Kinetic modelling of epitaxial film growth with up- and downward step barriers

The formation of three-dimensional structures during the epitaxial growth of films is associated to the reflection of diffusing particles in descending terraces due to the presence of the so-called Ehrlich-Schwoebel (ES) barrier. We generalize this concept in a solid-on-solid growth model, in which a barrier dependent on the particle coordination (number of lateral bonds) exists whenever the particle performs an interlayer diffusion. The rules do not distinguish explicitly if the particle is executing a descending or an ascending interlayer diffusion. We show that the usual model, with a step barrier in descending steps, produces spurious, columnar, and highly unstable morphologies if the growth temperature is varied in a usual range of mound formation experiments. Our model generates well-behaved mounded morphologies for the same ES barriers that produce anomalous morphologies in the standard model. Moreover, mounds are also obtained when the step barrier has an equal value for all particles independently if they are free or bonded. Kinetic roughening is observed at long times, when the surface roughness w and the characteristic length $\xi$ scale as $w ~ t^\beta$ and $\xi ~ t^\zeta$ where $\beta \approx 0.31$ and $\zeta \approx 0.22$, independently of the growth temperature.Comment: 15 pages, 7 figure

Fission fragment angular distribution measurements of U-235 and U-238 at CERN n_TOF facility

Neutron-induced fission cross sections of U-238 and U-235 are used as standards in the fast neutron region up to 200 MeV. A high accuracy of the standards is relevant to experimentally determine other neutron reaction cross sections. Therefore, the detection efficiency should be corrected by using the angular distribution of the fission fragments (FFAD), which are barely known above 20 MeV. In addition, the angular distribution of the fragments produced in the fission of highly excited and deformed nuclei is an important observable to investigate the nuclear fission process. In order to measure the FFAD of neutron-induced reactions, a fission detection setup based on parallel-plate avalanche counters (PPACs) has been developed and successfully used at the CERN-n_TOF facility. In this work, we present the preliminary results on the analysis of new U-235(n,f) and U-238(n,f) data in the extended energy range up to 200 MeV compared to the existing experimental data.Postprint (published version

Spin-orbit mode transfer via a classical analog of quantum teleportation

We translate the quantum teleportation protocol into a sequence of coherent operations involving three degrees of freedom of a classical laser beam. The protocol, which we demonstrate experimentally, transfers the polarisation state of the input beam to the transverse mode of the output beam. The role of quantum entanglement is played by a non-separable mode describing the path and transverse degrees of freedom. Our protocol illustrates the possibility of new optical applications based on this intriguing classical analogue of quantum entanglement.Comment: 5 pages, 7 figure

Surface-Invariants in 2D Classical Yang-Mills Theory

We study a method to obtain invariants under area-preserving diffeomorphisms associated to closed curves in the plane from classical Yang-Mills theory in two dimensions. Taking as starting point the Yang-Mills field coupled to non dynamical particles carrying chromo-electric charge, and by means of a perturbative scheme, we obtain the first two contributions to the on shell action, which are area-invariants. A geometrical interpretation of these invariants is given.Comment: 17 pages, 2 figure

Orientation dynamics of weakly Brownian particles in periodic viscous flows

Evolution equations for the orientation distribution of axisymmetric particles in periodic flows are derived in the regime of small but non-zero Brownian rotations. The equations are based on a multiple time scale approach that allows fast computation of the relaxation processes leading to statistical equilibrium. The approach has been applied to the calculation of the effective viscosity of a thin disk suspension in gravity waves.Comment: 16 pages, 7 eps figures include

Banco de sementes em sistemas de produção de agricultura com queima e sem queima no município de Marapanim, Pará.

