21,850 research outputs found
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
Cosmology with Varying Constants
The idea of possible time or space variations of the `fundamental' constants
of nature, although not new, is only now beginning to be actively considered by
large numbers of researchers in the particle physics, cosmology and
astrophysics communities. This revival is mostly due to the claims of possible
detection of such variations, in various different contexts and by several
groups. Here, I present the current theoretical motivations and expectations
for such variations, review the current observational status, and discuss the
impact of a possible confirmation of these results in our views of cosmology
and physics as a whole.Comment: 14 pages, no figures. Essay to appear in Phil. Trans. Roy. Soc. Lond.
A Triennial Series (Christmas 2002 Issue
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
Removal of chromium from wastewater by adsorption and by biosorption
The objective of this work is the definition of an efficient biosorption system based on the ability of some
microorganisms, metabolica1ly active or not, to retain heavy metals, specially the hexavalent fonn of Cr.
Metallic solutions were passed through mini-colunms in which a biotilm was developed at pre-established
conditions. As industrial wastewater usually contains organic and inorganic compounds, besides metallic ions,
it would be desirable to co-extract several pollutants at the same time. The utilization of granular activated
carbon (GAC) as a support for the biofilm seems to be advantageous, as it can retain other substances while
the biofilm removes the heavy metal. To quantify the contribution of GAC to the overall removal of
chromium, adsorption studies were carried out with the determination the effect of pH and of the presence of
concurrent species on the adsorption isotherms. Biosorption studies were done with three different bacteria,
Pseudomonasjluorescens, Escherichia coli and Arthrobacter viscosus. Among microorganisms, bacteria are
particularly interesting for the purpose as they are able to excrete polysaccharides allowing a good adhesion
to the support, implementing the retention capacity of the biosystem and protecting the cells from the
xenobiotic effect of the heavy metal ions. Removal efficiencies were compared between the three biosystems
On the Stability of Fundamental Couplings in the Galaxy
Astrophysical tests of the stability of Nature's fundamental couplings are a
key probe of the standard paradigms in fundamental physics and cosmology. In
this report we discuss updated constraints on the stability of the
fine-structure constant and the proton-to-electron mass ratio
within the Galaxy. We revisit and improve upon the analysis by
Truppe {\it et al.} by allowing for the possibility of simultaneous variations
of both couplings and also by combining them with the recent measurements by
Levshakov {\it et al.} By considering representative unification scenarios we
find no evidence for variations of at the 0.4 ppm level, and of
at the 0.6 ppm level; if one uses the Levshakov bound on as a prior,
the bound is improved to 0.1 ppm. We also highlight how these
measurements can constrain (and discriminate among) several fundamental physics
paradigms.Comment: 7 pages, 1 figur
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
Constraining the evolution of the CMB temperature with SZ measurements from Planck data
The CMB temperature-redshift relation, T_CMB(z)=T_0(1+z), is a key prediction
of the standard cosmology, but is violated in many non standard models.
Constraining possible deviations to this law is an effective way to test the
LambdaCDM paradigm and to search for hints of new physics. We have determined
T_CMB(z), with a precision up to 3%, for a subsample (104 clusters) of the
Planck SZ cluster catalog, at redshift in the range 0.01-- 0.94, using
measurements of the spectrum of the Sunyaev Zel'dovich effect obtained from
Planck temperature maps at frequencies from 70 to 353 GHz. The method adopted
to provide individual determinations of T_CMB(z) at cluster redshift relies on
the use of SZ intensity change, Delta I_SZ(nu), at different frequencies, and
on a Monte-Carlo Markov Chain approach. By applying this method to the sample
of 104 clusters, we limit possible deviations of the form
T_CMB(z)=T_0(1+z)^(1-beta) to be beta= 0.022 +/- 0.018, at 1 sigma uncertainty,
consistent with the prediction of the standard model. Combining these
measurements with previously published results we get beta=0.016+/-0.012.Comment: submitted to JCAP, 21 pages, 8 figure
Dimension minimization of a quantum automaton
A new model of a Quantum Automaton (QA), working with qubits is proposed. The
quantum states of the automaton can be pure or mixed and are represented by
density operators. This is the appropriated approach to deal with measurements
and dechorence. The linearity of a QA and of the partial trace super-operator,
combined with the properties of invariant subspaces under unitary
transformations, are used to minimize the dimension of the automaton and,
consequently, the number of its working qubits. The results here developed are
valid wether the state set of the QA is finite or not. There are two main
results in this paper: 1) We show that the dimension reduction is possible
whenever the unitary transformations, associated to each letter of the input
alphabet, obey a set of conditions. 2) We develop an algorithm to find out the
equivalent minimal QA and prove that its complexity is polynomial in its
dimension and in the size of the input alphabet.Comment: 26 page
Anisotropic linear and non-linear excitonic optical properties of buckled monolayer semiconductors
The optical properties of two-dimensional materials are exceptional in
several respects. They are highly anisotropic and frequently dominated by
excitonic effects. Dipole-allowed second order non-linear optical properties
require broken inversion symmetry. Hence, several two-dimensional materials
show strong in-plane (IP) non-linearity but negligible out-of-plane (OOP)
response due to vertical symmetry. By considering buckled hexagonal monolayers,
we analyze the critical role of broken vertical symmetry on their excitonic
optical response. Both linear as well as second order shift current and second
harmonic response are studied. We demonstrate that substantial OOP non-linear
response can be obtained, in particular, through off-diagonal tensor elements
coupling IP excitation to OOP response. Our findings are explained by excitonic
selection rules for OOP response and the impact of dielectric screening on
excitons is elucidated.Comment: 20 pages, 8 figure
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