1,564 research outputs found
Magnetic-field-induced Stoner transition in a dilute quantum Hall system
In a recent paper [Phys.Rev.B.\textbf{84}, 161307 (2011)], experimental data
on spin splitting in the integer quantum Hall effect has been reported in a
high mobility dilute 2D electron gas with electron density as low as 0.2
10 cm . In this work, we show that an excellent
\emph{quantitative} description of these data can be obtained within the model
of the magnetic-field-induced Stoner transition in the quantum Hall regime.
This provides a powerful tool to probe the non-trivial density dependance of
electron-electron interactions in the dilute regime of the 2D electron gas
Interplay among spin, orbital effects and localization in a GaAs two-dimensional electron gas in a strong in-plane magnetic field
The magnetoresistance of a low carrier density, disordered GaAs based
two-dimensional (2D) electron gas has been measured in parallel magnetic fields
up to 32 T. The feature in the resistance associated with the complete spin
polarization of the carriers shifts down by more than 20 T as the electron
density is reduced, consistent with recent theories taking into account the
enhancement of the electron-electron interactions at low densities.
Nevertheless, the magnetic field for complete polarization, Bp, remains 2-3
times smaller than predicted for a disorder free system. We show, in particular
by studying the temperature dependance of Bp to probe the effective size of the
Fermi sea, that localization plays an important role in determining the spin
polarization of a 2D electron gas.Comment: Published in the Physical Review
Critical point for the CAF-F phase transition at charge neutrality in bilayer graphene
We report on magneto-transport measurements up to 30 T performed on a bilayer
graphene Hall bar, enclosed by two thin hexagonal boron nitride flakes. Our
high mobility sample exhibits an insulating state at neutrality point which
evolves into a metallic phase when a strong in-plane field is applied, as
expected for a transition from a canted antiferromagnetic to a ferromagnetic
spin ordered phase. For the first time we individuate a temperature-independent
crossing in the four-terminal resistance as a function of the total magnetic
field, corresponding to the critical point of the transition. We show that the
critical field scales linearly with the perpendicular component of the field,
as expected from the underlying competition between the Zeeman energy and
interaction-induced anisotropies. A clear scaling of the resistance is also
found and an universal behavior is proposed in the vicinity of the transition
Conversion of a transverse density modulation into a longitudinal phase space modulation using an emittance exchange technique
We report on an experiment to produce a train of sub-picosecond microbunches
using a transverse-to-longitudinal emittance exchange technique. The generation
of a modulation on the longitudinal phase space is done by converting an
initial horizontal modulation produced using a multislits mask. The preliminary
experimental data clearly demonstrate the conversion process. To date only the
final energy modulation has been measured. However numerical simulations, in
qualitative agreement with the measurements, indicate that the conversion
process should also introduce a temporal modulation.Comment: 4 pages, 6 figures. Submitted to the proceedings of the Physics and
Applications of High-Brightness Electron Beams (HBEB09), Nov. 16-19, 2009,
Maui H
Longitudinal phase space manipulation in energy recovering linac-driven free-electron lasers
Energy recovering an electron beam after it has participated in a
free-electron laser (FEL) interaction can be quite challenging because of the
substantial FEL-induced energy spread and the energy anti-damping that occurs
during deceleration. In the Jefferson Lab infrared FEL driver-accelerator, such
an energy recovery scheme was implemented by properly matching the longitudinal
phase space throughout the recirculation transport by employing the so-called
energy compression scheme. In the present paper,after presenting a
single-particle dynamics approach of the method used to energy-recover the
electron beam, we report on experimental validation of the method obtained by
measurements of the so-called "compression efficiency" and "momentum
compaction" lattice transfer maps at different locations in the recirculation
transport line. We also compare these measurements with numerical tracking
simulations.Comment: 31 pages, 13 figures, submitted to Phys. Rev. Special Topics A&
Using the de Haas-van Alphen effect to map out the closed three-dimensional Fermi surface of natural graphite
The Fermi surface of graphite has been mapped out using de Haas van Alphen
(dHvA) measurements at low temperature with in-situ rotation. For tilt angles
between the magnetic field and the c-axis, the majority
electron and hole dHvA periods no longer follow the behavior
demonstrating that graphite has a 3 dimensional closed Fermi surface. The Fermi
surface of graphite is accurately described by highly elongated ellipsoids. A
comparison with the calculated Fermi surface suggests that the SWM trigonal
warping parameter is significantly larger than previously thought
Exploring Minimal Scenarios to Produce Transversely Bright Electron Beams Using the Eigen-Emittance Concept
Next generation hard X-ray free electron lasers require electron beams with
low transverse emittance. One proposal to achieve these low emittances is to
exploit the eigen-emittance values of the beam. The eigen-emittances are
invariant under linear beam transport and equivalent to the emittances in an
uncorrelated beam. If a correlated beam with two small eigen-emittances can be
produced, removal of the correlations via appropriate optics will lead to two
small emittance values, provided non-linear effects are not too large. We study
how such a beam may be produced using minimal linear correlations. We find it
is theoretically possible to produce such a beam, however it may be more
difficult to realize in practice. We identify linear correlations that may lead
to physically realizable emittance schemes and discuss promising future
avenues.Comment: 7 pages, 2 figures, to appear in NIM
La réglementation de l'audit est-elle dans l'intérêt public : quelques enseignements du modèle français
Cet article s'appuie sur les travaux académiques des dix dernières années pour évaluer les effets de la réglementation française visant à accroître l'indépendance des auditeurs. Pour les sociétés cotées en bourse, l'obligation de recourir à deux auditeurs se solde notamment par une moindre concentration du marché de l'audit : les Big Four détiennent une part de marché plus faible. Pour autant, les honoraires ne sont pas plus faibles, en raison vraisemblablement, d'une part, des coûts de coordination entre les deux auditeurs qui excèdent les bénéfices escomptés résultant d'un marché plus concurrentiel, d'autre part, de l'impossibilité de changer d'auditeur pendant la durée légale de six ans. Par ailleurs, la plus grande indépendance supposée, induite par cette réglementation spécifique, ne se traduit pas par une moindre gestion des résultats par les dirigeants français, malgré l'interdiction de facturer des honoraires de conseil. Ces constats empiriques nous conduisent à avancer que des assouplissements réglementaires du marché de l'audit pourraient s'avérer bénéfiques pour les actionnaires des entreprises françaisesAudit ; Réglementation; France ; Concentration ; Honoraires ; Gestion des résultats ; Audit ; Regulation ; France ; Concentration ; Audit fees ; Earnings management
Current-induced nuclear-spin activation in a two-dimensional electron gas
Electrically detected nuclear magnetic resonance was studied in detail in a
two-dimensional electron gas as a function of current bias and temperature. We
show that applying a relatively modest dc-current bias, I_dc ~ 0.5 microAmps,
can induce a re-entrant and even enhanced nuclear spin signal compared with the
signal obtained under similar thermal equilibrium conditions at zero current
bias. Our observations suggest that dynamic nuclear spin polarization by small
current flow is possible in a two-dimensional electron gas, allowing for easy
manipulation of the nuclear spin by simple switching of a dc current.Comment: 5 pages, 3 fig
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