643 research outputs found
CML: the commonKADS conceptual modelling language
We present a structured language for the specification of knowledge models according to the CommonKADS methodology. This language is called CML (Conceptual Modelling Language) and provides both a structured textual notation and a diagrammatic notation for expertise models. The use of our CML is illustrated by a variety of examples taken from the VT elevator design system
Non-Gaussian fluctuations of mesoscopic persistent currents
The persistent current in an ensemble of normal-metal rings shows Gaussian
distributed sample-to-sample fluctuations with non-Gaussian corrections, which
are precursors of the transition into the Anderson localized regime. We here
report a calculation of the leading non-Gaussian correction to the current
autocorrelation function, which is of third order in the current. Although the
third-order correlation function is small, inversely proportional to the
dimensionless conductance of the ring, the mere fact that it is nonzero is
remarkable, since it is an odd moment of the current distribution.Comment: 4+ pages, 2 figure
Self-dual Ginzburg-Landau vortices in a disk
We study the properties of the Ginzburg-Laundau model in the self-dual point
for a two-dimensional finite system . By a numerical calculation we analyze the
solutions of the Euler-Lagrange equations for a cylindrically symmetric ansatz.
We also study the self-dual equations for this case. We find that the minimal
energy configurations are not given by the Bogomol'nyi equations but by
solutions to the Euler Lagrange ones. With a simple approximation scheme we
reproduce the result of the numerical calculation.Comment: 8 pages, 4 figures, RevTex macro
Coherent Backscattering of Ultracold Atoms
We report on the direct observation of coherent backscattering (CBS) of
ultracold atoms, in a quasi-two-dimensional configuration. Launching atoms with
a well-defined momentum in a laser speckle disordered potential, we follow the
progressive build up of the momentum scattering pattern, consisting of a ring
associated with multiple elastic scattering, and the CBS peak in the backward
direction. Monitoring the depletion of the initial momentum component and the
formation of the angular ring profile allows us to determine microscopic
transport quantities. The time resolved evolution of the CBS peak is studied
and is found a fair agreement with predictions, at long times as well as at
short times. The observation of CBS can be considered a direct signature of
coherence in quantum transport of particles in disordered media. It is
responsible for the so called weak localization phenomenon, which is the
precursor of Anderson localization.Comment: 5 pages, 4 figure
Anomalously large conductance fluctuations in weakly disordered graphene
We have studied numerically the mesoscopic fluctuations of the conductance of
a graphene strip (width W large compared to length L), in an ensemble of
samples with different realizations of the random electrostatic potential
landscape. For strong disorder (potential fluctuations comparable to the
hopping energy), the variance of the conductance approaches the value predicted
by the Altshuler-Lee-Stone theory of universal conductance fluctuations. For
weaker disorder the variance is greatly enhanced if the potential is smooth on
the scale of the atomic separation. There is no enhancement if the potential
varies on the atomic scale, indicating that the absence of backscattering on
the honeycomb lattice is at the origin of the anomalously large fluctuations.Comment: 5 pages, 8 figure
Speed of sound in disordered Bose-Einstein condensates
Disorder modifies the sound-wave excitation spectrum of Bose-Einstein
condensates. We consider the classical hydrodynamic limit, where the disorder
correlation length is much longer than the condensate healing length. By
perturbation theory, we compute the phonon lifetime and correction to the speed
of sound. This correction is found to be negative in all dimensions, with
universal asymptotics for smooth correlations. Considering in detail optical
speckle potentials, we find a quite rich intermediate structure. This has
consequences for the average density of states, particularly in one dimension,
where we find a "boson dip" next to a sharp "boson peak" as function of
frequency. In one dimension, our prediction is verified in detail by a
numerical integration of the Gross-Pitaevskii equation.Comment: final, extended version with 2 new figure
Vortex nucleation through edge states in finite Bose-Einstein condensates
We study the vortex nucleation in a finite Bose-Einstein condensate. Using a
set of non-local and chiral boundary conditions to solve the
Schrdinger equation of non-interacting bosons in a rotating trap, we
obtain a quantitative expression for the characteristic angular velocity for
vortex nucleation in a condensate which is found to be 35% of the transverse
harmonic trapping frequency.Comment: 24 pages, 8 figures. Both figures and the text have been revise
Persistent Current of Free Electrons in the Plane
Predictions of Akkermans et al. are essentially changed when the Krein
spectral displacement operator is regularized by means of zeta function.
Instead of piecewise constant persistent current of free electrons on the plane
one has a current which varies linearly with the flux and is antisymmetric with
regard to all time preserving values of including . Different
self-adjoint extensions of the problem and role of the resonance are discussed.Comment: (Comment on "Relation between Persistent Currents and the Scattering
Matrix", Phys. Rev. Lett. {\bf 66}, 76 (1991)) plain latex, 4pp., IPNO/TH
94-2
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