5,913 research outputs found
Spontaneous and Stimulated Raman Scattering near Metal Nanostructures in the Ultrafast, High-Intensity regime
The inclusion of atomic inversion in Raman scattering can significantly alter
field dynamics in plasmonic settings. Our calculations show that large local
fields and femtosecond pulses combine to yield: (i) population inversion within
hot spots; (ii) gain saturation; and (iii) conversion efficiencies
characterized by a switch-like transition to the stimulated regime that spans
twelve orders of magnitude. While in Raman scattering atomic inversion is
usually neglected, we demonstrate that in some circumstances full accounting of
the dynamics of the Bloch vector is required
Strain controlled oxygen vacancy formation and ordering in CaMnO
We use first-principles calculations to investigate the stability of
bi-axially strained \textit{Pnma} perovskite CaMnO towards the formation of
oxygen vacancies. Our motivation is provided by promising indications that
novel material properties can be engineered by application of strain through
coherent heteroepitaxy in thin films. While it is usually assumed that such
epitaxial strain is accommodated primarily by changes in intrinsic lattice
constants, point defect formation is also a likely strain relaxation mechanism.
This is particularly true at the large strain magnitudes (4%) which
first-principles calculations often suggest are required to induce new
functionalities. We find a strong dependence of oxygen vacancy defect formation
energy on strain, with tensile strain lowering the formation energy consistent
with the increasing molar volume with increasing oxygen deficiency. In
addition, we find that strain differentiates the formation energy for different
lattice sites, suggesting its use as a route to engineering vacancy ordering in
epitaxial thin films.Comment: 7 pages, 7 figure
Comment on "Bosons as the origin for giant magnetic properties of organic monolayers"
© 2006 The American Physical SocietyDepto. de FÃsica de MaterialesFac. de Ciencias FÃsicasTRUEpu
Graphene-based perfect optical absorbers harnessing guided mode resonances
We numerically and experimentally investigate graphene-based optical
absorbers that exploit guided mode resonances (GMRs) achieving perfect
absorption over a bandwidth of few nanometers (over the visible and
near-infrared ranges) with a 40-fold increase of the monolayer graphene
absorption. We analyze the influence of the geometrical parameters on the
absorption rate and the angular response for oblique incidence. Finally, we
experimentally verify the theoretical predictions in a one-dimensional,
dielectric grating and placing it near either a metallic or a dielectric
mirror
Graphene-based absorber exploiting guided mode resonances in one-dimensional gratings
A one-dimensional dielectric grating, based on a simple geometry, is proposed
and investigated to enhance light absorption in a monolayer graphene exploiting
guided mode resonances. Numerical findings reveal that the optimized
configuration is able to absorb up to 60% of the impinging light at normal
incidence for both TE and TM polarizations resulting in a theoretical
enhancement factor of about 26 with respect to the monolayer graphene
absorption (about 2.3%). Experimental results confirm this behaviour showing
CVD graphene absorbance peaks up to about 40% over narrow bands of few
nanometers. The simple and flexible design paves the way for the realization of
innovative, scalable and easy-to-fabricate graphene-based optical absorbers
Cosmologies with a time dependent vacuum
The idea that the cosmological term, Lambda, should be a time dependent
quantity in cosmology is a most natural one. It is difficult to conceive an
expanding universe with a strictly constant vacuum energy density, namely one
that has remained immutable since the origin of time. A smoothly evolving
vacuum energy density that inherits its time-dependence from cosmological
functions, such as the Hubble rate or the scale factor, is not only a
qualitatively more plausible and intuitive idea, but is also suggested by
fundamental physics, in particular by quantum field theory (QFT) in curved
space-time. To implement this notion, is not strictly necessary to resort to ad
hoc scalar fields, as usually done in the literature (e.g. in quintessence
formulations and the like). A "running" Lambda term can be expected on very
similar grounds as one expects (and observes) the running of couplings and
masses with a physical energy scale in QFT. Furthermore, the experimental
evidence that the equation of state of the dark energy could be evolving with
time/redshift (including the possibility that it might currently behave
phantom-like) suggests that a time-variable Lambda term (possibly accompanied
by a variable Newton's gravitational coupling G=G(t)) could account in a
natural way for all these features. Remarkably enough, a class of these models
(the "new cosmon") could even be the clue for solving the old cosmological
constant problem, including the coincidence problem.Comment: LaTeX, 15 pages, 4 figure
Susceptibility to Infection and Immune Response in Insular and Continental Populations of Egyptian Vulture: Implications for Conservation
BACKGROUND:A generalized decline in populations of Old World avian scavengers is occurring on a global scale. The main cause of the observed crisis in continental populations of these birds should be looked for in the interaction between two factors -- changes in livestock management, including the increased use of pharmaceutical products, and disease. Insular vertebrates seem to be especially susceptible to diseases induced by the arrival of exotic pathogens, a process often favored by human activities, and sedentary and highly dense insular scavengers populations may be thus especially exposed to infection by such pathogens. Here, we compare pathogen prevalence and immune response in insular and continental populations of the globally endangered Egyptian vulture under similar livestock management scenarios, but with different ecological and evolutionary perspectives. METHODS/PRINCIPAL FINDINGS:Adult, immature, and fledgling vultures from the Canary Islands and the Iberian Peninsula were sampled to determine a) the prevalence of seven pathogen taxa and b) their immunocompetence, as measured by monitoring techniques (white blood cells counts and immunoglobulins). In the Canarian population, pathogen prevalence was higher and, in addition, an association among pathogens was apparent, contrary to the situation detected in continental populations. Despite that, insular fledglings showed lower leukocyte profiles than continental birds and Canarian fledglings infected by Chlamydophila psittaci showed poorer cellular immune response. CONCLUSIONS/SIGNIFICANCE:A combination of environmental and ecological factors may contribute to explain the high susceptibility to infection found in insular vultures. The scenario described here may be similar in other insular systems where populations of carrion-eaters are in strong decline and are seriously threatened. Higher susceptibility to infection may be a further factor contributing decisively to the extinction of island scavengers in the present context of global change and increasing numbers of emerging infectious diseases
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