The discovery of superfluidity

Superfluidity is a remarkable manifestation of quantum mechanics at the macroscopic level. This article describes the history of its discovery, which took place at a particularly difficult period of the twentieth century. A special emphasis is given to the role of J.F. Allen, D. Misener, P. Kapitza, F. London, L. Tisza and L.D. Landau. The nature and the importance of their respective contributions are analyzed and compared. Of particular interest is the controversy between Landau on one side, London and Tisza on the other, concerning the relevance of Bose-Einstein condensation to the whole issue, and also on the nature of thermal excitations in superfluid helium 4. In order to aid my understanding of this period, I have collected several testimonies which inform us about the work and attitude of these great scientists.Comment: 30 page

Steps and facets at the surface of soft crystals

We consider the shape of crystals which are soft in the sense that their elastic modulus $\mu$ is small compared to their surface tension $\gamma$, more precisely $\mu a < \gamma$ where $a$ is the lattice spacing. We show that their surface steps penetrate inside the crystal as edge dislocations. As a consequence, these steps are broad with a small energy which we calculate. We also calculate the elastic interaction between steps a distance $d$ apart, which is a $1/d^2$ repulsion. We finally calculate the roughening temperatures of successive facets in order to compare with the remarkable shapes of lyotropic crystals recently observed by P. Pieranski et al. Good agreement is found.Comment: 8 Pages, 1 Figure. To appear on Eur. Phys. Journal.

Nucleation in a Fermi liquid at negative pressure

Experimental investigation of cavitation in liquid helium 3 has revealed a singular behaviour in the degenerate region at low temperature. As the temperature decreases below 80 mK, the cavitation pressure becomes significantly more negative. To investigate this, we have extrapolated the Fermi parameters in the negative pressure region. This allowed us to calculate the zero sound velocity, which we found to remain finite at the spinodal limit where the first sound velocity vanishes. We discuss the impact on the nucleation of the gas phase in terms of a quantum stiffness of the Fermi liquid. As a result we predict a cavitation pressure which is nearer to the spinodal line than previously thought.Comment: 6 pages, 2 figures. To be published in Journal of Low Temperature Physics, Proceedings of the International Symposium on Quantum Fluids and Solids QFS200

Foucault's point of heresy : 'Quasi-Transcendentals' and the transdisciplinary function of the episteme

Major difficulties for readers of Foucault’s The Order of Things concern the historical function and the logical construction of the episteme. Our proposal is to link it with another notion, the ‘point of heresy’, less frequently addressed. This leads to asserting that irreconcilable dilemmas are in fact determined by the type of rationality governing the emergence of common objects of knowledge. It also introduces a possibility of ‘walking on two roads’: a dialogical adventure within rationality. Foucault is not content with either accepting or rejecting the ‘transcendental’ question ‘What is Man?’: with the help of quasi-transcendental categories performing a ‘transdisciplinary’ function, he wants to reach the ‘heretical’ point where anthropology becomes historicity within the horizon of finitude. </jats:p

Los dilemas históricos de la democracia y su relevancia contemporánea para la ciudadanía

Este ensayo analiza la relación dialéctica entre los conceptos de democracia y de ciudadanía a la luz de los debates actuales, en los que se combinan una transformación de la tradición filosófica y una evaluación de las situaciones en que se cuestiona la distinción jurídica entre el «ciudadano» y el «nacional». A partir de consideraciones sobre las tensiones semánticas de las categorías griegas y latinas (politeia, demokratia, isonomía, ius civitatis), se analizan las aporías de la democracia, entendida como modelo o ideología, que filósofos como Jacques Rancière y Hannah Arendt nos permiten superar mediante la definición de la democracia como un proceso de insurrección permanente «anti-oligárquico» en lugar de un régimen estable. No es la propagación de la democracia, por lo tanto, lo que constituye el objeto primordial de la teoría política, sino la democratización de la democracia, de sí misma, especialmente en la forma de eliminación de la exclusión interna. Esta teoría se ilustra y se perfecciona aún más al referirse a los debates sobre la discriminación de clase y raza, las luchas violentas en busca del reconocimiento que afectan a las instituciones republicanas, las formas de ciudadanía nómadas o diaspóricas, y la relación entre derechos sociales y políticos.This essay discusses the dialectical relationship between the concepts of democracy and citizenship, by relating to current debates that combine a transformation of the philo­sophical tradition and an evaluation of situations where the legal distinction between the citizen and the national is challenged. Starting with considerations on the semantic tensions of the Greek and Roman categories (politeia, demokratia, isonomia, ius civitatis), it discusses the aporias of democracy as a model or an ideology, which philosophers like Jacques Rancière and Hannah Arendt allow us to overcome by defining democracy as a process of permanent antioligarchic insurrection rather than a stable regime. It is not the spread of democracy, therefore, that constitutes the primordial object of political theory, but the democratization of democracy itself, especially in the form of the elimination of its internal exclusions. This theory is illustrated and further refined by referring to debates about class and race discrimination, violent struggles for recognition affecting republican institutions, nomadic or diasporic forms of citizenship, and the relationship between social and political rights