The comfortable roller coaster -- on the shape of tracks with constant normal force

A particle that moves along a smooth track in a vertical plane is influenced by two forces: gravity and normal force. The force experienced by roller coaster riders is the normal force, so a natural question to ask is: what shape of the track gives a normal force of constant magnitude? Here we solve this problem. It turns out that the solution is related to the Kepler problem; the trajectories in velocity space are conic sections.Comment: 10 pages, 4 figure

CATÁLOGO DE LAS PLANTAS VASCULARES DE RÁQUIRA (BOYACÁ), FLORA ANDINA EN UN ENCLAVE SECO DE COLOMBIA

Increasing Andean ecosystem loss and its conservation priority reinforce the need for documenting Tropical Andean biodiversity. This research provides an updated list of vascular plant species from Ráquira in the Department of Boyacá, based on a database and herbarium review along with field collections between 2007 and 2010. Four hundred and twenty six vascular plant species were found, representing 284 genera and 99 families. Asteraceae was the most diverse family with 54 species and 36 genera, followed by Fabaceae (33 species, 21 genera) and Poaceae (33 species, 21 genera). Herbs were the most common life form followed by woody life forms (trees and shrubs). The greatest number of species was found in open lands and near river margins. Species such as Juncus ramboi were rare because of over harvesting, while other species such as Quercus humboldtii are classified as threatened in Colombia. Improving knowledge of vascular plants in Andean forests is essential, especially in dry ecosystems, which are classified as globally important and diverse biomes.La creciente pérdida de ecosistemas andinos y la importancia de su conservación indican la relevancia de efectuar esfuerzos encaminados a la documentación y publicación de la biodiversidad allí presente. Esta investigación provee un listado de las especies de plantas vasculares del municipio de Ráquira (Boyacá), mediante una revisión (consulta directa en herbarios y bases de datos) y levantamientos de información en campo (inventarios y colecciones botánicas), desarrollados durante el 2007 y el 2010. Se encontró un total de 426 especies vasculares, distribuidas en 284 géneros y 99 familias. Desde el punto de vista florístico la familia Asteraceae es la más diversificada con 54 especies distribuidas en 36 géneros, seguida por Fabaceae (33 especies, 21 géneros) y Poaceae (24 especies, 18 géneros). El hábito correspondiente a hierbas fue el más predominante, en términos de riqueza de especies, seguido por los hábitos leñosos (árboles y arbustos). El mayor número de especies se encontró en zonas abiertas y riberas de quebradas. Se registran especies que por su frecuente uso y aprovechamiento han comenzado a extinguirse localmente, como es el caso de Juncus ramboi (esparto) y otras que se encuentran en algún grado de amenaza como Quercus humboldtii (roble). Es importante ampliar el conocimiento sobre la flora vascular de los bosques andinos, en especial, de ecosistemas situados en enclaves secos, puesto que estos se clasifican como zonas de importancia en diversidad de especies de flora en el ámbito mundial

Tunneling and transmission resonances of a Dirac particle by a double barrier

We calculate the tunneling process of a Dirac particle across two square barriers separated a distance $d$, as well as the scattering by a double cusp barrier where the centers of the cusps are separated a distance larger than their screening lengths. Using the scattering matrix formalism, we obtain the transmission and reflection amplitudes for the scattering processes of both configurations. We show that, the presence of transmission resonances modifies the Lorentizian shape of the energy resonances and induces the appearance of additional maxima in the transmission coefficient in the range of energies where transmission resonances occur. We calculate the Wigner time-delay and show how their maxima depend on the position of the transmission resonance.Comment: To appear in Physica Script

Inertial domain wall characterization in layered multisublattice antiferromagnets

The motion of a N\'eel-like ${180}^{\circ}$ domain wall induced by a time-dependent staggered spin-orbit field in the layered collinear antiferromagnet Mn$_2$Au is explored. Through an effective version of the two sublattice nonlinear $\sigma$-model which does not take into account the antiferromagnetic exchange interaction directed along the tetragonal c-axis, it is possible to replicate accurately the relativistic and inertial traces intrinsic to the magnetic texture dynamics obtained through atomistic spin dynamics simulations for quasistatic processes. In the case in which the steady-state magnetic soliton motion is extinguished due to the abrupt shutdown of the external stimulus, its stored relativistic exchange energy is transformed into a complex translational mobility, being the rigid domain wall profile approximation no longer suitable. Although it is not feasible to carry out a detailed follow-up of its temporal evolution in this case, it is possible to predict the inertial-based distance travelled by the domain wall in relation to its steady-state relativistic mass. This exhaustive dynamical characterization for different time-dependent regimes of the driving force is of potential interest in antiferromagnetic domain wall-based device applications

