693 research outputs found
Three-dimensionality in quasi-two dimensional flows: recirculations and barrel effects
A scenario is put forward for the appearance of three-dimensionality both in
quasi-2D rotating flows and quasi-2D magnetohydrodynamic (MHD) flows. We show
that 3D recirculating flows and currents originate in wall boundary layers and
that, unlike in ordinary hydrodynamic flows, they cannot be ignited by
confinement alone. They also induce a second form of three-dimensionality with
quadratic variations of velocities and current across the channel. This
scenario explains both the common tendency of these flows to two-dimensionality
and the mechanisms of the recirculations through a single formal analogy
covering a wide class of flow including rotating and MHD flows. These
trans-disciplinary effects are thus active in atmospheres, oceans or the
cooling blankets of nuclear fusion reactors.Comment: 6 pages, 1 Figur
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
Geometrical dependence of decoherence by electronic interactions in a GaAs/GaAlAs square network
We investigate weak localization in metallic networks etched in a two
dimensional electron gas between mK and mK when electron-electron
(e-e) interaction is the dominant phase breaking mechanism. We show that, at
the highest temperatures, the contributions arising from trajectories that wind
around the rings and trajectories that do not are governed by two different
length scales. This is achieved by analyzing separately the envelope and the
oscillating part of the magnetoconductance. For K we find
\Lphi^\mathrm{env}\propto{T}^{-1/3} for the envelope, and
\Lphi^\mathrm{osc}\propto{T}^{-1/2} for the oscillations, in agreement with
the prediction for a single ring \cite{LudMir04,TexMon05}. This is the first
experimental confirmation of the geometry dependence of decoherence due to e-e
interaction.Comment: LaTeX, 5 pages, 4 eps figure
Quantum oscillations in mesoscopic rings and anomalous diffusion
We consider the weak localization correction to the conductance of a ring
connected to a network. We analyze the harmonics content of the
Al'tshuler-Aronov-Spivak (AAS) oscillations and we show that the presence of
wires connected to the ring is responsible for a behaviour different from the
one predicted by AAS. The physical origin of this behaviour is the anomalous
diffusion of Brownian trajectories around the ring, due to the diffusion in the
wires. We show that this problem is related to the anomalous diffusion along
the skeleton of a comb. We study in detail the winding properties of Brownian
curves around a ring connected to an arbitrary network. Our analysis is based
on the spectral determinant and on the introduction of an effective perimeter
probing the different time scales. A general expression of this length is
derived for arbitrary networks. More specifically we consider the case of a
ring connected to wires, to a square network, and to a Bethe lattice.Comment: 17 pages, 7 eps figure
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
An effective lowest Landau level treatment of demagnetization in superconducting mesoscopic disks
Demagnetization, which is inherently present in the magnetic response of
small finite-size superconductors, can be accounted for by an effective
within a two-dimensional lowest Landau level approximation of the
Ginzburg-Landau functional. We show this by comparing the equilibrium
magnetization of superconducting mesoscopic disks obtained from the numerical
solution of the three-dimensional Ginzburg-Landau equations with that obtained
in the ``effective'' LLL approximation.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
Phi_0 - Periodic Aharonov-Bohm Oscillations Survive Ensemble Averaging
We have demonstrated that Phi_0 periodic Aharonov--Bohm oscillations measured
in a ensemble of rings may survive after ensemble averaging procedure. The
central point is the difference between the preparation stage of the ensemble
and the subsequent measurement stage. The robustness of the effect under finite
temperature and non--zero charging energy of rings is discussed.Comment: 11 pages, 2 figures, RevTex 3.0,WIS-93/84/Aug.-P
Resonant Josephson current through a quantum dot
We calculate the DC Josephson current through a semiconducting quantum dot
which is weakly coupled by tunnel barriers to two superconducting reservoirs. A
Breit-Wigner resonance in the conductance corresponds to a resonance in the
critical current, but with a different (non-lorentzian) lineshape.Comment: 5 pages including 1 figure; this paper was published in the
proceedings of SQUID'91; it is archived here because of its relevance to
cond-mat/011148
Vortices in Ginzburg-Landau billiards
We present an analysis of the Ginzburg-Landau equations for the description
of a two-dimensional superconductor in a bounded domain. Using the properties
of a special integrability point of these equations which allows vortex
solutions, we obtain a closed expression for the energy of the superconductor.
The role of the boundary of the system is to provide a selection mechanism
for the number of vortices.
A geometrical interpretation of these results is presented and they are
applied to the analysis of the magnetization recently measured on small
superconducting disks. Problems related to the interaction and nucleation of
vortices are discussed.Comment: RevTex, 17 pages, 3 eps figure
Effect of Magnetic Impurities on Energy Exchange between Electrons
In order to probe quantitatively the effect of Kondo impurities on energy
exchange between electrons in metals, we have compared measurements on two
silver wires with dilute magnetic impurities (manganese) introduced in one of
them. The measurement of the temperature dependence of the electron phase
coherence time on the wires provides an independent determination of the
impurity concentration. Quantitative agreement on the energy exchange rate is
found with a theory by G\"{o}ppert et al. that accounts for Kondo scattering of
electrons on spin-1/2 impurities.Comment: 4 page
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