Formation of magnetic skyrmions with tunable properties in PdFe bilayer deposited on Ir(111)

We perform an extensive study of the spin-configurations in a PdFe bilayer on Ir(111) in terms of ab initio and spin-model calculations. We use the spin-cluster expansion technique to obtain spin model parameters, and solve the Landau-Lifshitz-Gilbert equations at zero temperature. In particular, we focus on effects of layer relaxations and the evolution of the magnetic ground state in external magnetic field. In the absence of magnetic field, we find a spin-spiral ground state, while applying external magnetic field skyrmions are generated in the system. Based on energy calculations of frozen spin configurations with varying magnetic field we obtain excellent agreement for the phase boundaries with available experiments. We find that the wave length of spin-spirals and the diameter of skyrmions decrease with increasing inward Fe layer relaxation which is correlated with the increasing ratio of the nearest-neighbor Dzyaloshinskii-Moriya interaction and the isotropic exchange coupling, $D/J$. Our results also indicate that the applied field needed to stabilize the skyrmion lattice increases when the diameter of individual skyrmions decreases. Based on our observations, we suggest that the formation of the skyrmion lattice can be tuned by small structural modification of the thin film.Comment: 7 pages, 5 figures, 2 table

Tres miradas al conflicto colombo-venezolano

ResumenEste artículo ofrece tres perspectivas al conflicto colombo-venezolano: una comparación de los divergentes modelos de integración escogidos por los dos vecinos, un examen de sus posturas hacia Estados Unidos (EU), una de alineación activa y la otra antagónica, así como una clasificación de los flujos transfronterizos y de las instituciones que los obstaculizan y estimulan.Palabras clave: Integración, autonomía, flujos transfronterizos, regionalismo, inversión, propiedad intelectual, petropolítica

Template driven self-assembly of the pentacene structure on the Si(553)-Pb surface

The self-assembly of the pentacene molecules on the Pb-ordered Si(553) surface is studied with scanning tunneling microscopy and density functional theory methods. Within the surface coverage up to a single monolayer, pentacene was found to form two different chain-like structures. They vary in molecular density, but both exhibit long-range, one-dimensional ordering with longer molecular axes aligned along step edges. The low-density phase consists of single discontinuous molecular rows and adapts the template surface periodicity, while the high-density phase features triple molecular chains with the unit cell determined by the length of the pentacene molecules. Such an arrangement of the molecules is controlled by a subtle balance between molecule–molecule and molecule–substrate interactions

Competencias de gobernanza regional: Alianza del Pacífico-Mercosur y Unasur-OEA - estrategias colombianas

This essay analyzes two competitions on regional governance: the competition between the Pacific Alliance (PA) and Mercosur, and the competition between UNASUR and the OEA - exploring Colombian strategies. While the PA, combined with the cross-regionalism of its members, seemed to prevail in the competition with Mercosur to provide the preferred model of regional economic governance, Unasur not only lost the competition with the OEA for exercising the mandate of the security management in South America, but came under the threat of extinction. The Unasur-OEA competition –between two nested organizations– was much more intensive than the PA-Mercosur competition, attenuated with convergence efforts. While the Unasur-OEA competition emanated from a quest for South American autonomy, based on the rise of Brazil (up to 2013), the PA-Mercosur competition included institutional contestation by Colombia at Brazil´s bid for regional leadership.Este artículo de reflexión analiza dos competencias de gobernanza regional, la competencia entre la Alianza del Pacífico (AP) y el Mercosur, y la competencia entre la Unasur y la OEA, explorando las estrategias colombianas. Mientras la AP, combinada con el transregionalismo de sus miembros, parecía prevalecer en la competencia con el Mercosur para surtir el modelo privilegiado de gobernanza económica regional, la Unasur no solo perdió la competencia con la OEA por ejercer el mandato de la gestión de seguridad en América del Sur, sino quedó amenazada de extinción. La competencia Unasur-OEA – entre dos organizaciones anidadas– era mucho más intensa que la competencia AP-Mercosur, atenuada con esfuerzos de convergencia. Mientras la competencia Unasur-OEA emanaba de una búsqueda de autonomía suramericana, apoyada en el ascenso de Brasil (hasta 2013), la competencia AP-Mercosur abarcó la contestación institucional colombiana frente a su aspiración al liderazgo regional

Tres miradas al conflicto colombo-venezolano

Este artículo ofrece tres perspectivas al conflicto colombo-venezolano: una comparación de los divergentes modelos de integración escogidos por los dos vecinos, un examen de sus posturas hacia Estados Unidos (EU), una de alineación activa y la otra antagónica, así como una clasificación de los flujos transfronterizos y de las instituciones que los obstaculizan y estimulan.Palabras clave: Integración, autonomía, flujos transfronterizos, regionalismo, inversión, propiedad intelectual, petropolítica