Ising model for the alloys according to the effective field theory

FeAl alloys in their disordered structural phase have been investigated through an Ising model where besides exchange interactions between nearest-neighbors Fe atoms, a superexchange interaction mediated by Al atoms is also taken into account. The model has been approximately treated according to the effective field theory. Although the phase diagram, as a function of Al concentration, is similar to the one previously obtained from Bogoliubov variational approach for the free energy, a different behavior for the superexchange interaction is achieved, which can also be physically accepted for this system.     

Modelling heavy tails and asymmetry using ARCH-type models with stable Paretian distributions

Several approaches have been considered to model the heavy tails and asymmetric effect on stocks returns volatility. The most commonly used models are the Exponential Generalized AutoRegressive Conditional Heteroskedasticity (EGARCH), the Threshold GARCH (TGARCH), and the Asymmetric Power ARCH (APARCH) which, in their original form, assume a Gaussian distribution for the innovations. In this paper we propose the estimation of all these asymmetric models on empirical distributions of the Standard & Poor’s (S&P) 500 and the Financial Times Stock Exchange (FTSE) 100 daily returns, assuming the Student’s t and the stable Paretian with (α < 2) distributions for innovations. To the authors’ best knowledge, analysis of the EGARCH and TGARCH assuming innovations with α-stable distribution have not yet been reported in the literature. The results suggest that this kind of distributions clearly outperforms the Gaussian case. However, when α-stable and Student’s t distributions are compared, a general conclusion should be avoided as the goodness-of-fit measures favor the α-stable distribution in the case of S&P 500 returns and the Student’s t distribution in the case of FTSE 100.     

Water structure and selective permeation of cellulose-based membranes

The techniques used in the study of water in pores are extensively reviewed. Particular importance is given to ATR-FTIR, which allows the study of the active layer in the wet state. Taking further advantage of this technique, mathematical simulation studies on the ATR-FTIR spectra of water in cellulose acetate and cellulose acetate butyrate asymmetric membranes are performed. The OH-stretching band in the spectra of cellulose acetate membranes is deconvoluted into four components, which are ascribed to water clusters of different sizes. Results support the idea that the apparent rejection to salts may be associated to the size and number of water clusters whereas the flux depends on other factors, e.g. the thickness of the active layer.

The effect of salts, such as MgCl₂ and KCl, on the structure of water in the pores is also studied. The ATR-FTIR spectra of membranes previously equilibrated with solutions of such salts are deconvoluted.     

On the approximation of the solutions of the Riemann problem for a discontinuous conservation law

For a class of discontinuous flux functions introduced in [3] (cf. also [4]), we prove, for the Riemann problem, an extension of the existence result proved in [2] for a Lipschitz continuous flux function. In the last section, and based in the previous results, we apply the Lax-Friedrichs approximation method and the limiters technique (cf.[6]) to compute the quoted solution in a numerical example. For related results see [5].     

Analytical and computational modelling for wave energy systems: the example of oscillating wave surge converters

The development of new wave energy converters has shed light on a number of unanswered questions in fluid mechanics, but has also identified a number of new issues of importance for their future deployment. The main concerns relevant to the practical use of wave energy converters are sustainability, survivability, and maintainability. Of course, it is also necessary to maximize the capture per unit area of the structure as well as to minimize the cost. In this review, we consider some of the questions related to the topics of sustain-ability, survivability, and maintenance access, with respect to sea conditions, for generic wave energy converters with an emphasis on the oscillating wave surge converter. New analytical models that have been developed are a topic of par-ticular discussion. It is also shown how existing numerical models have been pushed to their limits to provide answers to open questions relating to the operation and characteristics of wave energy converters.     

Delocalized 2-magnons eigenstates in long-range correlated random Heisenberg chains

We consider the one-dimensional quantum disordered Heisenberg ferromagnetic chain model with long-range correlated exchange couplings and study the nature of collective two-spin excitations. By using an exact diagonalization of the Hamiltonian in the two-spin flip subspace, we compute the spin wave participation number to characterize the localized or delocalized nature of the two-magnon states. For strongly correlated random exchange couplings, extended two-spin excitations with finite energy appear. Integrating the time-dependent Schroedinger equation, we follow the time-evolution of an initially localized two-spin state. We find that, associated with the emergence of extended spin waves, the wave-packet mean-square displacement develops a ballistic spread. Further, the single-spin wave-packet acquires an asymmetric profile due to the kinematic interaction between the excited spins.     

Probing light pseudoscalar particles using synchrotron light

The need for purely laboratory-based light pseudoscalar particles searches has been emphasized many times in the literature, since astrophysical bounds on these particles rely on several assumptions to calculate the flux produced in stellar plasmas. In this Letter we study the use of light from synchrotron accelerators as a source for a photon regeneration experiment also know as “light shining through a wall”. Such an experiment can significantly improve present limits on the pseudoscalar particle mass and the pseudoscalar–photon coupling constant obtained from laser experiments. This is possible even using a small number of powerful magnets (B∼10 T$B\sim 10\text{T}$), due to the large incident photon flux. On the other hand, the use of a broadband incident photon-beam instead of infrared or optical lasers allows a significant improvement in the mass reach of the experiment (it is possible to test masses up to 0.01 eV without a drop in sensitivity). Large, but still feasible, configurations can explore in a quite model-independent way a large part of the parameter space examined by solar searches and HB stars in globular clusters. Additionally, the proposal may be useful for testing string motivated effective theories containing light and weakly interacting particles.     

Collapse of solitary waves near the transition from supercritical to subcritical bifurcations

The nonlinear stage of the instability of one-dimensional solitons within a small vicinity of the transition point from supercritical to subcritical bifurcations has been studied both analytically and numerically using the generalized nonlinear Schrödinger equation. It is shown that the pulse amplitude and its width near the collapsing time demonstrate a self-similar behavior with a small asymmetry at the pulse tails due to self-steepening. This theory is applied to solitary interfacial deep-water waves, envelope water waves with a finite depth, and short optical pulses in fibers.     

Charge storage behavior of nanostructures based on SiGe nanocrystals embedded in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> matrix

The charge storage behavior of nanostructures based on Si_1?x Ge _x (0 ≤ x ≤ 1) nanocrystals (NCs) in an Al₂O₃ matrix was investigated. The structures have been grown by RF magnetron sputtering and subsequently annealed at temperatures ranging from 700 °C to 1000 °C for 30 min in nitrogen ambient. The stoichiometry of the SiGe NCs and the alumina crystalline structure were found to be significantly dependent on the RF power and the annealing temperature. The sizes of the SiGe NCs and their distribution were investigated by grazing incidence small angle X-ray scattering (GISAXS). The capacitance-voltage (C-V) and conductance-voltage (G-V) measurements were performed to investigate the charge trapping characteristics of the memory structures. The C-V hysteresis width depends on variations in the crystalline structure resulting from different annealing temperatures. It is also shown that charge injection is governed by the Fowler-Nordheim tunnel mechanism for higher electric fields.