Operator ordering in quantum radiative processes

In this work we reexamine quantum electrodynamics of atomic electrons in the Coulomb gauge in the dipole approximation and calculate the shift of atomic energy levels in the context of Dalibard, Dupont-Roc and Cohen-Tannoudji formalism by considering the variation rates of physical observable. We then analyze the physical interpretation of the ordering of operators in the dipole approximation interaction Hamiltonian in terms of field fluctuations and self-reaction of atomic electrons, discussing the arbitrariness in the statistical functions in second-order bound-state perturbation theory. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Studying nonlinear effects on the early stage of phase ordering using a decomposition method

Nonlinear effects on the early stage of phase ordering are studied using Adomian's decomposition method for the Ginzburg-Landau equation for a nonconserved order parameter. While the long-time regime and the linear behavior at short times of the theory are well understood, the onset of nonlinearities at short times and the breaking of the linear theory at different length scales are less understood. In the Adomians decomposition method, the solution is systematically calculated in the form of a polynomial expansion for the order parameter, with a time dependence given as a series expansion. The method is very accurate for short times, which allows to incorporate the short-time dynamics of the nonlinear terms in a analytical and controllable way. (c) 2005 Elsevier B.V. All rights reserved.

Stress induced ordering due to interstitial elements in Nb and Ta by frequency analysis

It is well known that the interstitial elements present in solid solution in metals interact with the matrix by a relaxation process known as stress induced ordering. Traditionally this relaxation process is observed in the internal friction measurements. It is a common practice that researchers present the results of the frequency together with internal friction without giving any analysis. In this work we apply an expression which relates the variation of frequency with temperature and analyse the experimental results cited in the literature of the relaxation process due to the stress induced ordering of oxygen and nitrogen present in niobium and tantalum.

Stress induced ordering due interstitials in Nb-4.7 at.%Ta

The presence of interstitial elements in metals cause strong changes in their physical, chemical or mechanical properties. These interstitial impurities interact with the metallic matrix atoms by a relaxation process known as stress induced ordering. Relaxation processes give rise to a peak in the internal friction spectrum, known as Snock effect. The presence of substitutional solutes has a strong influence on Snoek effect, particularly if the substitutional solute element is the one, which interacts with the interstitial element. Anelastic spectroscopy measurements provide information of the behavior of these impurities in the metallic matrix. In this paper, polycrystalline samples of Nb-4.7 at.%Ta alloy have been analyzed in the as-received condition. Measurements of anelastic spectroscopy were carried out using an inverted torsion pendulum, operating with frequency of 2.0-30.0 Hz and in a temperature range between 300 and 700 K. It was observed the presence of a relaxation structure that have been attributed to stress induced ordering due to interstitial atoms around atoms of the metallic matrix. The relaxation structure have been decomposed in its constituent peaks, what it allowed to identify the following relaxation processes: Ta-O, Nb-O and Nb-N. (c) 2005 Elsevier B.V. All rights reserved.

Self-assembled FePt nanocrystals with large coercivity: Reduction of the fcc-to-L1(0) ordering temperature

Herein we report the synthesis and properties of Fe55Pt45 nanoparticles, both monodisperse and self-assembled into hexagonal close-packed and cubic arrays of 4.0 +/- 0.2 nm size in an L1(0) structure, obtained by a modified polyol process. The new synthetic route improved the control over the particle composition, thereby reducing the temperature required to convert from face-centered cubic (fcc) to face-centered tetragonal (fct) phase by some 30-50 degrees C without additives. Annealing at 550 degrees C for 30 min converts the self-assembled nanoparticles into ferromagnetic nanocrystals with large coercivity, H-C = 11.1 kOe. Reducing the fcc-to-fct (L1(0)) ordering temperature avoided particle coalescence and decreased the loss in particle positional order without compromising the magnetic properties, as is generally observed when additives are used.

Ordering ambiguity versus representation

In this work we show that the ordering ambiguity on quantization depends on the representation choice. This property is then used to solve unambiguously some particular systems. Finally, we speculate on the consequences for more involved cases.

