126 research outputs found
Large N spin quantum Hall effect
We introduce a large N version of the spin quantum Hall transition problem.
It is formulated as a problem of Dirac fermions coupled to disorder, whose
Hamiltonian belong to the symmetry class C. The fermions carry spin degrees of
freedom valued in the algebra sp(2N), the spin quantum Hall effect
corresponding to N=1. Arguments based on renormalization group transformations
as well as on a sigma model formulation, valid in the large N limit, indicate
the existence of a crossover as N varies. Contrary to the N=1 case, the large N
models are shown to lead to localized states at zero energy. We also present a
sigma model analysis for the system of Dirac fermions coupled to only sp(2N)
random gauge potentials, which reproduces known exact results.Comment: 29 pages; few references added, statement about the density of states
improved; published versio
Interaction effects on 2D fermions with random hopping
We study the effects of generic short-ranged interactions on a system of 2D
Dirac fermions subject to a special kind of static disorder, often referred to
as ``chiral.'' The non-interacting system is a member of the disorder class BDI
[M. R. Zirnbauer, J. Math. Phys. 37, 4986 (1996)]. It emerges, for example, as
a low-energy description of a time-reversal invariant tight-binding model of
spinless fermions on a honeycomb lattice, subject to random hopping, and
possessing particle-hole symmetry. It is known that, in the absence of
interactions, this disordered system is special in that it does not localize in
2D, but possesses extended states and a finite conductivity at zero energy, as
well as a strongly divergent low-energy density of states. In the context of
the hopping model, the short-range interactions that we consider are
particle-hole symmetric density-density interactions. Using a perturbative
one-loop renormalization group analysis, we show that the same mechanism
responsible for the divergence of the density of states in the non-interacting
system leads to an instability, in which the interactions are driven strongly
relevant by the disorder. This result should be contrasted with the limit of
clean Dirac fermions in 2D, which is stable against the inclusion of weak
short-ranged interactions. Our work suggests a novel mechanism wherein a clean
system, initially insensitive to interaction effects, can be made unstable to
interactions upon the inclusion of weak static disorder.Comment: 16 pages, 10 figures; References added, figures enlarged; to be
published in Phys. Rev.
Pour une anthropologie générale
Lâassociation de lâanthropologie biologique, de lâanthropologie sociale-ethnologie et de la prĂ©histoire, sous la forme de pĂŽles de recherche et dâenseignement conjoints ou dâinstances dâĂ©valuation partagĂ©es, a-t-elle toujours un sens en ce dĂ©but de xxie siĂšcle, compte tenu des nouveaux objets que tendent Ă construire les chercheurs se revendiquant de ces champs scientifiques ? Si la rĂ©ponse demeure positive, autour de quels objectifs et projets communs peuvent-ils se rĂ©unir ? Si la rĂ©ponse es..
Cosmic Ray particle production
The status of some popular models to simulate hadronic and nuclear
interactions at Cosmic Ray energies is reviewed. The models predict the rise of
all the hadronic and nuclear cross sections with energy and a smooth
(logarithmic) rise of average multiplicities, rapidity plateaus and average
transverse momenta with the energy. The importance is stressed to put more
effort into the models and especially a better understanding of the minijet
component at the highest energies. Likewise, experimental data on particle
production are needed at the highest possible energies, to guide the models.Comment: 13 pages, LATEX, espcrc2.sty, 21 figures included as ps file
Le numĂ©rique Ă lâĂ©cole : Ă©volution ou rĂ©volution pĂ©dagogique ?
Ce dĂ©bat entre experts et praticiens fait apparaĂźtre des lignes de fracture et de force : fracture entre le temps long de lâĂ©cole et lâimmĂ©diatetĂ© numĂ©rique de la jeunesse, tensions sur les mĂ©susages technologiques, mais aussi travail dâĂ©quipe, dĂ©veloppement de projets, enrichissement de la culture humaniste. Le numĂ©rique bouscule lâĂ©cole, ses certitudes, son organisation, ses instruments. Son enseignement ne saurait, selon les participants, se rĂ©duire Ă celui dâune discipline, et requiert une approche transversale de la littĂ©ratie numĂ©rique, associant usage responsable des instruments, connaissance des grand concepts du numĂ©rique, et dĂ©veloppement dâune rĂ©flexion scientifique, Ă©thique et philosophique.This debate between experts and practitioners reveals the main areas of discussion and divergence: the gap between the long timeframe of school and the digital immediacy of youth, tensions surrounding the misuse of technologies, but also teamwork, project development and an enriched culture of humanism. Digital technologies are disrupting schoolâs certainties, organisation and -equipment. According to the speakers, the teaching of digital technologies cannot be reduced to a single discipline and requires a multisectoral approach to digital literacy â one that combines the responsible use of equipment, knowledge of major digital concepts and the development of scientific, ethical and philosophical reflections.Este debate entre expertos y practicantes hace aparecer unas lĂneas de fractura : fractura entre el tiempo largo de la escuela y la inmediatez digital de la juventud, tensiones entre los malos usos tecnolĂłgicos, pero tambiĂ©n trabajo colectivo, desarrollo de proyectos, enriquecimiento de la cultura humanista. Lo digital empuja a la escuela, sus certidumbres, su organizaciĂłn, sus instrumentos. SegĂșn los participantes, su enseñanza no podrĂa reducirse al de una asignatura, y requiere una aproximaciĂłn transversal de la alfabetizaciĂłn digital, que asocie uso responsable de los instrumentos, conocimiento de los grandes conceptos de lo digital y desarrollo de una reflexiĂłn cientĂfica, Ă©tica y filosĂłfica
