lsim1. Such a
was then analyzed in terms of the
mean-field--like Gaussian-Ginzburg-Landau (GGL) approach extended to the
high- region by means of the introduction of a total-energy cutoff,
which takes into account both the kinetic energy and the quantum localization
energy of each fluctuating mode. Our results strongly suggest that at all
temperatures above Tc, including the high reduced-temperature region, the
doping mainly affects in LSxCO thin films the normal-state properties and that
its influence on the superconducting fluctuations is relatively moderate: Even
in the high- region, the in-plane paraconductivity is found to be
independent of the opening of a pseudogap in the normal state of the underdoped
films.Comment: 35 pages including 10 figures and 1 tabl
Generalized parton distributions and double distributions for q q-bar pions
We consider two simple covariant models for pions (one with scalar and the
other with spin-1/2 constituents). Pion generalized parton distributions are
derived by integration over the light-cone energy. The model distributions are
consistent with all known properties of generalized parton distributions,
including positivity. We also construct the corresponding double distributions
by appealing to Lorentz invariance. These ostensibly constructed double
distributions lead to different generalized parton distributions that need not
respect the positivity constraints. This inconsistency arises from the
ambiguity inherent in defining double distributions in a one-component
formalism (even in the absence of the Polyakov-Weiss term). We demonstrate that
the correct model double distributions can be calculated from non-diagonal
matrix elements of twist-two operators.Comment: 10 pages, 7 figures, RevTex4, reference added, typos correcte
Cosmic acceleration from second order gauge gravity
We construct a phenomenological theory of gravitation based on a second order
gauge formulation for the Lorentz group. The model presents a long-range
modification for the gravitational field leading to a cosmological model
provided with an accelerated expansion at recent times. We estimate the model
parameters using observational data and verify that our estimative for the age
of the Universe is of the same magnitude than the one predicted by the standard
model. The transition from the decelerated expansion regime to the accelerated
one occurs recently (at ).Comment: RevTex4 15 pages, 1 figure. Accepted for publication in Astrophysics
& Space Scienc
Exploring skewed parton distributions with two body models on the light front II: covariant Bethe-Salpeter approach
We explore skewed parton distributions for two-body, light-front wave
functions. In order to access all kinematical regimes, we adopt a covariant
Bethe-Salpeter approach, which makes use of the underlying equation of motion
(here the Weinberg equation) and its Green's function. Such an approach allows
for the consistent treatment of the non-wave function vertex (but rules out the
case of phenomenological wave functions derived from ad hoc potentials). Our
investigation centers around checking internal consistency by demonstrating
time-reversal invariance and continuity between valence and non-valence
regimes. We derive our expressions by assuming the effective qq potential is
independent of the mass squared, and verify the sum rule in a non-relativistic
approximation in which the potential is energy independent. We consider
bare-coupling as well as interacting skewed parton distributions and develop
approximations for the Green's function which preserve the general properties
of these distributions. Lastly we apply our approach to time-like form factors
and find similar expressions for the related generalized distribution
amplitudes.Comment: 25 pages, 12 figures, revised (minor changes but essential to
consistency
Consumer perception of Brazilian traced beef
The objective of this study was to determine consumers understanding of beef traceability, identifying how consumers value this meat and traceability elements to be presented on retail shelves. The method used in this study was a survey through the internet applying the Sphinx software. The sample consisted of 417 consumers, mostly living in Porto Alegre, Brazil. Consumers are aware of certified beef, consider it important, but this is not a demand. As to traced beef, most consumers (62.4%) are in favor of mandatory traceability of beef cattle in Brazil, but 86.6% disagree with the destination of traced beef only to the foreign market. The majority of people are willing to pay more for traced beef and consider traceability a market opportunity, used as a differentiating tool
Consistent treatment of spin-1 mesons in the light-front formalism
We analyze the matrix element of the electroweak current between q \qb
vector meson states in the framework of a covariant extension of the
light-front formalism. The light-front matrix element of a one-body current is
naturally associated with zero modes, which affect some of the form factors
that are necessary to represent the Lorentz structure of the light-front
integral. The angular condition contains some information on zero modes, i.e.,
only if the effect of zero modes is accounted for correctly, is it satisfied.
With plausible assumptions we derive from the angular condition several
consistency conditions which can be used quite generally to determine the zero
mode contribution of form factors. The correctness of this method is tested by
the phenomenological success of the derived form factors. We compare the
predictions of our formalism with those of the standard light-front approach
and with available data. As examples we discuss the magnetic moment of the
, the coupling constant , and the coupling constants of
the pseudoscalar density, and , which provide a phenomenological
link between constituent and current quark masses.Comment: 36 pages, figure 1 is include
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