Charge-orbital ordering and phase separation in the two-orbital model for manganites: Roles of Jahn-Teller phononic and Coulombic interactions

The main properties of realistic models for manganites are studied using analytic mean-field approximations and computational numerical methods, focusing on the two-orbital model with electrons interacting through Jahn-Teller (JT) phonons and/or Coulombic repulsions. Analyzing the model including both interactions by the combination of the mean-field approximation and the exact diagonalization method, it is argued that the spin-charge-orbital structure in the insulating phase of the purely JT-phononic model with a large Hund couphng J(H) is not qualitatively changed by the inclusion of the Coulomb interactions. As an important application of the present mean-held approximation, the CE-type antiferromagnetic state, the charge-stacked structure along the z axis, and (3x(2) - r(2))/(3y(2) - r(2))-type orbital ordering are successfully reproduced based on the JT-phononic model with large JH for the half-doped manganite, in agreement with recent Monte Carlo simulation results. Topological arguments and the relevance of the Heisenberg exchange among localized t(2g) spins explains why the inclusion of the nearest-neighbor Coulomb interaction does not destroy the charge stacking structure. It is also verified that the phase-separation tendency is observed both in purely JT-phononic (large JH) and purely Coulombic models in the vicinity of the hole undoped region, as long as realistic hopping matrices are used. This highlights the qualitative similarities of both approaches and the relevance of mixed-phase tendencies in the context of manganites. In addition, the rich and complex phase diagram of the two-orbital Coulombic model in one dimension is presented. Our results provide robust evidence that Coulombic and JT-phononic approaches to manganites are not qualitatively different ways to carry out theoretical calculations, but they share a variety of common features.

Stress-induced ordering due heavy interstitial atoms in Nb-0.3 wt.% Ti alloys

The mechanical properties of metals with bee structure, such as niobium and their alloys, are changed of a significant way by the introduction of heavy interstitial elements. These interstitial elements (oxygen, for example) present in the metallic matrix occupy octahedral sites and constitute an elastic dipole of tetragonal symmetry and might produce anelastic relaxation. Polycrystalline samples of Nb-0.3 wt.% Ti (Nb-Ti) alloy with oxygen in solid solution were analysed. The anelastic spectroscopy measurements had been made in a torsion pendulum, with frequencies in the Hz range, in a temperature range between 300 and 700 K. The results showed thermally activated relaxation structures were identified four relaxation process attributed to stress-induced ordering of single oxygen, nitrogen and carbon atoms around niobium and stress-induced ordering of single oxygen atoms around titanium atoms. (c) 2005 Elsevier B.V. All rights reserved.

Charge-ordering transition in (TMTTF)2X explored via dilatometry

Charge-ordering phenomena have been highly topical over the past few years. A phase transition towards a charge-ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one-dimensional organic charge-transfer salts (TMTTF)2X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anion (X = PF6, AsF6 and SbF6). However, the relationship between the electron localization phenomena and the role of the lattice distortion in stabilizing the charge-ordering pattern is poorly documented in the literature. Here we present a brief overview of selected literature results, with emphasis placed on recent thermal expansion experiments probing the charge-ordering transition of these salts. © 2013 IOP Publishing Ltd.

Ordering of representational level adverbial modifiers in spoken Brazilian Portuguese

Supported by the Functional Discourse Grammar theoretical model, as proposed by Hengeveld (2005), this paper aims to show that the order of modifiers of the Representational Level in spoken Brazilian Portuguese is determined by scope relations according to the layers of property, state-of-affairs and propositional content. This kind of distribution indicates that, far from being free-ordered as suggested by traditional grammarians, modifiers have a preferred position determined by semantic relations that may be only changed for pragmatic and structural reasons.

Precision of distances and ordering of microsatellite markers in consensus linkage maps of chromosomes 1, 3 and 4 from two reciprocal chicken populations using bootstrap sampling

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Intrinsic ordering, combinatorial numbers and reliability engineering

[EN]A new algorithm for evaluating the top event probability of large fault trees (FTs) is presented. This algorithm does not require any previous qualitative analysis of the FT. Indeed, its efficiency is independent of the FT logic, and it only depends on the number n of basic system components and on their failure probabilities. Our method provides exact lower and upper bounds on the top event probability by using new properties of the intrinsic order relation between binary strings. The intrinsic order enables one to select binary n-tuples with large occurrence probabilities without necessity to evaluate them. This drastically reduces the complexity of the problem from exponential (2n binary n-tuples) to linear (n Boolean variables)...

