6,337 research outputs found
Nonsingular black holes in nonlinear gravity coupled to Euler-Heisenberg electrodynamics
We study static, spherically symmetric black holes supported by
Euler-Heisenberg theory of electrodynamics and coupled to two different
modified theories of gravity. Such theories are the quadratic model and
Eddington-inspired Born-Infeld gravity, both formulated in metric-affine
spaces, where metric and affine connection are independent fields. We find
exact solutions of the corresponding field equations in both cases,
characterized by mass, charge, the Euler-Heisenberg coupling parameter and the
modified gravity one. For each such family of solutions, we characterize its
horizon structure and the modifications in the innermost region, finding that
some subclasses are geodesically complete. The singularity regularization is
achieved under two different mechanisms: either the boundary of the manifold is
pushed to an infinite affine distance, not being able to be reached in finite
time by any geodesic, or the presence of a wormhole structure allows for the
smooth extension of all geodesics overcoming the maximum of the potential
barrier.Comment: 14 pages, 8 figures, revtex4-1 style. v2: some new discussion and
minor corrections. Version to appear in Phys. Rev.
Relaciones internacionales España-Cuba. ¿que sucedería tras el fin del bloqueo?
Màster Oficial d'Internacionalització, Facultat d'Economia i Empresa, Universitat de Barcelona, Curs: 2014-2015, Tutor: Susana GordilloEste trabajo analiza la evolución del comercio y las inversiones de España hacia Cuba
en diversos períodos de la apertura de la economía cubana. En la primera parte del
trabajo se detalla la evolución de las políticas y la situación económica de Cuba desde
la Revolución de 1959 hasta la actualidad, mientras que en la segunda parte se
analiza la evolución del comercio con Cuba y las inversiones de España en la isla,
observando las variaciones de ambas variables según el ritmo de las reformas
aplicadas.
La principal motivación del autor para realizar este trabajo reside en el interés en el
análisis económico como medio para entender las relaciones internacionales entre
países. Y entre todos los países que se podían elegir para realizar el estudio se optó
por Cuba debido a la especial relación que históricamente ha tenido España con la isla
caribeña. El hecho de que Cuba haya sido un país con un modelo económico especial
y crítico con el capitalismo global agregaba un elemento adicional a esta investigación,
cuyo objetivo principal apunta a entender cómo afectaría la apertura de Cuba y el
anunciado fin del bloqueo norteamericano en las relaciones económicas entre ambos
países..
Spontaneous mass generation and the small dimensions of the Standard Model gauge groups U(1), SU(2) and SU(3)
The gauge symmetry of the Standard Model is SU(3)_c x SU(2)_L x U(1)_Y for
unknown reasons. One aspect that can be addressed is the low dimensionality of
all its subgroups. Why not much larger groups like SU(7), or for that matter,
SP(38) or E7? We observe that fermions charged under large groups acquire much
bigger dynamical masses, all things being equal at a high e.g. GUT scale, than
ordinary quarks. Should such multicharged fermions exist, they are too heavy to
be observed today and have either decayed early on (if they couple to the rest
of the Standard Model) or become reliquial dark matter (if they don't). The
result follows from strong antiscreening of the running coupling for those
larger groups (with an appropriately small number of flavors) together with
scaling properties of the Dyson-Schwinger equation for the fermion mass.Comment: 15 pages, 17 plots. This version incorporates community as well as
referee comments. Accepted for publication in Nuclear Physics
Evaluation of reproductive sucess and conservation strategies for Senecio coincyi (Asteraceae), a narrow and threatened species
Senecio coincyi is a threatened endemic plant of central western Spain, with a very narrow extent of occurrence. The reproductive success and germination behaviour of this species were studied. The area of occupancy, habitat types and size of 13 known subpopulations of S. coincyi were evaluated. The number of individuals that form all these subpopulations was counted. In addition, the number of flowers and cypselas per fruit head and the number of fruit heads per individual plant were recorded in a subset of subpopulations. Germination tests were carried out to evaluate the effect of temperature and light regimes on, and possible intraspecific variation in, cypsela germination. Cypselas reached very high germination percentages (90?100%) from 15?C to 30?C. However, the germination decreased (19%) at 10?C. The light conditions assayed (16-h light photoperiod and complete darkness) did not significantly affect cypsela germination. In general, there was no intraspecific variability in germination patterns of S. coincyi cypselas. Livestock is the most important factor that can be a threat for this species. S. coincyi showed high reproductive success and, therefore, its conservation problems are not due to agents related to its reproduction, but rather to other factors such as alteration of its habitat caused by the presence of livestock
Recent advances to understand morphology stability of organic photovoltaics
Organic photovoltaic devices are on the verge of commercialization with power conversion efficiencies
exceeding 10 % in laboratory cells and above 8.5 % in modules. However, one of the main limitations hindering their mass
scale production is the debatable inferior stability of organic photovoltaic devices in comparison to other technologies.
