22,851 research outputs found
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
Characterization of weak convergence of Birkhoff sums for Gibbs-Markov maps
We investigate limit theorems for Birkhoff sums of locally H\"older functions
under the iteration of Gibbs-Markov maps. Aaronson and Denker have given
sufficient conditions to have limit theorems in this setting. We show that
these conditions are also necessary: there is no exotic limit theorem for
Gibbs-Markov maps. Our proofs, valid under very weak regularity assumptions,
involve weak perturbation theory and interpolation spaces. For L^2 observables,
we also obtain necessary and sufficient conditions to control the speed of
convergence in the central limit theorem.Comment: 35 pages v2: minor modifications, clarified titl
Chiral symmetry and spectrum of Euclidean Dirac operator
After recalling some connections between the Spontaneous Breakdown of Chiral
Symmetry (SBChS) and the spectrum of the Dirac operator for Euclidean QCD on a
torus, we use this tool to reconsider two related issues : the Zweig rule
violation in the scalar channel and the dependence of SBChS order parameters on
the number N_f of massless flavours. The latter would result into a great
variety of SBChS patterns in the (N_f,N_c) plane, which could be studied
through so-called Leutwyler-Smilga sum rules in association with lattice
computations of the Dirac spectrum.Comment: 6 pages, no figure, class file included. Talk given at the XVII
International School of Physics "QCD: Perturbative or Nonperturbative",
Lisbon, Portugal, 29 September - 4 October 1999, to appear in the Proceeding
Theory for helical turbulence under fast rotation
Recent numerical simulations have shown the strong impact of helicity on
\ADD{homogeneous} rotating hydrodynamic turbulence. The main effect can be
summarized through the following law, , where and are respectively the power law indices of the one-dimensional energy and
helicity spectra. We investigate this rotating turbulence problem in the small
Rossby number limit by using the asymptotic weak turbulence theory derived
previously. We show that the empirical law is an exact solution of the helicity
equation where the power law indices correspond to perpendicular (to the
rotation axis) wave number spectra. It is proposed that when the cascade
towards small-scales tends to be dominated by the helicity flux the solution
tends to , whereas it is when the energy flux
dominates. The latter solution is compatible with the so-called maximal
helicity state previously observed numerically and derived theoretically in the
weak turbulence regime when only the energy equation is used, whereas the
former solution is constrained by a locality condition.Comment: 4 page
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