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

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

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

Ab initio electronic structure, optical, and magneto-optical properties of MnGaAs digital ferromagnetic heterostructures

We report on a theoretical study of the electronic, optical, and magneto-optical properties of digital ferromagnetic heterostructures based on Mn delta-doped GaAs. We consider different structures corresponding to Mn contents within the range 12%-50% and we study how the system changes as a function of the doping concentration. Our first-principles approach includes the spin-orbit interaction in a fully relativistic pseudopotential scheme and the local-field effect in the description of the optical absorption. We show that Mn d-doped GaAs shares many properties with the uniformly doped Ga1-xMnxAs system, i.e., half-metallicity, similar absorption spectra, and moderate Kerr rotation angles in the visible spectral region