Intrinsic stokes parameters for 3D and 2D polarization states

The second-order characterization of a three-dimensional (3D) state of polarization is provided either by the corresponding 3D coherency matrix or (equivalently) by the associated 3D Stokes parameters. The analysis of the polarization properties that are invariant under orthogonal transformations of the laboratory reference frame allows to define a set of six intrinsic Stokes parameters which provides a simplified interpretation of 3D states of polarization in terms of meaningful physical properties. The rotationally invariant properties of 2D states of polarization are straightforwardly retrieved in a consistent way, so that the 2D intrinsic Stokes parameters are constituted by the intensity, the degree of linear polarization and the degree of circular polarization

Sources of Asymmetry and the Concept of Nonregularity ofn-Dimensional Density Matrices

The information contained in ann-dimensional (nD) density matrix rho is parametrized and interpreted in terms of its asymmetry properties through the introduction of a family of components of purity that are invariant with respect to arbitrary rotations of thenD Cartesian reference frame and that are composed of two categories of meaningful parameters of different physical nature: the indices of population asymmetry and the intrinsic coherences. It is found that the components of purity coincide, up to respective simple coefficients, with the intrinsic Stokes parameters, which are also introduced in this work, and that determine two complementary sources of purity, namely the population asymmetry and the correlation asymmetry, whose weighted square average equals the overall degree of purity of rho. A discriminating decomposition of rho as a convex sum of three density matrices, viz. the pure, the fully random (maximally mixed) and the discriminating component, is introduced, which allows for the definition of the degree of nonregularity of rho as the distance from rho to a density matrix of a system composed of a pure component and a set of 2D, 3D, horizontal ellipsis andnD maximally mixed components. The chiral properties of a state rho are analyzed and characterized from its intimate link to the degree of correlation asymmetry. The results presented constitute a generalization tonD systems of those established and exploited for polarization density matrices in a series of previous works

Analysis of magnetic and structural properties in La0.6Sr 0.4MnO3 ferromagnetic particles under the influence of mechanical ball milling effect

We have investigated the magnetic, structural and morphological properties of La0.6Sr0.4MnO3 (LSMO-40) manganite particles, synthesized by solid state reaction method. The resulting LSMO-40 powders were milled in air atmosphere during 3, 6 and 12 hours, by using a planetary ball milling. Samples obtained were characterized by X-ray diffraction, scanning electron microscopy – SEM and magnetization measurements as a function of temperature and magnetic field. A Rietveld analysis was carried on each XRD pattern, and was observed a reduction in crystallite average size (Dv) with increased ball milling time, tM. This is associated with a decrease in particle size. A characteristic rhombohedral crystal structure for the LSMO-40 phase was identified (space group R3C), independent of the milling time of the powders. However, from SEM microstructure was observed more homogeneity in the grain distribution by milling process. The results of magnetic characterization, showed that samples with higher tM (smaller grain size), presented the lowest value of the saturation magnetization, which is attributed to surface effects that induce magnetically disordered states with decreasing particle sizes. This magnetic anisotropy surface is evidenced also on the changes of coercive fields, HC, measured at low temperatures, which increased with increasing tM

Alternative power flow method for direct current resistive grids with constant power loads: A truncated Taylor-based method

The power flow in electrical system permits analyzing and studying the steady-state behavior of any grid. Additionally, the power flow helps with the proper planning and management of the system. Therefore, it is increasingly necessary to propose power flows with fast convergence and high efficiency in their results. For this reason, this paper presents an alternative power flow approach for direct current networks with constant power loads based on a truncated Taylor-based approximation. This approach is based on a first-order linear approximation reformulated as a recursive, iterative method. It works with a slope variable concept based on derivatives, which allow few iterations and low processing times. Numerical simulations permit identifying the best power flow approaches reported in the specialized literature for radial and mesh dc grids, including the proposed approach. All the simulations were conducted in MATLAB 2015a. © Published under licence by IOP Publishing Ltd.Universidad Tecnológica de Pereira, UTP: C2018P020 Department of Science, Information Technology and Innovation, Queensland Government, DSITI: ColcienciasThis work was supported in part by the Administrative Department of Science, Technology and Innovation of Colombia (Colciencias) through the National Scholarship Program under Grant 727-2015 and in part by the Universidad Tecnológica de Bolívar under Project C2018P020

Polarimetric nonregularity of evanescent waves

Three-dimensional polarization states of random light can be classified into regular and nonregular according to the structure of the related 3 × 3 polarization matrix. Here we show that any purely evanescent wave excited in total internal reflection of a partially polarized plane-wave field is always in a nonregular polarization state. The degree of nonregularity of such evanescent waves is also studied in terms of a recently advanced measure. Nonregular evanescent waves uncover new aspects of the polarimetric structure and dimensional character of electromagnetic near fields, with potential applications in nanoscale surface optics

