Antireflection coatings from analogy between electron scattering and spin precession

We use the analogy between scattering of a wave from a potential, and the precession of a spin-half particle in a magnetic field, to gain insight into the design of an antireflection coating for electrons in a semiconductor superlattice. It is shown that the classic recipes derived for optics are generally not applicable due to the different dispersion law for electrons. Using the stability conditions we show that a Poisson distribution of impedance steps is a better approximation than is a Gaussian distribution. Examples are given of filters with average transmissivity exceeding 95% over an allowed band.

Networks of noisy oscillators with correlated degree and frequency dispersion

We investigate how correlations between the diversity of the connectivity of networks and the dynamics at their nodes affect the macroscopic behavior. In particular, we study the synchronization transition of coupled stochastic phase oscillators that represent the node dynamics. Crucially in our work, the variability in the number of connections of the nodes is correlated with the width of the frequency distribution of the oscillators. By numerical simulations on Erdös-Rényi networks, where the frequencies of the oscillators are Gaussian distributed, we make the counterintuitive observation that an increase in the strength of the correlation is accompanied by an increase in the critical coupling strength for the onset of synchronization. We further observe that the critical coupling can solely depend on the average number of connections or even completely lose its dependence on the network connectivity. Only beyond this state, a weighted mean-field approximation breaks down. If noise is present, the correlations have to be stronger to yield similar observations.

Bernoulli correction to viscous losses: Radial flow between two parallel discs

For a massless fluid (density = 0), the steady flow along a duct is governed exclusively by viscous losses. In this paper, we show that the velocity profile obtained in this limit can be used to calculate the pressure drop up to the first order in density. This method has been applied to the particular case of a duct, defined by two plane-parallel discs. For this case, the first-order approximation results in a simple analytical solution which has been favorably checked against numerical simulations. Finally, an experiment has been carried out with water flowing between the discs. The experimental results show good agreement with the approximate solution

Persistent random walk model for transport trough thin slabs

We present a model for transport in multiply scattering media based on a three-dimensional generalization of the persistent random walk. The model assumes that photons move along directions that are parallel to the axes. Although this hypothesis is not realistic, it allows us to solve exactly the problem of multiple scattering propagation in a thin slab. Among other quantities, the transmission probability and the mean transmission time can be calculated exactly. Besides being completely solvable, the model could be used as a benchmark for approximation schemes to multiple light scattering.

La voluntad del retorno: correspondencia desde el exilio catalán

[spa] El presente artículo ofrece una aproximación al complejo fenómeno del exilio provocado por la Guerra Civil española desde un punto de vista poco analizado hasta este momento: el estudio a través de los epistolarios existentes de los sentimientos personales que hizo aflorar este abandono forzado del país (miedos, deseos, esperanzas, incertidumbres, preocupaciones e inquietudes, ánimos, etc.). Para conseguir esta visión más personal hemos basado nuestra investigación en el análisis de unas fuentes que consideramos poco trabajadas hasta el momento como son los epistolarios. El estudio de éstos, ya sean fuentes primarias depositadas en fondos de archivos diversos de Catalunya como los epistolarios editados, nos ha permitido obtener un testimonio personal y de primera mano de un fenómeno, el del exilio, tratado principalmente hasta ahora desde un punto de vista colectivo. El análisis, centrado básicamente en personajes catalanes y en el periodo comprendido entre 1939 y 1946 (cuando se desvanece definitivamente la esperanza que la victoria aliada provoque el fin del régimen) pone de manifiesto una voluntad común: el ansia del retorno.