Monte Carlo technique with a quantified time step: Application to the motion of magnetic moments

The viability of the time quantified Metropolis Monte Carlo technique to describe the dynamics of magnetic systems is discussed. Similar to standard Brownian motion, the method is introduced basing on the comparison between the Monte Carlo trial step and the mean squared deviation of the direction of the magnetic moment. The Brownian dynamics approach to the time evolution of a magnetic moment is investigated and expressions for the mean square deviations are obtained. However, the principle difference between the standard Brownian motion and the magnetic moments dynamics is the presence of the spin precession which constitutes the reversible part of the dynamics. Although some part of the precession contributes to the diffusion coefficient, it also gives rise to athermal, energy conserving motion which cannot be taken into account by Monte Carlo methods. It is found that the stochastic motion of a magnetic moment falls into one of two possible regimes: (i) precession dominated motion, (ii) nonprecessional motion, according to the value of the damping constant and anisotropy strength and orientation. Simple expressions for the diffusion coefficient can be obtained in both cases for diffusion dominated motion, i.e., where the athermal precessional contribution can be neglected. These simple expressions are used to convert the Monte Carlo steps to real time units. The switching time for magnetic particles obtained by the Monte Carlo with time quantification is compared with the numerical integration of the Landau-Lifshitz-Gilbert equations with thermal field contribution and with some well known asymptotic formulas

Extremely broadband ultralight thermally emissive metasurfaces

We report the design, fabrication and characterization of ultralight highly emissive metaphotonic structures with record-low mass/area that emit thermal radiation efficiently over a broad spectral (2 to 35 microns) and angular (0-60 degrees) range. The structures comprise one to three pairs of alternating nanometer-scale metallic and dielectric layers, and have measured effective 300 K hemispherical emissivities of 0.7 to 0.9. To our knowledge, these structures, which are all subwavelength in thickness are the lightest reported metasurfaces with comparable infrared emissivity. The superior optical properties, together with their mechanical flexibility, low outgassing, and low areal mass, suggest that these metasurfaces are candidates for thermal management in applications demanding of ultralight flexible structures, including aerospace applications, ultralight photovoltaics, lightweight flexible electronics, and textiles for thermal insulation

Shape-invariant quantum Hamiltonian with position-dependent effective mass through second order supersymmetry

Second order supersymmetric approach is taken to the system describing motion of a quantum particle in a potential endowed with position-dependent effective mass. It is shown that the intertwining relations between second order partner Hamiltonians may be exploited to obtain a simple shape-invariant condition. Indeed a novel relation between potential and mass functions is derived, which leads to a class of exactly solvable model. As an illustration of our procedure, two examples are given for which one obtains whole spectra algebraically. Both shape-invariant potentials exhibit harmonic-oscillator-like or singular-oscillator-like spectra depending on the values of the shape-invariant parameter.Comment: 16 pages, 5 figs; Present e-mail of AG: [email protected]

Acacia farnesiana (L.) Willd. (Fabaceae: Leguminosae), una especie exótica con potencial invasivo en los bosques secos de la isla de Providencia (Colombia