Na agricultura de corte e queima, a capoeira tem como papel principal a fertilização da terra após a queima de sua biomassa vegetal. Geralmente as repetidas queimadas acabam destruindo todas as formas de regeneração, incluindo o banco de sementes. Pensando nessa problemática, o Projeto Studies of Human Impact on Forests and Floodplains in the Tropics (SHIFT), no qual esta pesquisa se insere, após uma série de estudos básicos sobre as capoeiras da região Bragantina, vem desenvolvendo uma alternativa de produção agrícola em que a capoeira é usada como fonte de nutrientes e matéria orgânica para o solo em sistemas de produção de ?agricultura sem queima?. O trabalho desenvolvido no município de Marapanim foi um experimento participativo, uma nova abordagem do projeto, onde as áreas foram definidas pelos agricultores. O objetivo do trabalho foi avaliar o potencial do banco de sementes de áreas submetidas a diferentes práticas agrícolas, caracterizando qualitativa e quantitativamente sua composição florística. Foram testadas as hipóteses de que o fogo diminui o potencial do banco de sementes e cria condições favoráveis à regeneração de espécies invasoras herbáceas. A cobertura triturada minimiza os impactos provocados pela queima sobre a diversidade vegetal e aumenta a regeneração de espécies acumuladoras de biomassa. O potencial de germinação do banco de sementes nos tratamentos foi monitorado durante 270 dias. Nos tratamentos queima e cobertura triturada, foram avaliadas a densidade, riqueza de espécies, famílias e formas de vida predominantes. No banco de sementes antes do manejo do solo, a densidade média na queima foi de 284 indivíduos/m2 e na cobertura triturada 328 indivíduos/m2. Depois do manejo do solo na queima, foram encontrados em média 23 indivíduos/m2 e na cobertura triturada 139 indivíduos/m2. As espécies mais freqüentes nos dois momentos e nos dois tratamentos foram Borreria latifolia, Fimbristylis meliacea e Cyperus diffusus. As famílias Cyperaceae, Rubiaceae e Poaceae apareceram em alta frequência em todas as áreas e nos dois tratamentos. Além dessas, destacaram-se as famílias Euphorbiaceae, Melastomataceae, Malvaceae, Lamiaceae, Scrophulariaceae e Asteraceae. As formas de vida mais comuns encontradas nos tratamentos foram as ervas. Os diferentes tratamentos afetaram a abundância e riqueza florística, diminuindo o número de sementes de espécies arbóreas e arbustivas, acumuladoras de biomassa

Modelling of epitaxial film growth with a Ehrlich-Schwoebel barrier dependent on the step height

The formation of mounded surfaces in epitaxial growth is attributed to the presence of barriers against interlayer diffusion in the terrace edges, known as Ehrlich-Schwoebel (ES) barriers. We investigate a model for epitaxial growth using a ES barrier explicitly dependent on the step height. Our model has an intrinsic topological step barrier even in the absence of an explicit ES barrier. We show that mounded morphologies can be obtained even for a small barrier while a self-affine growth, consistent with the Villain-Lai-Das Sarma equation, is observed in absence of an explicit step barrier. The mounded surfaces are described by a super-roughness dynamical scaling characterized by locally smooth (faceted) surfaces and a global roughness exponent $\alpha>1$. The thin film limit is featured by surfaces with self-assembled three-dimensional structures having an aspect ratio (height/width) that may increase or decrease with temperature depending on the strength of step barrier.Comment: To appear in J. Phys. Cond. Matter; 3 movies as supplementary materia

Aptitud anaerobia en deportistas de combate del sexo femenino

Se estudia en nivel de preparación anaerobia en tres selecciones femeninas de deportes de combate de alta calificación: 15 judocas, 7 esgrimistas de la modalidad sable y 16 taekwondoistas. Fueron medidos indicadores neuromusculares, al final de la preparación especial, utilizando los tests de ergosalto y de saltabilidad de Bosco, Se determinan las estadísticas descriptivas, significación de las diferencias entre deportes y la correlación entre indicadores. Se obtienen los resultados: Pala (15,7 ± 2,1-19,2 ± 1,5 W/Kg), Pana (14,5 ± 1,9-17,7 ± 1,2 W/Kg), Los SCAB, SSAB, SCPD, SCPCD y SDPC mas altos fueron los de las esgrimistas y la mejor coordinación se encontró en las competidoras de esgrima, mientras que los mayores valores de CELAS fueron los de las judocas. Los resultados obtenidos se corresponden con las características del gesto deportivo que prevalece en cada uno de los deportes estudiados

Interacting Particles and Strings in Path and Surface Representations

Non-relativistic charged particles and strings coupled with abelian gauge fields are quantized in a geometric representation that generalizes the Loop Representation. We consider three models: the string in self-interaction through a Kalb-Ramond field in four dimensions, the topological interaction of two particles due to a BF term in 2+1 dimensions, and the string-particle interaction mediated by a BF term in 3+1 dimensions. In the first case one finds that a consistent "surface-representation" can be built provided that the coupling constant is quantized. The geometrical setting that arises corresponds to a generalized version of the Faraday's lines picture: quantum states are labeled by the shape of the string, from which emanate "Faraday`s surfaces". In the other models, the topological interaction can also be described by geometrical means. It is shown that the open-path (or open-surface) dependence carried by the wave functional in these models can be eliminated through an unitary transformation, except by a remaining dependence on the boundary of the path (or surface). These feature is closely related to the presence of anomalous statistics in the 2+1 model, and to a generalized "anyonic behavior" of the string in the other case.Comment: RevTeX 4, 28 page