Intensity and spin anisotropy of three-dimensional polarization states

Anisotropy is a natural feature of polarization states, and only fully random three-dimensional (3D) states exhibit complete isotropy. In general, differences between the strengths of the electric field components along the three orthogonal directions give rise to intensity anisotropy. Moreover, polarization states involve an average spin whose inherent vectorial nature constitutes a source of spin anisotropy. In this work, appropriate descriptors are identified to characterize quantitatively the levels of intensity anisotropy and spin anisotropy of a general 3D polarization state, leading to a novel interpretation for the degree of polarimetric purity as a measure describing the overall polarimetric anisotropy of a 3D optical field. The mathematical representation, as well as the physical features of completely intensity-isotropic 3D polarization states with a maximum spin anisotropy, are also examined. The results provide new insights into the polarimetric field structure of random 3D electromagnetic light states. (C) 2019 Optical Society of Americ

Drought and Forest Decline in the Iberian Peninsula: A Simple Explanation for a Complex Phenomenom?

Different episodes of forest decline have been reported in several areas of the Northern Hemisphere during the 20th century. The explanation of this process included anthropogenic (air pollution), biotic (pathogens) and climatic factors. Among the climatic factors, the effects of acute or chronic droughts have been the most common explanation for the massive dieback observed. In the Iberian Peninsula, besides the pathogenic explanation which gives a paramount relevance to the fungus Phytophtora cinnamomi in many situations, the role of an increment in aridity (chronic) or the consequences of severe droughts (acute) have been empirically supported. The evident synchronism between forest decline and abnormally adverse climatic conditions, in addition to the reversibility of the process when precipitation intensity increased, are two major arguments for supporting the climatic involvement in the phenomenon. Nevertheless, the two most affected species in the episodes of oak decline in the Iberian Peninsula were Quercus ilex and Q. suber, which are representatives of the so called Mediterranean woody flora and which have been considered as more drought resistant than the temperate oak species that co-occur in this territory. The climatic complexity ofthe Iberian Peninsula, the many mechanisms for coping with water stress that have been described within the sclerophyllous Mediterranean flora and the effects of human management partially explain this paradox. Other forest species have also been affected by massive decline. Pinus sylvestris and Abies alba, which have their southern distribution limit in the mountain ranges of the Iberian Peninsula, are two examples of this situation. Both cases, besides the inmediate effects of some climatic perturbations, need the incorporation of different predisposing factors – mainly historical aspects to obtain a complete image of the process

Delineating limits: Confronting predicted climatic suitability to field performance in mistletoe populations

1. Biotic stressors such as hemiparasites have a profound impact on forest functioning. However, predicting the future incidence of these stressors on forests remains challenging because climate-based distribution does not consider tree-hemiparasite interactions or the impacts of extreme climate events on stressors'' performance. 2. We use species distribution models (SDMs) and ecophysiological and demographic field data to assess whether climatic suitability is a proxy for the performance of the hemiparasite mistletoe (Viscum album) in two forests with contrasting climatic conditions. Two host tree species representing wet-cold (Scots pine) and dry-warm (Aleppo pine) conditions were selected. We fitted SDMs based on climate variables, and measured different ecophysiological variables capturing cold- (photoinhibition) and drought tolerance (intrinsic water-use efficiency, iWUE). We also assessed demographic variables related to seed germination and establishment rates of mistletoe through a translocation experiment. 3. Species distribution models showed a high climatic suitability of mistletoe in both forests. Mistletoes living in the Scots pines site presented a higher cold tolerance, while those inhabiting the Aleppo pine site showed a higher iWUE. Seedlings coming from local seeds showed a lower mortality than seedlings coming from translocated seeds. Germination and seedling establishment showed temporal mismatches when comparing local and translocated seeds. 4. Synthesis. Habitat suitability predicted by SDMs based on climate data and field performance were related in this mistletoe species. However, ecophysiological and demographic variables indicated a lower fitness of mistletoe in the dry-warm site associated with drought stress. In conclusion, predicted climate suitability based on SDMs forecasts should be refined using field data on actual performance and considering plant-to-plant interactions and extreme climate events

Fundamentos de Control Automático de Sistemas Continuos y Muestreados

La Ingeniería de Control versa sobre el diseño de leyes matemáticas que gobiernen sistemas físicos conforme a una serie de especificaciones. Esta disciplina es, por tanto, esencial para la automatización de sistemas o procesos industriales. Este manual presenta, en un primer gran bloque, los métodos tradicionales de análisis y diseño de controladores de sistemas continuos (o analógicos). La segunda parte describe las mismas herramientas para sistemas muestreados (o digitales)