Recent progress on the notion of global hyperbolicity

Global hyperbolicity is a central concept in Mathematical Relativity. Here, we review the different approaches to this concept explaining both, classical approaches and recent results. The former includes Cauchy hypersurfaces, naked singularities, and the space of the causal curves connecting two events. The latter includes structural results on globally hyperbolic spacetimes, their embeddability in Lorentz-Minkowski, and the recently revised notions of both, causal and conformal boundaries. Moreover, two criteria for checking global hyperbolicity are reviewed. The first one applies to general splitting spacetimes. The second one characterizes accurately global hyperbolicity and spacelike Cauchy hypersurfaces for standard stationary spacetimes, in terms of a naturally associated Finsler metric.Comment: 18 pages, 1 figure. Extended and updated contribution to the meeting "New Developments in Lorentzian Geometry" Berlin, Nov. 200

Direct probing of band-structure Berry phase in diluted magnetic semiconductors

We report on experimental evidence of the Berry phase accumulated by the charge carrier wave function in single-domain nanowires made from a (Ga,Mn)(As,P) diluted ferromagnetic semiconductor layer. Its signature on the mesoscopic transport measurements is revealed as unusual patterns in the magnetoconductance, that are clearly distinguished from the universal conductance fluctuations. We show that these patterns appear in a magnetic field region where the magnetization rotates coherently and are related to a change in the band-structure Berry phase as the magnetization direction changes. They should be thus considered as a band structure Berry phase fingerprint of the effective magnetic monopoles in the momentum space. We argue that this is an efficient method to vary the band structure in a controlled way and to probe it directly. Hence, (Ga,Mn)As appears to be a very interesting test bench for new concepts based on this geometrical phase.Comment: 7 pages, 6 figure

Slow Radiation-Driven Wind Solutions of A-Type Supergiants

The theory of radiation-driven winds succeeded in describing terminal velocities and mass loss rates of massive stars. However, for A-type supergiants the standard m-CAK solution predicts values of mass loss and terminal velocity higher than the observed values. Based on the existence of a slow wind solution in fast rotating massive stars, we explore numerically the parameter space of radiation-driven flows to search for new wind solutions in slowly rotating stars, that could explain the origin of these discrepancies. We solve the 1-D hydrodynamical equation of rotating radiation-driven winds at different stellar latitudes and explore the influence of ionization's changes throughout the wind in the velocity profile. We have found that for particular sets of stellar and line-force parameters, a new slow solution exists over the entire star when the rotational speed is slow or even zero. In the case of slow rotating A-type supergiant stars the presence of this novel slow solution at all latitudes leads to mass losses and wind terminal velocities which are in agreement with the observed values. The theoretical Wind Momentum-Luminosity Relationship derived with these slow solutions shows very good agreement with the empirical relationship. In addition, the ratio between the terminal and escape velocities, which provides a simple way to predict stellar wind energy and momentum input into the interstellar medium, is also properly traced.Comment: 7 Pages, 3 figures, Astrophysical Journal, Accepte

Localized magnetoplasmon modes arising from broken translational symmetry in semiconductor superlattices

The electromagnetic propagator associated with the localized collective magnetoplasmon excitations in a semiconductor superlattice with broken translational symmetry, is calculated analytically within linear response theory. We discuss the properties of these collective excitations in both radiative and non-radiative regimes of the electromagnetic spectra. We find that low frequency retarded modes arise when the surface density of carriers at the symmetry breaking layer is lower than the density at the remaining layers. Otherwise a doublet of localized, high-frequency magnetoplasmon-like modes occurs.Comment: Revtex file + separate pdf figure

Angular Momentum Loss Rates in Be Stars Determined by the Viscous Decretion Disc Model

Circumstellar discs around Be stars are formed by the material ejected by the central star. This process removes excess angular momentum from the star as viscosity facilitates the mass and angular momentum transfer within the disc and its growth. The angular momentum loss rates (AMLR) of Be stars is a subject of debate in the literature. Through the modelling of the disc formation and dissipation phases observed from Be stars, their average AMLR can be determined and this is the goal of this work. We use the viscous decretion disc (VDD) model to provide a range of the average AMLR for Be stars and compare these rates with predicted values from the literature. We explore the reasons for discrepancies between the predicted values of average AMLR using the VDD and Geneva stellar evolution (GSE) models that were previously reported in literature and find that the largest differences occur when Be stars are rotating below their critical speeds. We show that the time over which the mass reservoir builds up is inversely proportional to the average AMLR. Also, we determine a revised value of the average AMLR for the Galactic Be star omega CMa of 4.7x10^36 g cm^2/s^2, which is in better agreement with the values expected for a typical B2 type star. Finally, the effect of disc truncation due to the presence of a companion star is investigated and we find that this has a minimal effect on the average AMLR

