860 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
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
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
A Single-pass Cr:ZnSe Amplifier for Broadband Infared Undulator Radiation
An amplifier based on a highly-doped Chromium Zinc-Selenide (Cr:ZnSe) crystal
is proposed to increase the pulse energy emitted by an electron bunch after it
passes through an undulator magnet. The primary motivation is a possible use of
the amplified undulator radiation emitted by a beam circulating in a particle
accelerator storage ring to increase the particle beam's phase-space
density---a technique dubbed Optical Stochastic Cooling (OSC). This paper uses
a simple four energy level model to estimate the single-pass gain of Cr:ZnSe
and presents numerical calculations combined with wave-optics simulations of
undulator radiation to estimate the expected properties of the amplified
undulator wave-packet
The hole Fermi surface in BiSe probed by quantum oscillations
Transport and torque magnetometry measurements are performed at high magnetic
fields and low temperatures in a series of p-type (Ca-doped) BiSe
crystals. The angular dependence of the Shubnikov-de Haas and de Haas-van
Alphen quantum oscillations enables us to determine the Fermi surface of the
bulk valence band states as a function of the carrier density. At low density,
the angular dependence exhibits a downturn in the oscillations frequency
between and , reflecting a bag-shaped hole Fermi surface.
The detection of a single frequency for all tilt angles rules out the existence
of a Fermi surface with different extremal cross-sections down to ~meV.
There is therefore no signature of a camel-back in the valence band of our bulk
samples, in accordance with the direct band gap predicted by calculations.Comment: A supplemental material file giving a more detailed description of
our work is available upon reques
Fractional quantum Hall effect in CdTe
The fractional quantum Hall (FQH) effect is reported in a high mobility CdTe
quantum well at mK temperatures. Fully-developed FQH states are observed at
filling factor 4/3 and 5/3 and are found to be both spin-polarized ground state
for which the lowest energy excitation is not a spin-flip. This can be
accounted for by the relatively high intrinsic Zeeman energy in this single
valley 2D electron gas. FQH minima are also observed in the first excited (N=1)
Landau level at filling factor 7/3 and 8/3 for intermediate temperatures.Comment: Submitte
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