This article reports Acacia farnesiana as a potential invasive alien species in the Providence Island, Colombia. According to the evaluation of the establishment risk, the potential of invasion and the control feasibility, based on I3N methodology, this species has been defined as a high-risk invasive plant to the biodiversity (6,97), it fulfills more than 86 % of the risk factor variables, which have been using in this methodology, to measure those above mentioned. Therefore, eradication and control should start immediately in the island, in order to mitigate latent damage and future greater consequences that have been showed in other Caribbean islands and continental ecosystems. Finally, we suggest some recommendations to eradicate and mitigate the effects of the potential invasion.Este artículo reporta Acacia farnesiana como una especie con potencial invasor en la isla de Providencia, Colombia. De acuerdo con la evaluación del riesgo de establecimiento, del potencial invasor y de la factibilidad de control, propuestas en la metodología I3N, esta especie ha sido definida como una invasora de alto riesgo (6,97) para la biodiversidad en la isla, cumpliendo con más del 80% de las variables de riesgo establecidas en esta propuesta. Por lo tanto, es necesario emprender acciones inmediatas que permitan su control y futura erradicación, con el fin de mitigar los efectos latentes y las consecuencias identificadas actualmente en otras islas del Caribe y ecosistemas continentales. Finalmente, se presentan recomendaciones para erradicar y mitigar los efectos de una posible invasión en la isla

Agenda de investigación y monitoreo en bosques secos de Colombia (2013-2015): fortaleciendo redes de colaboración para su gestión integral en el territorio

Tropical dry forests are the most threatened, yet among the least studied ecosystems in the Neotropics. These forests hold a unique combination of plants, animals and microorganisms that have adapted to the rain seasonality that characterizes tropical dry forest, thus holding many endemic species. In addition, this ecosystem provides key environmental services such as water regulation, soil retention and carbon sequestration, which regulates local climate, water and nutrients availability. In Colombia, dry forests occur in six regions: the Caribbean, the inter-Andean valleys of Cauca and Magdalena, the Northeastern Andes, the Patía valley, and the Llanos (Orinoquia region). In this working paper we discuss the three lines of research that the Instituto Alexander von Humboldt has been leading (2013-2015) to enhance tropical dry forest research, longtermmonitoring, and sustainable management: i) distribution and conservation status, ii) composition, ecological dynamics and functioning, and iii) characterization and conservation of tropical dry forests associated biodiversity.El bosque seco tropical tiene una diversidad única de plantas, animales y microorganismos que se han adaptado a condiciones de estrés hídrico, por lo cual presenta altos niveles de endemismo. Presta además servicios fundamentales como la regulación hídrica, la retención de suelos y la captura de carbono que regula el clima y la disponibilidad de agua y nutrientes. En Colombia el bosque seco se encuentra en seis regiones biogeográficas: el Caribe, los valles interandinos del Cauca y el Magdalena, la región Norandina en Santander y Norte de Santander, el valle del río Patía y afloramientos rocosos en los departamentos de Arauca y Vichada en los Llanos Orientales. Pese a su importancia para la conservación de la biodiversidad y los beneficios derivados para las sociedades humanas, este ecosistema es considerado el más amenazado del neotrópico y cuenta con muy poca información disponible, producto de la investigación científica. Bajo este contexto, el Instituto Humboldt, bajo el liderazgo de los Programas de Ciencias de la Biodiversidad y de Gestión Territorial, ha liderado la consolidación de una Agenda de Investigación y Monitoreo a escala de país para fortalecer la gestión integral de este ecosistema a partir del conocimiento científico. En este documento de trabajo se analizan las tres líneas de investigación que se han venido liderando desde 2013 hasta 2015 con el objetivo de fomentar la investigación, el monitoreo a largo plazo y un manejo integral del bosque seco en Colombia: i) distribución espacial y estado de conservación, ii) composición, dinámica y funcionamiento, y iii) caracterización y conservación de la biodiversidad asociada

X-ray emission and dynamics from large diameter superbubbles: The case of N 70 superbubble

The morphology, dynamics and thermal X-ray emission of the superbubble N70 is studied by means of 3D hydrodynamical simulations, carried out with the {\sc{yguaz\'u-a}} code. We have considered different scenarios: the superbubble being the product of a single supernova remnant, of the stellar winds from an OB association, or the result of the joint action of stellar winds and a supernova event. Our results show that, in spite that all scenarios produce bubbles with the observed physical size, only those where the bubble is driven by stellar winds and a SN event are successful to explain the general morphology, dynamics and the X-ray luminosity of N70. Our models predict temperatures in excess of $10^8 \mathrm{K}$ at the interior of the superbubble, however the density is too low and the emission in thermal X-ray above $2 \mathrm{keV}$ is too faint to be detected.Comment: 9 pages, 11 figures, ApJ accepte