664 research outputs found
Synthesis and Characterization of LnAg(WO4)(MoO4)
Polycrystalline LnAg(WO4)(MoO4) powders, with Ln = La to Lu and Y, have been obtained by ceramic method. Rietveld refinement for all compounds reveals that they present tetragonal symmetry, space group I41/a (No. 88), where the Ln3+/Ag+ ions are located in the 4a atomic positions, since the W/Mo are randomly distributed into 4b crystal sites.
In these compounds, a and b lattice parameters take values between those corresponding to tungstate and molybdate compounds. A progressive decrease in the lattice parameters is observed in going from La to Lu derivatives as a consequence of the well-known lanthanide contraction
Semileptonic decays, magnetic moments and spin distributions of spin-1/2 baryons with sea contribution
Spin-1/2 baryons are considered as a composite system made out of a "core" of
three quarks surrounded by a "sea" (of gluons and -pairs) which is
specified by its total quantum numbers. Specifically, we assume this sea to be
a flavor octet with spin-0 or 1 but no color. We show our model can provide
very goods fits to magnetic moments and semileptonic decay data using
experimental errors. The predictions for spin distributions are in reasonable
agreement with experiment.Comment: To be published in Int. J. of Mod. Phys.
Spectroscopic Characteristics of Carbon Dots (C-Dots) Derived from Carbon Fibers and Conversion to Sulfur-Bridged C-Dots Nanosheets
We synthesized sub‐10 nm carbon nanoparticles (CNPs) consistent with photoluminescent carbon dots (C-dots) from carbon fiber starting material. The production of different C-dots fractions was monitored over seven days. During the course of the reaction, one fraction of C-dots species with relatively high photoluminescence was short-lived, emerging during the first hour of reaction but disappearing after one day of reaction. Isolation of this species during the first hour of the reaction was crucial to obtaining higher-luminescent C-dots species. When the reaction proceeded for one week, the appearance of larger nanostructures was observed over time, with lateral dimensions approaching 200 nm. The experimental evidence suggests that these larger species are formed from small C-dot nanoparticles bridged together by sulfur-based moieties between the C-dot edge groups, as if the C-dots polymerized by cross-linking the edge groups through sulfur bridges. Their size can be tailored by controlling the reaction time. Our results highlight the variety of CNP products, from sub‐10 nm C-dots to ~200 nm sulfur-containing carbon nanostructures, that can be produced over time during the oxidation reaction of the graphenic starting material. Our work provides a clear understanding of when to stop the oxidation reaction during the top-down production of C-dots to obtain highly photoluminescent species or a target average particle size
Refinement Type Inference via Horn Constraint Optimization
We propose a novel method for inferring refinement types of higher-order
functional programs. The main advantage of the proposed method is that it can
infer maximally preferred (i.e., Pareto optimal) refinement types with respect
to a user-specified preference order. The flexible optimization of refinement
types enabled by the proposed method paves the way for interesting
applications, such as inferring most-general characterization of inputs for
which a given program satisfies (or violates) a given safety (or termination)
property. Our method reduces such a type optimization problem to a Horn
constraint optimization problem by using a new refinement type system that can
flexibly reason about non-determinism in programs. Our method then solves the
constraint optimization problem by repeatedly improving a current solution
until convergence via template-based invariant generation. We have implemented
a prototype inference system based on our method, and obtained promising
results in preliminary experiments.Comment: 19 page
Mirror matter admixtures in K_L \to \gamma\gamma
Based on possible albeit tiny, admixtures of mirror matter in ordinary mesons
we study the K_L \to \gamma\gamma transition. We find that this process can be
described with a small SU(3) symmetry breaking of only 3%. We also determine
the eta-eta' mixing angle and the pseudoscalar decay constants. The results for
these parameters are consistent with some obtained in the literature. They
favor two recent determinations; one based on two analytical constraints, and
another one based on next-to-leading order power corrections
Narrow Band Halpha Photometry of the Super-Earth GJ 1214b with GTC/OSIRIS Tunable Filters
The super-earth planet GJ 1214b has recently been the focus of several
studies, using the transit spectroscopy technique, trying to determine the
nature of its atmosphere. Here we focus on the Halpha line as a tool to further
restrict the nature of GJ1214's atmosphere. We used the Gran Telescopio
Canarias (GTC) OSIRIS instrument to acquire narrow band photometry with tunable
filters. With our observations, we were able to observe the primary transit of
the super-Earth GJ 1214b in three bandpasses: two centered in the continuum
around Halpha (653.5 nm and 662.0 nm) and one centered at the line core (656.3
nm). We measure the depth of the planetary transit at each wavelength
interval.By fitting analytic models to the measured light curves we were able
to compute the depth of the transit at the three bandpasses. Taking the
difference in the computed planet to star radius ratio between the line and the
comparison continuum filters, we find Delta (Rp/Rstar)_{Halpha-653.5} = (6.60
+/- 3.54) 10^-3 and Delta (Rp/Rstar)_{Halpha-662.0} = (3.30 +/- 3.61) 10^-3.
Although the planet radius is found to be larger in the Halpha line than in the
surrounding continuum, the quality of our observations and the sigma level of
the differences (1.8 and 1.0, respectively) does not allow us to claim an
Halpha excess in GJ1214's atmosphere. Further observations will be needed to
resolve this issue.Comment: Accepted for publication in A&A, language and typos correcte
Complexity of Bradley-Manna-Sipma Lexicographic Ranking Functions
In this paper we turn the spotlight on a class of lexicographic ranking
functions introduced by Bradley, Manna and Sipma in a seminal CAV 2005 paper,
and establish for the first time the complexity of some problems involving the
inference of such functions for linear-constraint loops (without precondition).
We show that finding such a function, if one exists, can be done in polynomial
time in a way which is sound and complete when the variables range over the
rationals (or reals). We show that when variables range over the integers, the
problem is harder -- deciding the existence of a ranking function is
coNP-complete. Next, we study the problem of minimizing the number of
components in the ranking function (a.k.a. the dimension). This number is
interesting in contexts like computing iteration bounds and loop
parallelization. Surprisingly, and unlike the situation for some other classes
of lexicographic ranking functions, we find that even deciding whether a
two-component ranking function exists is harder than the unrestricted problem:
NP-complete over the rationals and -complete over the integers.Comment: Technical report for a corresponding CAV'15 pape
Magnitude estimation of the 1900 earthquake in Venezuela based on its coseismic effects
The 29 October, 1900, earthquake occurred in Venezuela triggered six landslides and six liquefactions located in the center-north region of Venezuela and La Tortuga island. Due to the location of the coseismic effects, the barycenter and the focal depth related to this earthquake, it was possible to calculate the magnitude by using several statistical methods. The results show a magnitude in the range 7.4-7.7 Mw, with an average value equal to 7.6 Mw, which is consistent with the instrumental magnitude of 7.6 Mw obtained by Fiedler (1988) and the macrosismic magnitude by Vásquez et al. (2018) equal to 7.5 ± 0.3 Mw estimated by using the Bakun and Wentworth (1997) method
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