1,173 research outputs found
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
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&
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
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
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
Fractional Quantum Hall Effect in a Diluted Magnetic Semiconductor
We report the observation of the fractional quantum Hall effect in the lowest
Landau level of a two-dimensional electron system (2DES), residing in the
diluted magnetic semiconductor Cd(1-x)Mn(x)Te. The presence of magnetic
impurities results in a giant Zeeman splitting leading to an unusual ordering
of composite fermion Landau levels. In experiment, this results in an
unconventional opening and closing of fractional gaps around filling factor v =
3/2 as a function of an in-plane magnetic field, i.e. of the Zeeman energy. By
including the s-d exchange energy into the composite Landau level spectrum the
opening and closing of the gap at filling factor 5/3 can be modeled
quantitatively. The widely tunable spin-splitting in a diluted magnetic 2DES
provides a novel means to manipulate fractional states
Classical percolation fingerprints in the high-temperature regime of the integer quantum Hall effect
We have performed magnetotransport experiments in the high-temperature regime
(up to 50 K) of the integer quantum Hall effect for two-dimensional electron
gases in semiconducting heterostructures. While the magnetic field dependence
of the classical Hall law presents no anomaly at high temperatures, we find a
breakdown of the Drude-Lorentz law for the longitudinal conductance beyond a
crossover magnetic field B_c ~ 1 T, which turns out to be correlated with the
onset of the integer quantum Hall effect at low temperatures. We show that the
high magnetic field regime at B > B_c can be understood in terms of classical
percolative transport in a smooth disordered potential. From the temperature
dependence of the peak longitudinal conductance, we extract scaling exponents
which are in good agreement with the theoretically expected values. We also
prove that inelastic scattering on phonons is responsible for dissipation in a
wide temperature range going from 1 to 50 K at high magnetic fields.Comment: 14 pages + 8 Figure
Amp\`ere-Class Pulsed Field Emission from Carbon-Nanotube Cathodes in a Radiofrequency Resonator
Pulsed field emission from cold carbon-nanotube cathodes placed in a
radiofrequency resonant cavity was observed. The cathodes were located on the
backplate of a conventional -cell resonant cavity operating at
1.3-GHz and resulted in the production of bunch train with maximum average
current close to 0.7 Amp\`ere. The measured Fowler-Nordheim characteristic,
transverse emittance, and pulse duration are presented and, when possible,
compared to numerical simulations. The implications of our results to
high-average-current electron sources are briefly discussed.Comment: 5 pages, 6 figures; submitted to Applied Physics Letter
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