Adequate donor/acceptor morphology of the active layer is required to provide carrier separation and transport to the
electrodes. Unfortunately, the beneficial morphology for device performance is usually a kinetically frozen state which has
not reached thermodynamic equilibrium. During the last 5 years, special efforts have been dedicated to isolate the effects
related to morphology changes taking place within the active layer and compare to those affecting the interfaces with the
external electrodes. The current review discusses some of the factors affecting the donor/acceptor morphology evolution as
one of the major intrinsic degradation pathways. Special attention is paid to factors in the nano- and microscale domain.
For example, phase segregation of the polymer and fullerene domains due to Ostwald ripening is a major factor in the
microscale domain and is affected by the presence of additives, glass transition temperature of the polymers or use of
crosslinkers in the active layer. Alternatively, the role of vertical segregation profile toward the external electrodes is key
for device operation, being a clear case of nanoscale morphology evolution. For example, donor and acceptor molecules
actually present at the external interfaces will determine the leakage current of the device, energy-level alignment, and
interfacial recombination processes. Different techniques have been developed over the last few years to understand its
relationship with the device efficiency. Of special interest are those techniques which enable in situ analysis being nondestructive
as they can be used to study accelerated degradation experiments and some will be discussed here.This work was partially supported by FP7
European collaborative project SUNFLOWER (FP7-ICT-2011-7-
contract No. 287594), the Spanish Ministerio de Economı´a y Competitividad
(project MAT2013-47192-C3-1-R), and Generalitat
Valenciana (project ISIC/2012/008 Institute of Nanotechnologies for
Clean Energies). A.G. would like to thank the Spanish Ministerio de
Economı´a y Competitividad for a Ramo´n y Cajal Fellowship (RYC-
2014-16809)
The gradual extinction of transferred avoidance stimulus functions
We investigated the transfer of conditioned avoidance functions through equivalence relations, and the extinction of these functions, facilitated by verbal prompts. Nine participants acquired three 4-member stimulus equivalence classes using a matching-to-sample procedure. One class stimulus was paired, by classical conditioning, with an aversive tone, which was used in avoidance training of a distinct response. There were two groups: A established the equivalence classes before avoidance training and vice versa for B. During some avoidance trials, each stimulus presentation was followed by the request for a verbal estimation of the probability of the tone. The last trials, run in extinction, included a verbal prompt to corroborate the provided estimation. One participant in each group received no verbal prompts. To negate the necessary reliance on instructions-governed performance, an additional participant completed the experiment with minimal instructions. All participants who had the equivalence training prior to the conditioning showed within-class transfer of avoidance functions, in contrast to the others. All prompted participants who demonstrated transfer showed gradual response extinction, but with a differential gradient. Responding decreased more sharply to the indirectly related stimuli than to the directly paired stimuli. The clinical implications are discussed
Evaluation of reproductive success in Senecio coincyi Rouy, a threatened species from Spain
Senecio coincyi Rouy (Asteraceae) is a threatened endemic species from the mountains of Sierra de Gredos, centraf western Spain. This species is protected by the Autonomous Community of Castile and León (Spain) and is included in the "In danger of extinction" category in the protected flora catalogue of that region. It was later catalogued as Vulnerable (VU) on the Red List of Threatened Spanish Vascular Flora (Moreno et al., 2008). In this paper, the results of a two-year investigation of S. coincyi population biology are shown. The aim of this work was to study several aspects of S. coincyi reproductive biology
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