Shot Noise in Linear Macroscopic Resistors

We report on a direct experimental evidence of shot noise in a linear macroscopic resistor. The origin of the shot noise comes from the fluctuation of the total number of charge carriers inside the resistor associated with their diffusive motion under the condition that the dielectric relaxation time becomes longer than the dynamic transit time. Present results show that neither potential barriers nor the absence of inelastic scattering are necessary to observe shot noise in electronic devices.Comment: 10 pages, 5 figure

Shot-noise suppression in Schottky barrier diodes

We give a theoretical interpretation of the noise properties of Schottky barrier diodes based on the role played by the long range Coulomb interaction. We show that at low bias Schottky diodes display shot noise because the presence of the depletion layer makes negligible the effects of the Coulomb interaction on the current fluctuations. When the device passes from barrier to flat band conditions, the Coulomb interaction becomes active, thus introducing correlation between different current fluctuations. Therefore, the cross-over between shot and thermal noise represents the suppression due to long range Coulomb interaction of the otherwise full shot-noise. Similar ideas can be used to interpret the noise properties of others semiconductor devices.Comment: 3 page

Firms vs. insiders as traders of last resort

We explore the role of corporate insiders vs. firms as traders of last resort. We develop a simple model of insider trading in which insiders provide price support, as well as liquidity, in security markets. Consistent with the model predictions we find that in the US markets insiders’ trading activities have a clear impact on return distributions. Furthermore, we provide empirical evidence on insiders transactions and firm transactions affecting returns in a different manner. In particular, while insiders’ transactions (both purchases and sales) have a strong impact on skewness in the short run and to a lesser extent in short run volatility, company repurchases only have a clear impact on volatility, both in the short and the long run. We provide explanations for this asymmetry.Insider trading, liquidity, short-horizon variance, autocorrelation, skewness

El paper de les associacions de pares i mares d’alumnes en el sistema educatiu de les Illes Balears

La participació de les famílies en l’escola és un element cabdal en un sistema que promou la implicació col·lectiva de la comunitat educativa en la construcció d’una societat més justa, democràtica i igualitària. En aquest sentit, el paper que fan les associacions de pares i mares d’alumnes mereix una atenció especial tant de les institucions que sostenen el sistema educatiu com de la investigació en educació. L’article proposa, doncs, una ràpida mirada a les associacions i al seu pes en el sistema educatiu, que viu un moment socioeconòmic difícil.La participación de las familias en la escuela es un elemento crucial en un sistema que promueve la implicación colectiva de la comunidad educativa en la construcción de una sociedad más justa, democrática e igualitaria. En este sentido, el papel que juegan las asociaciones de padres y madres de alumnos merece una atención especial tanto por parte de las instituciones que sostienen el sistema educativo como por parte de la investigación en educación. El artículo propone, pues, una rápida mirada a las asociaciones y a su peso en el sistema educativo, que vive un momento socioeconómico difícil

Fairy circle landscapes under the sea

Short-scale interactions yield large-scale vegetation patterns that, in turn, shape ecosystem function across landscapes. Fairy circles, which are circular patches bare of vegetation within otherwise continuous landscapes, are characteristic features of semiarid grasslands. We report the occurrence of submarine fairy circle seascapes in seagrass meadows and propose a simple model that reproduces the diversity of seascapes observed in these ecosystems as emerging from plant interactions within the meadow. These seascapes include two extreme cases, a continuous meadow and a bare landscape, along with intermediate states that range from the occurrence of persistent but isolated fairy circles, or solitons, to seascapes with multiple fairy circles, banded vegetation, and "leopard skin" patterns consisting of bare seascapes patterns consisting of bare seascapes dotted with plant patches. The model predicts that these intermediate seascapes extending across kilometers emerge as a consequence of local demographic imbalances along with facilitative and competitive interactions among the plants with a characteristic spatial scale of 20 to 30 m, consistent with known drivers of seagrass performance. The model, which can be extended to clonal growth plants in other landscapes showing fairy rings, reveals that the different seascapes observed hold diagnostic power as to the proximity of seagrass meadows to extinction points that can be used to identify ecosystems at risks

Shot-noise anomalies in nondegenerate elastic diffusive conductors

We present a theoretical investigation of shot-noise properties in nondegenerate elastic diffusive conductors. Both Monte Carlo simulations and analytical approaches are used. Two new phenomena are found: (i) the display of enhanced shot noise for given energy dependences of the scattering time, and (ii) the recovery of full shot noise for asymptotic high applied bias. The first phenomenon is associated with the onset of negative differential conductivity in energy space that drives the system towards a dynamical electrical instability in excellent agreement with analytical predictions. The enhancement is found to be strongly amplified when the dimensionality in momentum space is lowered from 3 to 2 dimensions. The second phenomenon is due to the suppression of the effects of long range Coulomb correlations that takes place when the transit time becomes the shortest time scale in the system, and is common to both elastic and inelastic nondegenerate diffusive conductors. These phenomena shed new light in the understanding of the anomalous behavior of shot noise in mesoscopic conductors, which is a signature of correlations among different current pulses.Comment: 9 pages, 6 figures. Final version to appear in Phys. Rev.

Elementary Excitations of a Bose-Einstein Condensate in an Effective Magnetic Field

We calculate the low energy elementary excitations of a Bose-Einstein Condensate in an effective magnetic field. The field is created by the interplay between light beams carrying orbital angular momentum and the trapped atoms. We examine the role of the homogeneous magnetic field, familiar from studies of rotating condensates, and also investigate spectra for vector potentials with a more general radial dependence. We discuss the instabilities which arise and how these may be manifested.Comment: 8 pages, 4 figure