Chiral symmetry in linear Sigma model in magnetic environment
We study the chiral symmetry structure in a linear sigma model with fermions
in the presence of an external, uniform magnetic field in the 'effective
potential' approach at the one loop level. We also study the chiral phase
transition as a function of density in the core of magnetized neutron stars.Comment: LaTex2e file with six postscript figures. journal ref: Physical
Review D 62 (2000) 02502
The First Self-Assembled Trimetallic Lanthanide Helicates Driven by Positive Cooperativity
The segmental tris-tridentate ligand L7 reacts with stoichiometric quantities of Ln(III) (Ln=La-Lu) in acetonitrile to give the complexes [Ln(2)(L7)(3)](6+) and [Ln(3)(L7)(3)](9+). Formation constants point to negligible size-discriminating effects along the lanthanide series, but Scatchard plots suggest that the self-assembly of the trimetallic triple-stranded helicates [Ln(3)(L7)(3)](9+) is driven to completion by positive cooperativity, despite strong intermetallic electrostatic repulsions. Crystallization provides quantitatively [Ln(3)(L7)(3)](CF(3)SO(3))(9) (Ln=La, Eu, Gd, Tb, Lu) and the X-ray crystal structure of [Eu(3)(L7)(3)](CF(3)SO(3))(9).(CH(3)CN)(9).(H(2)O)(2) (Eu(3)C(216)H(226)N(48)O(35)F(27)S(9), triclinic, P1, Z=2) shows the three ligand strands wrapped around a pseudo-threefold axis defined by the three metal ions rigidly held at about 9 A. Each metal ion is coordinated by nine donor atoms in a pseudo-trigonal prismatic arrangement, but the existence of terminal carboxamide units in the ligand strands differentiates the electronic properties of the terminal and the central metallic sites. Photophysical data confirm that the three coordination sites possess comparable pseudo-trigonal symmetries in the solid state and in solution. High-resolution luminescence analyses evidence a low-lying LMCT state affecting the central EuN(9) site, so that multi-metal-centered luminescence is essentially dominated by the emission from the two terminal EuN(6)O(3) sites in [Eu(3)(L7)(3)](9+). New multicenter equations have been developed for investigating the solution structure of [Ln(3)(L7)(3)](9+) by paramagnetic NMR spectroscopy and linear correlations for Ln=Ce-Tb imply isostructurality for these larger lanthanides. NMR spectra point to the triple helical structure being maintained in solution, but an inversion of the magnitude of the second-rank crystal-field parameters, obtained by LIS analysis, for the LnN(6)O(3) and LnN(9) sites with respect to the parameters extracted for Eu(III) from luminescence data, suggests that the geometry of the central LnN(9) site is somewhat relaxed in solution
Low-Energy Compton Scattering of Polarized Photons on Polarized Nucleons
The general structure of the cross section of scattering with
polarized photon and/or nucleon in initial and/or final state is systematically
described and exposed through invariant amplitudes. A low-energy expansion of
the cross section up to and including terms of order is given which
involves ten structure parameters of the nucleon (dipole, quadrupole,
dispersion, and spin polarizabilities). Their physical meaning is discussed in
detail. Using fixed-t dispersion relations, predictions for these parameters
are obtained and compared with results of chiral perturbation theory. It is
emphasized that Compton scattering experiments at large angles can fix the most
uncertain of these structure parameters. Predictions for the cross section and
double-polarization asymmetries are given and the convergence of the expansion
is investigated. The feasibility of the experimental determination of some of
the struture parameters is discussed.Comment: 41 pages of text, 9 figures; minor revisions prior to publication in
Phys. Rev.
Density of states for the -flux state with bipartite real random hopping only: A weak disorder approach
Gade [R. Gade, Nucl. Phys. B \textbf{398}, 499 (1993)] has shown that the
local density of states for a particle hopping on a two-dimensional bipartite
lattice in the presence of weak disorder and in the absence of time-reversal
symmetry(chiral unitary universality class) is anomalous in the vicinity of the
band center whenever the disorder preserves the sublattice
symmetry. More precisely, using a nonlinear-sigma-model that encodes the
sublattice (chiral) symmetry and the absence of time-reversal symmetry she
argues that the disorder average local density of states diverges as
with some non-universal
positive constant and a universal exponent. Her analysis has been
extended to the case when time-reversal symmetry is present (chiral orthogonal
universality class) for which the same exponent was predicted.
Motrunich \textit{et al.} [O. Motrunich, K. Damle, and D. A. Huse, Phys. Rev. B
\textbf{65}, 064206 (2001)] have argued that the exponent does not
apply to the typical density of states in the chiral orthogonal universality
class. They predict that instead. We confirm the analysis of
Motrunich \textit{et al.} within a field theory for two flavors of Dirac
fermions subjected to two types of weak uncorrelated random potentials: a
purely imaginary vector potential and a complex valued mass potential. This
model is believed to belong to the chiral orthogonal universality class. Our
calculation relies in an essential way on the existence of infinitely many
local composite operators with negative anomalous scaling dimensions.Comment: 30 pages, final version published in PR
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