598 research outputs found
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
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
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 econĂ´mica agrĂcola e responsabilidade ambiental em unidades rurais de produção orgânica e convencional em Mundo Novo, MS.
bitstream/item/69064/1/099-recalde-viabilidade.pdfPublicado tambĂŠm no Cadernos de Agroecologia, v. 7, n.2, 2012
Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes
The gap oscillations caused by a magnetic flux penetrating a carbon nanotube
represent one of the most spectacular observation of the Aharonov-Bohm effect
at the nano--scale. Our understanding of this effect is, however, based on the
assumption that the electrons are strictly confined on the tube surface, on
trajectories that are not modified by curvature effects. Using an ab-initio
approach based on Density Functional Theory we show that this assumption fails
at the nano-scale inducing important corrections to the physics of the
Aharonov-Bohm effect. Curvature effects and electronic density spilled out of
the nanotube surface are shown to break the periodicity of the gap
oscillations. We predict the key phenomenological features of this anomalous
Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of
a large metallic phase in the low flux regime of Multi-walled nanotubes, also
suggesting possible experiments to validate our results.Comment: 7 figure
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