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One of the hallmark properties of fluids is their shear viscosity which is, among other things, responsible for parabolic flow profiles through narrow channels. In recent years, there has been a growing number of observations of said flow profiles in electronic transport measurements in a variety of material systems, most notably in graphene. In this paper, we investigate the shear viscosity of interacting graphene from a theoretical point of view. We study both a phenomenological as well as a microscopic model and find excellent agreement between the two. Our main finding is collective modes make a sizable contribution to the viscosity that can equal or even outweigh the electronic contribution that is usually assumed dominant. We comment on how this finding carries over to systems beyond graphene and related Dirac materials.

In this paper we study thermoelectric transport in interacting two-dimensional Dirac-type systems using a phenomenological Boltzmann approach. We consider a setup that can accommodate electrons, holes, and collective modes. In the first part of the paper we consider the electron-hole hydrodynamics, a model that is popular in the context of graphene, and its transport properties. In a second part, we propose a unique type of hydrodynamics. In that setup, the "fluid" consists of electrons, holes, and plasmons. We study its transport properties, especially the thermoelectric behavior. The results of this part can also be adapted to the study of a fluid consisting of electrons and phonons. This paper is accompanied by a technical paper [K. Pongsangangan et al., Phys. Rev. B 106, 205127 (2022)], in which we give a detailed derivation of the Boltzmann equations and the encoded conservation laws.

Mario Schulz analysiert schließlich die Komplexität des Kollektivs der deutsch-tschechischen und deutsch-slowakischen Historikerkommission. Um die Arbeits- und Wirkungsweise dieses transnationalen Akteurs mit Hilfe qualitativer Experteninterviews herauszuarbeiten, greift er auf Hansens Konzept der Multikollektivität zurück. Überzeugend wird belegt, dass eine zunächst methodisch notwenige Komplexitätserhöhung dazu dienen kann, kulturelle Komplexität in einem zweiten Schritt Verständnis fördernd zu senken. Die Perspektive Kollektivität erweist sich dabei erneut als viel versprechender Mittler einer komplexen Kulturwirklichkeit., The question how to describe complex collectives without unduly reducing their complexity gains in importance when dealing with transnational expert committees that are developing outputs with high societal relevance. This article takes the example of the German-Czech and German-Slovak historics commission to examine the mode of operation in such organizations and explains how complex collectives can produce robust solutions, that have not been possible in usual political interaction. Based on Hansen’s concept of collectives the author suggests a new model to describe the functioning of complex collectives that stays manageable while not falling into the trap of oversimplifying interactional dynamics.