Monte Carlo studies of antiferromagnetic spin models in three dimensions

We study several antiferromagnetic formulations of the O(3) spin model in three dimensions by means of Monte Carlo simulations. We discuss about the vacua properties and analyze the phase transitions. Using Finite Size Scaling analysis we conclude that all phase transitions found are of first orderComment: 4 pages, 2 Postscript figures. Contribution to Lattice '9

Time relaxation of interacting single--molecule magnets

We study the relaxation of interacting single--molecule magnets (SMMs) in both spatially ordered and disordered systems. The tunneling window is assumed to be, as in Fe8, much narrower than the dipolar field spread. We show that relaxation in disordered systems differs qualitatively from relaxation in fully occupied cubic and Fe_8 lattices. We also study how line shapes that develop in ''hole--digging'' experiments evolve with time t in these fully occupied lattices. We show (1) that the dipolar field h scales as t^p in these hole line shapes and show (2) how p varies with lattice structure. Line shapes are not, in general, Lorentzian. More specifically, in the lower portion of the hole, they behave as (h/t^p)^{(1/p)-1} if h is outside the tunnel window. This is in agreement with experiment and with our own Monte Carlo results.Comment: 21 LaTeX pages, 6 eps figures. Submitted to PRB on 15 June 2005. Accepted on 13 August 200

Interaction of oxygen with TiN (001): N↔O exchange and oxidation process

This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450K. At the lowest temperature, there was chemisorption of oxygen (O2,gas→2Oads) without significant surface oxidation. In contrast, at 450K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (ΔE=2.84eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2→TiOx+NO reaction is an important exit channel for nitrogen in the oxidation process.Ministerio de Educación y Ciencia de España, MEC. MAT2005-01872Junta de Andalucía. FQM-132División de Ciencias Químicas del Departamento de Energía de EE. UU. DE-AC02-98CH10086Fundación japonesa para la Ciencia de los Materiale

Antiferromagnetism in four dimensions: search for non-triviality

We present antiferromagnetism as a mechanism capable of modifying substantially the phase diagram and the critical behaviour of statistical mechanical models. This is particularly relevant in four dimensions, due to the connection between second order transition points and the continuum limit as a quantum field theory. We study three models with an antiferromagnetic interaction: the Ising and the O(4) Models with a second neighbour negative coupling, and the \RP{2} Model. Different conclusions are obtained depending on the model.Comment: 4 pages LateX. Contribution to Lat9

A theoretical insight into the catalytic effect of a mixed-metal oxide at the nanometer level: The case of the highly active metal/CeOx/TiO2(110) catalysts

The structural and electronic properties of CeOx species supported on the rutile TiO2 110 surface have been examined by means of periodic density-functional calculations that use a generalized gradient approximation functional including a Hubbard-like type correction. Deposition of Ce atoms leads in a first step to Ce3+ ions bound to the surface through bridge and in-plane oxygen atoms, the released electrons occupying the Ti 3d empty orbitals. Further addition of Ce and molecular oxygen gives place to Ce2O3 dimers diagonally arranged on the surface, in agreement with the spots observed in the scanning tunnel microscope images. The formation process of CeOx nanoparticles NPs on the TiO2 surface is highly exothermic and our calculations show that the redox properties of the Ce III-Ce IV couple are significantly altered when it is supported on TiO2. In particular the reactivity against CO/O2 indicates that on the surface the presence of Ce III is favored over Ce IV species. Our results also indicate that the CeOx /TiO2 interface should be seen like a real mixed-metal oxide rather than a supported NP of ceria. Finally, in the context of the high catalytic activity of the M /CeOx /TiO2 M =Au,Cu,Pt systems in the water-gas shift reaction, we have examined the dissociation of water on the CeOx /TiO2 surface and estimated a barrier as small as 0.04 eV, i.e. 8 times smaller than that computed for a TiO2 oxygen vacancy. This result agrees with the experimental superior catalytic activity of the M /CeOx /TiO2 systems over M /TiO2.Gobierno de España. Ministerio Ciencia e Innovación (MICINN) MAT2005-01872 y CSD2008-0023Junta de Andalucía FQM-132Departamento de Energía de EE. UU. División de Subvención de Ciencias Químicas DE-AC02-98CH1088

