194 research outputs found
Higher Threshold Parameters in pi pi Scattering
A family of threshold parameters which probe the stability of chiral
predictions is considered. The relevant criteria for the choice of threshold
parameters are discussed. Sum rules for these quantities are derived from
dispersion relations and evaluated from effective range formulae. Good
agreement with two-loop chiral estimates for many of these quantities is found
and interesting discrepancies are discussed.Comment: 15 pages, LaTe
pi K scattering inputs to ChPT
Experimental information on low energy scattering would shed light on
the poorly known OZI suppressed sector of ChPT. I describe recent work aimed at
generating such information based on available experimental data by setting up
and then solving with appropriate boundary conditions a non linear system of
equations of the Roy and Steiner type. First results of this analysis are
presented.Comment: Talk given at QCD'03 conference, 2-9 July 2003, Montpellie
Parity meter for charge qubits: an efficient quantum entangler
We propose a realization of a charge parity meter based on two double quantum
dots alongside a quantum point contact. Such a device is a specific example of
the general class of mesoscopic quadratic quantum measurement detectors
previously investigated by Mao et al. [Phys. Rev. Lett. 93, 056803 (2004)]. Our
setup accomplishes entangled state preparation by a current measurement alone,
and allows the qubits to be effectively decoupled by pinching off the parity
meter. Two applications of the parity meter are discussed: the measurement of
Bell's inequality in charge qubits and the realization of a controlled NOT
gate.Comment: 8 pages, 4 figures; v2: discussion of measurement time, references
adde
Comment on "Observation of Spin Injection at a Ferromagnet-Semiconductor Interface, by P.R. Hammar et al
In a recent Letter Hammar et al. claim the observation of injection of a
spin-polarized current in a two-dimensional electron gas (2DEG). This is an
important observation, since, despite considerable effort of several groups,
all attempts to realize spin-injection into a 2DEG using purely electrical
measurements have failed sofar. However, in my opinion the claim made is not
correct, and the observed behaviour can be explained by a combination of a
magneto resistance (Hall) effect (e.g. generated by the fringe magnetic fields
present at the edges of the ferromagnetic electrode), with a {\it
spin-independent} rectification effect due to the presence of a metal-
semiconductor junction.Comment: accepted for PRL, 1 pag
Quantum dot dephasing by edge states
We calculate the dephasing rate of an electron state in a pinched quantum
dot, due to Coulomb interactions between the electron in the dot and electrons
in a nearby voltage biased ballistic nanostructure. The dephasing is caused by
nonequilibrium time fluctuations of the electron density in the nanostructure,
which create random electric fields in the dot. As a result, the electron level
in the dot fluctuates in time, and the coherent part of the resonant
transmission through the dot is suppressed
Rashba effect induced localization in quantum networks
We study a quantum network extending in one-dimension (chain of square loops
connected at one vertex) made up of quantum wires with Rashba spin-orbit
coupling. We show that the Rashba effect may give rise to an electron
localization phenomenon similar to the one induced by magnetic field. This
localization effect can be attributed to the spin precession due to the Rashba
effect. We present results both for the spectral properties of the infinite
chain, and for linear transport through a finite-size chain connected to leads.
Furthermore, we study the effect of disorder on the transport properties of
this network.Comment: To appear in Phys. Rev. Let
Rashba effect in quantum networks
We present a formalism to study quantum networks made up by single-channel
quantum wires in the presence of Rashba spin-orbit coupling and magnetic field.
In particular, linear transport through one-dimensional and two-dimensional
finite-size networks is studied by means of the scattering formalism. In some
particular quantum networks, the action of the magnetic field or of the Rashba
spin-orbit coupling induces localization of the electron wave function. This
phenomenon, which relies on both the quantum-mechanical interference and the
geometry of the network, is manifested through the suppression of the
conductance for specific values of the spin-orbit-coupling strength or of the
magnetic field. Furthermore, the interplay of the Aharonov-Bohm phases and of
the non-Abelian phases introduced by spin-orbit coupling, is discussed in a
number of cases.Comment: 8 pages and 6 figure
Quelle est la proportion d'ombles chevaliers (Salvelinus alpinus L.) issus de reproduction naturelle ou de repeuplement, dans le Léman
La proportion d'ombles chevaliers, Salvelinus alpinus (L.), issus de fraye naturelle, des cohortes 1984 et 1985, a été estimée en marquant une partie des estivaux issus de pisciculture immergés dans le Léman. En 1984, entre 8.3 et 24.8 % seulement des estivaux présents dans les eaux suisses, au moment de la mise à l'eau des individus marqués, étaient issus de fraye naturelle. Ce chiffre est compris entre 19.1 et 34.9 % pour la cohorte 1985. Le repeuplement apparaît donc comme très efficace. Une étude des principaux sites de fraye de l'omble, de même qu'une analyse des statistiques de pêche, démontre également cette efficacité
Oscillatory Tunneling between Quantum Hall Systems
Electron tunneling between quantum Hall systems on the same two dimensional
plane separated by a narrow barrier is studied. We show that in the limit where
inelastic scattering time is much longer than the tunneling time, which can be
achieved in practice, electrons can tunnel back and forth through the barrier
continously, leading to an oscillating current in the absence of external
drives. The oscillatory behavior is dictated by a tunneling gap in the energy
spectrum. We shall discuss ways to generate oscillating currents and the
phenomenon of natural ``dephasing" between the tunneling currents of edge
states. The noise spectra of these junctions are also studied. They contain
singularites reflecting the existence of tunneling gaps as well as the inherent
oscillation in the system. (Figures will be given upon requests).Comment: 20 pages, OS
Is there a renormalization of the 1D conductance in Luttinger Liquid model?
Properties of 1D transport strongly depend on the proper choice of boundary
conditions. It has been frequently stated that the Luttinger Liquid (LL)
conductance is renormalized by the interaction as . To
contest this result I develop a model of 1D LL wire with the interaction
switching off at the infinities. Its solution shows that there is no
renormalization of the universal conductance while the electrons have a free
behavior in the source and drain reservoirs.Comment: 5 pages, RevTex 2.0, attempted repair of tex error
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