603 research outputs found
Non-equilibrium Bethe-Salpeter equation for transient photo-absorption spectroscopy
In this work we propose an accurate first-principle approach to calculate the
transient photo--absorption spectrum measured in Pump\&\,Probe experiments. We
formulate a condition of {\em adiabaticity} and thoroughly analyze the
simplifications brought about by the fulfillment of this condition in the
non--equilibrium Green's function (NEGF) framework. Starting from the
Kadanoff-Baym equations we derive a non--equilibrium Bethe--Salpeter equation
(BSE) for the response function that can be implemented in most of the already
existing {\em ab--initio} codes. In addition, the {\em adiabatic} approximation
is benchmarked against full NEGF simulations in simple model hamiltonians, even
under extreme, nonadiabatic conditions where it is expected to fail. We find
that the non--equilibrium BSE is very robust and captures important spectral
features in a wide range of experimental configurations.Comment: 13 pages, 5 captioned figure
First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra
We show that any {\em quasi-particle} or GW approximation to the self-energy
does not capture excitonic features in time-resolved (TR) photoemission
spectroscopy. In this work we put forward a first-principles approach and
propose a feasible diagrammatic approximation to solve this problem. We also
derive an alternative formula for the TR photocurrent which involves a single
time-integral of the lesser Green's function. The diagrammatic approximation
applies to the {\em relaxed} regime characterized by the presence of
quasi-stationary excitons and vanishing polarization. The main distinctive
feature of the theory is that the diagrams must be evaluated using {\em
excited} Green's functions. As this is not standard the analytic derivation is
presented in detail. The final result is an expression for the lesser Green's
function in terms of quantities that can all be calculated {\em ab initio}. The
validity of the proposed theory is illustrated in a one-dimensional model
system with a direct gap. We discuss possible scenarios and highlight some
universal features of the exciton peaks. Our results indicate that the exciton
dispersion can be observed in TR {\em and} angle-resolved photoemission.Comment: 15 pages, 8 figure
The method of mothers for non-overlapping non-matching DDM
In this paper we introduce a variant of the three-field formulation
where we use only two sets of variables. Considering, to fix the ideas, the
homogeneous Dirichlet problem for the Laplace operator in a bounded domain,
our variables are: 1) an approximation of the solution on the skeleton (the
union of the interfaces of the sub-domains) on an independent grid (that could
often be uniform), and 2) the approximations of the solution in each
sub-domain, each on its own grid. The novelty is in the way to derive, from the
approximation on the skeleton, the values of each trace of the approximations
in the subdomains. We do it by solving an auxiliary problem, that resembles the
mortar method but is more flexible. Under suitable assumptions, quasi-optimal
error estimates are proved, uniformly with respect to the number and size of
the subdomains
Spline Upwind for space--time Isogeometric Analysis of cardiac electrophysiology
We present an elaboration and application of Spline Upwind (SU) stabilization
method, designed in space--time Isogeometric Analysis framework, in order to
make this stabilization as suitable as possible in the context of cardiac
electrophysiology. Our aim is to propose a formulation as simple and efficient
as possible, effectual in preventing spurious oscillations present in plain
Galerkin method and also reasonable from the computational cost point of view.
For these reasons we validate the method's capability with numerical
experiments, focusing on accuracy and computational aspects
A parallel multigrid solver for multi-patch Isogeometric Analysis
Isogeometric Analysis (IgA) is a framework for setting up spline-based
discretizations of partial differential equations, which has been introduced
around a decade ago and has gained much attention since then. If large spline
degrees are considered, one obtains the approximation power of a high-order
method, but the number of degrees of freedom behaves like for a low-order
method. One important ingredient to use a discretization with large spline
degree, is a robust and preferably parallelizable solver. While numerical
evidence shows that multigrid solvers with standard smoothers (like Gauss
Seidel) does not perform well if the spline degree is increased, the multigrid
solvers proposed by the authors and their co-workers proved to behave optimal
both in the grid size and the spline degree. In the present paper, the authors
want to show that those solvers are parallelizable and that they scale well in
a parallel environment.Comment: The first author would like to thank the Austrian Science Fund (FWF)
for the financial support through the DK W1214-04, while the second author
was supported by the FWF grant NFN S117-0
Viabilidade econômica agrícola e responsabilidade ambiental em unidades rurais de produção orgânica e convencional em Mundo Novo, MS.
bitstream/item/69064/1/099-recalde-viabilidade.pdfPublicado também no Cadernos de Agroecologia, v. 7, n.2, 2012
Ecological factors affecting foraging behaviour during nestling rearing in a high-elevation species, the White-winged Snowfinch (Montifringilla nivalis)
During breeding, parents of avian species must increase their foraging efforts to collect food for their offspring, besides themselves. Foraging trips are thus a key aspect of the foraging ecology of central-place foragers when rearing their offspring. However, studies of the foraging ecology of high-elevation specialists inhabiting harsh environments are scarce. Here we report for the first time quantitative information on ecological determinants of foraging trips in the White-winged Snowfinch (Montifringilla nivalis), a high-elevation specialist threatened by climate warming. We focused on seasonal, meteorological, habitat and social factors affecting distance and duration of foraging trips performed during nestling rearing, recorded by visual observations in the Italian Alps. Based on 309 foraging trips from 35 pairs, we found that trips lasted 6.12 min and foraging areas were located at 175 m from the nest site on average. Trip duration was affected by snow cover (longer at intermediate cover), distance travelled and wind, while distance travelled was affected by snow cover (being higher at intermediate cover) and trip duration. Foraging individuals thus travelled farther and spent more time at areas characterized by intermediate snow cover, implying the presence of snow margins. It is likely that at such snow patches/margins snowfinches collected food for self-maintenance, besides that for their offspring, or collected more food items. Any reduction of snow cover during the breeding season, as expected under current climate warming, will severely alter foraging habitat suitability. Conserving suitable foraging habitats in the nest surroundings will be crucial to buffer such negative impacts
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