Hamilton's Principle as Variational Inequality forMechanical Systems with Impact

The classical form of Hamilton's principle holds for conservative systems with perfect bilateral constraints. Several attempts have been made in literature to generalise Hamilton's principle for mechanical systems with perfect unilateral constraints involving impulsive motion. This has led to a number of different variants of Hamilton's principle, some expressed as variational inequalities. Up to now, the connection between these different principles has been missing. The aim of this paper is to put these different principles of Hamilton in a unified framework by using the concept of weak and strong extrema. The difference between weak and strong variations of the motion is explained in detail. Each type of variation leads to a variant of the principle of Hamilton in the form of a variational inequality. The conclusion of the paper is that each type of variation leads to different necessary and sufficient conditions on the impact law. The principle of Hamilton with strong variations is valid for perfect unilateral constraints with a completely elastic impact law, whereas the weak form of Hamilton's principle only requires perfect unilateral constraints and no condition on the energ

Microscopic non-equilibrium theory of quantum well solar cells

We present a microscopic theory of bipolar quantum well structures in the photovoltaic regime, based on the non-equilibrium Green's function formalism for a multi band tight binding Hamiltonian. The quantum kinetic equations for the single particle Green's functions of electrons and holes are self-consistently coupled to Poisson's equation, including inter-carrier scattering on the Hartree level. Relaxation and broadening mechanisms are considered by the inclusion of acoustic and optical electron-phonon interaction in a self consistent Born approximation of the scattering self energies. Photogeneration of carriers is described on the same level in terms of a self energy derived from the standard dipole approximation of the electron-photon interaction. Results from a simple two band model are shown for the local density of states, spectral response, current spectrum, and current-voltage characteristics for generic single quantum well systems.Comment: 10 pages, 6 figures; corrected typos, changed caption Fig. 1, replaced Fig.

Robust fitting for generalized additive models for location, scale and shape

The validity of estimation and smoothing parameter selection for the wide class of generalized additive models for location, scale and shape (GAMLSS) relies on the correct specification of a likelihood function. Deviations from such assumption are known to mislead any likelihood-based inference and can hinder penalization schemes meant to ensure some degree of smoothness for nonlinear effects. We propose a general approach to achieve robustness in fitting GAMLSSs by limiting the contribution of observations with low log-likelihood values. Robust selection of the smoothing parameters can be carried out either by minimizing information criteria that naturally arise from the robustified likelihood or via an extended Fellner–Schall method. The latter allows for automatic smoothing parameter selection and is particularly advantageous in applications with multiple smoothing parameters. We also address the challenge of tuning robust estimators for models with nonlinear effects by proposing a novel median downweighting proportion criterion. This enables a fair comparison with existing robust estimators for the special case of generalized additive models, where our estimator competes favorably. The overall good performance of our proposal is illustrated by further simulations in the GAMLSS setting and by an application to functional magnetic resonance brain imaging using bivariate smoothing splines

Theory and simulation of photogeneration and transport in Si-SiOx superlattice absorbers

Si-SiOx superlattices are among the candidates that have been proposed as high band gap absorber material in all-Si tandem solar cell devices. Owing to the large potential barriers for photoexited charge carriers, transport in these devices is restricted to quantum-confined superlattice states. As a consequence of the finite number of wells and large built-in fields, the electronic spectrum can deviate considerably from the minibands of a regular superlattice. In this article, a quantum-kinetic theory based on the non-equilibrium Green's function formalism for an effective mass Hamiltonian is used for investigating photogeneration and transport in such devices for arbitrary geometry and operating conditions. By including the coupling of electrons to both photons and phonons, the theory is able to provide a microscopic picture of indirect generation, carrier relaxation, and inter-well transport mechanisms beyond the ballistic regime

Theory and simulation of quantum photovoltaic devices based on the non-equilibrium Green's function formalism

This article reviews the application of the non-equilibrium Green's function formalism to the simulation of novel photovoltaic devices utilizing quantum confinement effects in low dimensional absorber structures. It covers well-known aspects of the fundamental NEGF theory for a system of interacting electrons, photons and phonons with relevance for the simulation of optoelectronic devices and introduces at the same time new approaches to the theoretical description of the elementary processes of photovoltaic device operation, such as photogeneration via coherent excitonic absorption, phonon-mediated indirect optical transitions or non-radiative recombination via defect states. While the description of the theoretical framework is kept as general as possible, two specific prototypical quantum photovoltaic devices, a single quantum well photodiode and a silicon-oxide based superlattice absorber, are used to illustrated the kind of unique insight that numerical simulations based on the theory are able to provide.Comment: 20 pages, 10 figures; invited review pape