Computer simulation of ordering and dynamics in liquid crystals in the bulk and close to the surface

The aim of this PhD thesis is to investigate the orientational and dynamical properties of liquid crystalline systems, at molecular level and using atomistic computer simulations, to reach a better understanding of material behavior from a microscopic point view. In perspective this should allow to clarify the relation between the micro and macroscopic properties with the objective of predicting or confirming experimental results on these systems. In this context, we developed four different lines of work in the thesis. The first one concerns the orientational order and alignment mechanism of rigid solutes of small dimensions dissolved in a nematic phase formed by the 4-pentyl,4 cyanobiphenyl (5CB) nematic liquid crystal. The orientational distribution of solutes have been obtained with Molecular Dynamics Simulation (MD) and have been compared with experimental data reported in literature. we have also verified the agreement between order parameters and dipolar coupling values measured in NMR experiments. The MD determined effective orientational potentials have been compared with the predictions of Maier­Saupe and Surface tensor models. The second line concerns the development of a correct parametrization able to reproduce the phase transition properties of a prototype of the oligothiophene semiconductor family: sexithiophene (T6). T6 forms two crystalline polymorphs largely studied, and possesses liquid crystalline phases still not well characterized, From simulations we detected a phase transition from crystal to liquid crystal at about 580 K, in agreement with available experiments, and in particular we found two LC phases, smectic and nematic. The crystal­smectic transition is associated to a relevant density variation and to strong conformational changes of T6, namely the molecules in the liquid crystal phase easily assume a bent shape, deviating from the planar structure typical of the crystal. The third line explores a new approach for calculating the viscosity in a nematic through a virtual exper- iment resembling the classical falling sphere experiment. The falling sphere is replaced by an hydrogenated silicon nanoparticle of spherical shape suspended in 5CB, and gravity effects are replaced by a constant force applied to the nanoparticle in a selected direction. Once the nanoparticle reaches a constant velocity, the viscosity of the medium can be evaluated using Stokes' law. With this method we successfully reproduced experimental viscosities and viscosity anisotropy for the solvent 5CB. The last line deals with the study of order induction on nematic molecules by an hydrogenated silicon surface. Gaining predicting power for the anchoring behavior of liquid crystals at surfaces will be a very desirable capability, as many properties related to devices depend on molecular organization close to surfaces. Here we studied, by means of atomistic MD simulations, the flat interface between an hydrogenated (001) silicon surface in contact with a sample of 5CB molecules. We found a planar anchoring of the first layers of 5CB where surface interactions are dominating with respect to the mesogen intermolecular interactions. We also analyzed the interface 5CB­vacuum, finding a homeotropic orientation of the nematic at this interface.

Permutation-based stochastic ordering using pairwise comparisons

The topic of this work concerns nonparametric permutation-based methods aiming to find a ranking (stochastic ordering) of a given set of groups (populations), gathering together information from multiple variables under more than one experimental designs. The problem of ranking populations arises in several fields of science from the need of comparing G>2 given groups or treatments when the main goal is to find an order while taking into account several aspects. As it can be imagined, this problem is not only of theoretical interest but it also has a recognised relevance in several fields, such as industrial experiments or behavioural sciences, and this is reflected by the vast literature on the topic, although sometimes the problem is associated with different keywords such as: "stochastic ordering", "ranking", "construction of composite indices" etc., or even "ranking probabilities" outside of the strictly-speaking statistical literature. The properties of the proposed method are empirically evaluated by means of an extensive simulation study, where several aspects of interest are let to vary within a reasonable practical range. These aspects comprise: sample size, number of variables, number of groups, and distribution of noise/error. The flexibility of the approach lies mainly in the several available choices for the test-statistic and in the different types of experimental design that can be analysed. This render the method able to be tailored to the specific problem and the to nature of the data at hand. To perform the analyses an R package called SOUP (Stochastic Ordering Using Permutations) has been written and it is available on CRAN.