Magnetization Process of Single Molecule Magnets at Low Temperatures

We show that correlations established before quenching to very low temperatures, later drive the magnetization process of systems of single molecule magnets, after a magnetic field is applied at t=0. We also show that in SC lattices, m \propto sqrt(t), as observed in Fe_8, but only for 1+2*log_10(h_d/h_w) time decades, where h_d is a nearest neighbor dipolar magnetic field and a spin reversal can occur only if the field on it is within (-h_w,h_w). However, the sqrt(t) behavior is not universal. For BCC and FCC lattices, m \propto t^p, but p \simeq 0.7. The value to which m finally levels off is also given.Comment: 5 LaTeX pages, 2 figures. Submitted to Phys. Rev. Lett. on 26 January 2003, accepted on 24 July 200

New Universality Class in three dimensions: the Antiferromagnetic RP2RP^2 model

We present the results of a Monte Carlo simulation of the $RP^2$ model in three dimensions with negative coupling. We observe a second order phase transition between the disordered phase and an antiferromagnetic, unfrustrated, ordered one. We measure, with a Finite Size Scaling analysis, the thermal exponent, obtaining $\nu=0.784(8)$. We have found two magnetic-type relevant operators whose related $\eta$ exponents are $0.038(2)$ and $1.338(8)$ respectively.Comment: 10 pages, 2 Postscript figures. Revised version: references adde

'Where Have I Heard It?' Assessing the Recall of Traffic Safety Campaigns in the Dominican Republic

Although traffic crashes are the eighth leading cause of death in the world, and are linked to vehicle and infrastructure-related factors, crash-related fatality rates are much higher in low-income countries. Particularly, the Dominican Republic is the country with the highest accident rate in the whole American continent. Therefore, in the past few years, public agencies have been developing different measures aimed at reducing traffic fatalities, including road safety campaigns. The aim of the present study was to assess the recalling of such campaigns among the Dominican population, which may serve as an additional indicator to evaluate their effectiveness in this and other countries of the region presenting similar traffic safety issues. For this cross-sectional study, a nationwide sample composed of 1260 people (50% males and 50% females) with a mean age of 39.3 years was used. The data were collected through personal interviews. Overall, the recall of traffic safety campaigns was found to be very low (9%); male drivers who were employed, possessed a driver's license and habitually drove were the ones who could commonly remember these campaigns. The results of this study suggest that further evaluation and follow-up could help to maximize the impact of future traffic campaigns and advertisements in the Dominican Republic, as well as in other emerging countries of the region with similar characteristics. Further, key segments of the population such as the female, young, less formally educated and non-driving populations should be also targeted for further actions in this regar

Sport education model versus traditional model: Effects on motivation and sportsmanship

El objetivo de este trabajo fue comparar los efectos que provocan los modelos de enseñanza Tradicional y de Educación Deportiva en la motivación y deportividad del alumnado de educación física, incluyendo también un tercer nivel de tratamiento, variante del modelo de Educación Deportiva, en el que se empleó material autoconstruido. Se realizó un diseño cuasi-experimental con tres niveles de tratamiento y medidas pretest y postest de cuatro variables dependientes: metas de logro, metas de amistad, necesidades psicológicas básicas y la deportividad durante la enseñanza del ultimate. Accedieron a participar un total de 295 estudiantes de secundaria y bachillerato cuyas edades oscilaban entre los 12-17 años. Los resultados indicaron mejoras significativas en las metas de evitación del rendimiento, metas de amistad, necesidades psicológicas básicas y subescalas de deportividad en ambos grupos de Educación DeportivaThe goal of the present project was to compare the effects of three different instructional models: (Traditional, Sport Education and Sport Education with Self-Made Materials) on physical education students' motivation and sportsmanship. A quasi-experimental design with three levels of treatment was used. Pretest and posttest scores on achievement goals, friendship, psychological needs and sportsmanship were obtained after the application of a learning unit of ultimate in a school in the northern part of Spain. A total of 295 secondary school students, ages 12-17, agreed to participate. Results showed significant increases in performance-avoidance goals, friendship goals, basic psychological needs and sportsmanship in both Sport Education groups. Sport Education seems to offer more advantages than the traditional teaching method to develop adolescents achievement and social goals, as well as sportsmanship, in physical educatio