Robust Fitting for Generalized Additive Models for Location, Scale and Shape

The validity of estimation and smoothing parameter selection for the wide class of generalized additive models for location, scale and shape (GAMLSS) relies on the correct specification of a likelihood function. Deviations from such assumption are known to mislead any likelihood-based inference and can hinder penalization schemes meant to ensure some degree of smoothness for non-linear effects. We propose a general approach to achieve robustness in fitting GAMLSSs by limiting the contribution of observations with low log-likelihood values. Robust selection of the smoothing parameters can be carried out either by minimizing information criteria that naturally arise from the robustified likelihood or via an extended Fellner-Schall method. The latter allows for automatic smoothing parameter selection and is particularly advantageous in applications with multiple smoothing parameters. We also address the challenge of tuning robust estimators for models with non-linear effects by proposing a novel median downweighting proportion criterion. This enables a fair comparison with existing robust estimators for the special case of generalized additive models, where our estimator competes favorably. The overall good performance of our proposal is illustrated by further simulations in the GAMLSS setting and by an application to functional magnetic resonance brain imaging using bivariate smoothing splines

SU(2) symmetry in a Hubbard model with spin-orbit coupling

We study the underlying symmetry in a spin-orbit coupled tight-binding model with Hubbard interaction. It is shown that, in the absence of the on-site interaction, the system possesses the SU(2) symmetry arising from the timereversal symmetry. The influence of the on-site interaction on the symmetry depends on the topology of the networks: The SU(2) symmetry is shown to be the spin rotation symmetry of a simply-connected lattice, so it still holds in the presence of the Hubbard correlation. In contrary, the on-site interaction breaks the SU(2) symmetry of a multi-connected lattice.Comment: 5 pages, 2 figure

Sugerencias para introducir los biocombustibles en la norma nacional vigente para la determinación del poder calorífico

La energía que realmente podrá utilizarse de la biomasa, depende de parámetros concretos y de fácil medida, tales como el poder calorífico y la humedad. Una correcta elección de la técnica para determinar estas variables, hace fiables sus mediciones. En este trabajo se hace una revisión sobre calorímetros, se presenta la metodología para determinar el poder calorífico de biocombustibles, según la técnica definida por la norma IRAM 17016, haciéndose sugerencias para aclarar algunos conceptos. Después de varios ensayos realizados siguiendo el procedimiento establecido en la norma, se encontraron ciertas dificultades y carencias, por lo que resultó necesario proponer una modificación y actualización. Entre los inconvenientes surgidos durante las experiencias, se mencionan: una insuficiente descripción y escasa información gráfica, el sistema de medición de temperaturas no es claro y falta agregar esquemas del calorímetro con cada una de sus partes, entre otros. Por último, se comparó la Norma IRAM 17016 con últimas normas europeas con el fin de proponer una actualización que facilite la utilización y aprovechamiento de la misma.The energy that actually be used from biomass, depends on specific parameters and easy to measure, such as the calorific power and moisture content. A correct choice of technique for determining these variables makes its reliable measurements. This paper presents a review of calorimeters, and a methodology is presented to determine the heating value of biofuels according to the technique defined by the IRAM 17016 standard, making suggestions to clarify some concepts. After several trials following the procedure laid down in the standard, certain difficulties and shortcomings were found, so it was necessary to propose a modification and updating. Among the problems encountered during the experiments, are mentioned: insufficient description and graphical information sparse, the temperature measurement system is unclear and need to add calorimeter schemes with each of its parts, among other problems. Finally, the IRAM 17016 was compared to latest European standards in order to propose an update to facilitate the use and exploitation of it.Tema 6: Energía eólica, geotermia, biomasa y otras energías no convencionales.Facultad de Arquitectura y Urbanism

Sugerencias para introducir los biocombustibles en la norma nacional vigente para la determinación del poder calorífico

La energía que realmente podrá utilizarse de la biomasa, depende de parámetros concretos y de fácil medida, tales como el poder calorífico y la humedad. Una correcta elección de la técnica para determinar estas variables, hace fiables sus mediciones. En este trabajo se hace una revisión sobre calorímetros, se presenta la metodología para determinar el poder calorífico de biocombustibles, según la técnica definida por la norma IRAM 17016, haciéndose sugerencias para aclarar algunos conceptos. Después de varios ensayos realizados siguiendo el procedimiento establecido en la norma, se encontraron ciertas dificultades y carencias, por lo que resultó necesario proponer una modificación y actualización. Entre los inconvenientes surgidos durante las experiencias, se mencionan: una insuficiente descripción y escasa información gráfica, el sistema de medición de temperaturas no es claro y falta agregar esquemas del calorímetro con cada una de sus partes, entre otros. Por último, se comparó la Norma IRAM 17016 con últimas normas europeas con el fin de proponer una actualización que facilite la utilización y aprovechamiento de la misma.The energy that actually be used from biomass, depends on specific parameters and easy to measure, such as the calorific power and moisture content. A correct choice of technique for determining these variables makes its reliable measurements. This paper presents a review of calorimeters, and a methodology is presented to determine the heating value of biofuels according to the technique defined by the IRAM 17016 standard, making suggestions to clarify some concepts. After several trials following the procedure laid down in the standard, certain difficulties and shortcomings were found, so it was necessary to propose a modification and updating. Among the problems encountered during the experiments, are mentioned: insufficient description and graphical information sparse, the temperature measurement system is unclear and need to add calorimeter schemes with each of its parts, among other problems. Finally, the IRAM 17016 was compared to latest European standards in order to propose an update to facilitate the use and exploitation of it.Tema 6: Energía eólica, geotermia, biomasa y otras energías no convencionales.Facultad de Arquitectura y Urbanism