General response function for interacting quantum liquids

Linearizing the appropriate kinetic equation we derive general response functions including selfconsistent mean fields or density functionals and collisional dissipative contributions. The latter ones are considered in relaxation time approximation conserving successively different balance equations. The effect of collisions is represented by correlation functions which are possible to calculate with the help of the finite temperature Lindhard RPA expression. The presented results are applicable to finite temperature response of interacting quantum systems if the quasiparticle or mean field energy is parameterized within Skyrme - type of functionals including density, current and energy dependencies which can be considered alternatively as density functionals. By this way we allow to share correlations between density functional and collisional dissipative contributions appropriate for the special treatment. We present results for collective modes like the plasmon in plasma systems and the giant resonance in nuclei. The collisions lead in general to an enhanced damping of collective modes. If the collision frequency is close to the frequency of the collective mode, resonance occurs and the collective mode is enhanced showing a collisional narrowing

A finite volume scheme for nonlinear parabolic equations derived from one-dimensional local Dirichlet problems

In this paper, we propose a new method to compute the numerical flux of a finite volume scheme, used for the approximation of the solution of parabolic partial differential equation with nonlinear diffusion and convection terms a 1D, 2D or 3D domain. The nonlinear diffusion term be bounded away from zero except a finite number of values. The method is based on the solution, at each interface between two control volumes, of a nonlinear elliptic two point boundary value problem derived from the original equation with Dirichlet boundary conditions given by the values of the discrete approximation in both control volumes. We prove the existence of a solution to this two point boundary value problem. We show that the expression for the numerical flux can be yielded without referring to this solution. Furthermore, we prove that the so designed finite volume scheme has the expected stability properties and that its solution converges to the weak solution of the continuous problem. Numerical results show the increase of accuracy due to the use of this scheme, compared to some other schemes

Momentum conservation and local field corrections for the response of interacting Fermi gases

We reanalyze the recently derived response function for interacting systems in relaxation time approximation respecting density, momentum and energy conservation. We find that momentum conservation leads exactly to the local field corrections for both cases respecting only density conservation and respecting density and energy conservation. This rewriting simplifies the former formulae dramatically. We discuss the small wave vector expansion and find that the response function shows a high frequency dependence of $\omega^{-5}$ which allows to fulfill higher order sum rules. The momentum conservation also resolves a puzzle about the conductivity which should only be finite in multicomponent systems

Dimensionalisierung, Gruppenbildung und Wahrscheinlichkeitsübergang: zur Identifizier- und Interpretierbarkeit multivariater Beziehungen zwischen Milieu, Beruf und Weiterbildung an SOEP-Daten

'Der Artikel behandelt Fragen der Milieu- und Bildungsbeteiligungsforschung primär unter methodischen Gesichtspunkten der Auswertung aus verschiedenen Erhebungen des SOEP (Erhebungen 1994 und 1999). Im Zentrum steht die Frage, in welcher Weise Milieuzugehörigkeit und Weiterbildungsbeteiligung aufeinander bezogen werden können. Die Rekonstruktion des Zusammenhangs erfolgt auf der Grundlage multivariater Verfahren der Objekt- und Variablenklassifikation. Gezeigt wird, dass mittels einer gezielten, auf die Analyse von Milieus zugeschnittenen Kombination solcher Verfahren die Unterscheidung milieuspezifischer von milieuunspezifischen Bestimmungsfaktoren der Weiterbildungsbeteiligung gelingen kann. Auf diese Weise ist es möglich, eine inhaltliche und methodische Schwäche der bildungsbezogenen Milieuforschung sichtbar zu machen: Nicht nur die innere Unterscheidbarkeit von Milieus, sondern auch die Unterscheidung von Milieu und Nicht- Milieu ist eine für die Einschätzung von Milieus als unabhängige Variable inhaltlich und methodisch erforderliche Operation.' (Autorenreferat)'The article deals with issues from milieu research and research on participation in education, primarily looking at methodological aspects in the analysis of various surveys from the SOEP (surveys 1994 and 1999). The central question is how milieu and participation in further education are related to each another. The relationship is reconstructed on the basis of multivariate procedures for classifying objects and variables. It will be shown that milieu-specific factors for participation in further education can be distinguished from milieu-non-specific factors, namely by systematically combining these multivariate procedures in such a way that the combination is tailored to the analysis of milieus. Thus a weakness regarding content and methodology can be made visible in milieu research focusing on education: in order to assess milieus as an independent variable, both matter and methodology require not only a differentiation between various types of milieus, but also between milieu and non-milieu.' (author's abstract)

Der schiefe Turm von PISA: die logistischen Parameter des Rasch-Modells sollten revidiert werden

'Die Kompetenz- und Anforderungsmessungen in den PISA-Studien beruhen auf dem logistischen Rasch-Modell, welches der probabilistischen Testtheorie zu Grunde liegt. Dieses Modell weist Schwächen auf. Wegen der logistischen item response Funktion lässt es Sätze von Antworten zu, die zwar legitim aber mit den vorgesehenen Parametern nicht auswertbar sind. Es handelt sich um die uniform beantworteten Fragebögen. Mit deren Sonderstellung hängt zusammen, dass die Schätzer für besonders hohe wie auch besonders niedrige Kompetenzen systematisch vom wahren Wert des Parameters abweichen und dass die Fehlerintervalle beliebig groß werden. Dies erschwert die Interpretation der Schätzer sowie die sozialwissenschaftliche Verwendung der Resultate - z.B. in Regressionsanalysen. Es sind aber gerade die oberen und unteren Kompetenzniveaus und Schwierigkeitsstufen, denen das besondere Interesse der Bildungsforschung und Bildungspolitik gilt. Dieses durchaus bekannte Problem wurde bislang formal nicht gelöst. Im vorliegenden Aufsatz wird gezeigt, dass man es lösen kann, indem man zu einer anderen - der trigonometrischen - item response Funktion übergeht.' (Autorenreferat)'The attainment of persons and the difficulties of items are measured via the logistic Rasch model which is the basis of probabilistic test theory. This model has a weakness. Due to the logistic item response function it admits sets of answers that - although they are legitimate - cannot be analyzed in terms of the parameters of the model. The authors refer to persons that uniformly answer all items either correctly or incorrectly. As a consequence, the estimators of especially high and especially low attainment are systematically biased and the error intervals can become indefinitely large. This obscures the interpretation of the estimators, and makes the results difficult to use in the social sciences. However, the special interest of research and politics is focussed on the upper and lower levels of attainment. The problem is known but has remained unsolved so far. The authors show that one can solve it within the binomial framework of the Rasch model by introducing a new - the so-called trigonometrical - item response function.' (author's abstract)

Self-heating effects in organic semiconductor devices enhanced by positive temperature feedback

We studied the influence of heating effects in an organic device containing a layer sequence of n-doped / intrinsic / n-doped C60 between crossbar metal electrodes. A strong positive feedback between current and temperature occurs at high current densities beyond 100 A/cm2, as predicted by the extended Gaussian disorder model (EGDM) applicable to organic semiconductors. These devices give a perfect setting for studying the heat transport at high power densities because C60 can withstand temperatures above 200ʿ C. Infrared images of the device and detailed numerical simulations of the heat transport demonstrate that the electrical circuit produces a superposition of a homogeneous power dissipation in the active volume and strong heat sources localized at the contact edges ..