ArgumentThe historiography of botanical maps has mainly concentrated on their alleged “golden age,” on maps drawn by famous first-generation plant geographers. This article instead describes botanical maps after the age of discovery, and detects both a quantitative explosion and qualitative modification in the late nineteenth century. By spotlighting the case of the plant geographer Oscar Drude (1852–1933), I argue that the dynamics of botanical mappings were closely linked to a specific milieu of knowledge production: the visual culture of Imperial (...) Germany. The scientific upgrading of maps was stimulated by a prospering commercial cartographical market as well as a widespread practice of mediating between professionals and amateurs via maps in the public sphere. In transferring skills and practices from these “popular” fields of knowledge to scientific domains, botanists like Oscar Drude established maps as an indispensable element of botanical observation. This wholesale dissemination of botanical maps had thus a formative influence on collective perception – the botanist's “period eye” – regarding plant distribution. (shrink)

Einstein's early thoughts about superconductivity are discussed as a case study of how theoretical physics reacts to experimental findings that are incompatible with established theoretical notions. One such notion that is discussed is the model of electric conductivity implied by Drude's electron theory of metals, and the derivation of the Wiedemann-Franz law within this framework. After summarizing the experimental knowledge on superconductivity around 1920, the topic is then discussed both on a phenomenological level in terms of implications of Maxwell's (...) equations for the case of infinite conductivity, and on a microscopic level in terms of suggested models for superconductive charge transport. Analyzing Einstein's manuscripts and correspondence as well as his own 1922 paper on the subject, it is shown that Einstein had a sustained interest in superconductivity and was well informed about the phenomenon. It is argued that his appointment as special professor in Leiden in 1920 was motivated to a considerable extent by his perception as a leading theoretician of quantum theory and condensed matter physics and the hope that he would contribute to the theoretical direction of the experiments done at Kamerlingh Onnes' cryogenic laboratory. Einstein tried to live up to these expectations by proposing at least three experiments on the phenomenon, one of which was carried out twice in Leiden. Compared to other theoretical proposals at the time, the prominent role of quantum concepts was characteristic of Einstein's understanding of the phenomenon. The paper concludes with comments on Einstein's epistemological reflections on the problem. (shrink)

The Hubbard model is studied at half filling, using two complementary variational wave functions, the Gutzwiller ansatz for the metallic phase at small values of the interaction parameter U and its analog for the insulating phase at large values of U. The metallic phase is characterized by the Drude weight, which exhibits a jump at the critical point Uc. In the insulating phase the system behaves as a collection of dipoles which increase both in number and in size as (...) U gets smaller. The two wave functions are able to describe the two asymptotic regimes (small and large values of U, respectively), but they can no longer be trusted in the region of the Mott transition (U≈Uc). More powerful methods are needed to study, for instance, the divergence of the electric susceptibility for U→Uc. (shrink)