Anisotropic Magnetoresistance in Manganites: Model and Experiment

We present measurements of anisotropic magnetoresistance of La_{0.75}Sr_{0.25}MnO_3 films deposited on (001) SrTiO_3 substrates, and develop a model to describe the low temperature AMR in manganites. We measure an AMR of the order of 10^{-3} for the current I parallel to the [100] axis of the crystal and vanishing AMR for I//[110], in agreement with the model predictions.Comment: 4 pages, 4 figure

Análisis sociológico de la Ley 4/2000, de 11 de enero, sobre derechos y libertades de los extranjeros en España y su integración social. Sus reformas (8/2000, 11/2003, 14/2003, 2/2009) y su implicación social

La conocida como Ley de extranjería ha sido y es uno de los elementos que mayor atención y debate ha suscitado en el marco de la política migratoria española. Sirvan como ejemplo las sucesivas reformas de las que ha sido objeto en la última década, puesto que se han llegado a contabilizar hasta cuatro. La última tuvo lugar en el mes de diciembre de 2009. Con el objeto de profundizar en esta realidad, se ha desarrollado el proyecto de investigación I+D concedido por el Ministerio de Ciencia y Tecnología y titulado SEJ 2006-05222. Inmigración y Bienestar Social, en cuyo contexto se enmarca el presente artículo. Se ha analizado el articulado de la Ley y sus implicaciones sociales a través de los principios de la Grounded Theory. Para cada reforma, se ha contextualizado, además, el marco político y social que justificaba la misma. Entre otras conclusiones, se observa que la Ley es un intento de dar respuesta a la nueva realidad de la extranjería en España desde un planteamiento meramente técnico, sin mayor pretensión ideológica. No obstante, la lógica discursiva pone el acento de manera mayoritaria en la necesidad de «controlar socialmente» al extranjero. Todo ello provoca una suerte de metonimia en el articulado de la Ley entre extranjero e inmigrante, lo que convierte a la Ley en coadyuvante de una fractura social cada vez mayor entre extranjeros y nacionales, con todas las implicaciones que ello supone en los procesos de identidad y alteridad.Act 4/2000, commonly known as the Aliens Act, has caused considerable debate in Spanish society about migration policy. The last decade, the Act has been amended repeatedly- a total of four amendments have been made. The most recent amendment was proposed in December 2009. This article reports the findings of research project I + D SEJ 2006-05222. Social Welfare and Immigration, funded by the Spanish Ministry of Science and Technology, and aims to examine this issue in depth. The provisions of the Act and their social implications were analysed using Grounded Theory. In addition, this article contextualises the political and social framework for justifying the Act for each set of amendments. We conclude, inter alia, that the Act is an attempt to respond to changes in the situation of aliens in Spain from a technical perspective, without an ideological objective. Nevertheless, the discursive logic of the Act focuses mainly on the need of "social control" over foreigners. The Act's provisions cause a sort of metonymy between foreigners and immigrants. Thus, the Act contributes to an ever greater social divide between foreigners and nationals, with wide-ranging implications on identity and otherness processes

Rate compatible modulation for non-orthogonal multiple access

We propose a new Non-Orthogonal Multiple Access (NOMA) coding scheme based on the use of a Rate Compatible Modulation (RCM) encoder for each user. By properly designing the encoders and taking advantage of the additive nature of the Multiple Access Channel (MAC), the joint decoder from the inputs of all the users can be represented by a bipartite graph corresponding to a standard point-topoint RCM structure with certain constraints. Decoding is performed over this bipartite graph utilizing the sum-product algorithm. The proposed scheme allows the simultaneous transmission of a large number of uncorrelated users at high rates, while the decoding complexity is the same as that of standard point-to-point RCM schemes. When Rayleigh fast fading channels are considered, the BER vs SNR performance improves as the number of simultaneous users increases, as a result of the averaging effect