8,099 research outputs found
Reentrant nu = 1 quantum Hall state in a two-dimensional hole system
We report the observation of a reentrant quantum Hall state at the Landau
level filling factor nu = 1 in a two-dimensional hole system confined to a
35-nm-wide (001) GaAs quantum well. The reentrant behavior is characterized by
a weakening and eventual collapse of the nu = 1 quantum Hall state in the
presence of a parallel magnetic field component B||, followed by a
strengthening and reemergence as B|| is further increased. The robustness of
the nu = 1 quantum Hall state during the transition depends strongly on the
charge distribution symmetry of the quantum well, while the magnitude of B||
needed to invoke the transition increases with the total density of the system
Anisotropic Fermi Contour of (001) GaAs Holes in Parallel Magnetic Fields
We report a severe, spin-dependent, Fermi contour anisotropy induced by
parallel magnetic field in a high-mobility (001) GaAs two-dimensional hole
system. Employing commensurability oscillations created by a unidirectional,
surface-strain-induced, periodic potential modulation, we directly probe the
anisotropy of the two spin subband Fermi contours. Their areas are obtained
from the Fourier transform of the Shubnikov-de Haas oscillations. Our findings
are in semi-quantitative agreement with the results of parameter-free
calculations of the energy bands.Comment: 4 pages, 4 figure
Anomalous spin-resolved point-contact transmission of holes due to cubic Rashba spin-orbit coupling
Evidence is presented for the finite wave vector crossing of the two lowest
one-dimensional spin-split subbands in quantum point contacts fabricated from
two-dimensional hole gases with strong spin-orbit interaction. This phenomenon
offers an elegant explanation for the anomalous sign of the spin polarization
filtered by a point contact, as observed in magnetic focusing experiments.
Anticrossing is introduced by a magnetic field parallel to the channel or an
asymmetric potential transverse to it. Controlling the magnitude of the
spin-splitting affords a novel mechanism for inverting the sign of the spin
polarization.Comment: 4 pages, 3 figure
Even-denominator Fractional Quantum Hall Effect at a Landau Level Crossing
The fractional quantum Hall effect (FQHE), observed in two-dimensional (2D)
charged particles at high magnetic fields, is one of the most fascinating,
macroscopic manifestations of a many-body state stabilized by the strong
Coulomb interaction. It occurs when the filling factor () of the quantized
Landau levels (LLs) is a fraction which, with very few exceptions, has an odd
denominator. In 2D systems with additional degrees of freedom it is possible to
cause a crossing of the LLs at the Fermi level. At and near these crossings,
the FQHE states are often weakened or destroyed. Here we report the observation
of an unusual crossing of the two \emph{lowest-energy} LLs in high-mobility
GaAs 2D systems which brings to life a new \emph{even-denominator} FQHE
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Tuning of Fermi Contour Anisotropy in GaAs (001) 2D Holes via Strain
We demonstrate tuning of the Fermi contour anisotropy of two-dimensional (2D)
holes in a symmetric GaAs (001) quantum well via the application of in-plane
strain. The ballistic transport of high-mobility hole carriers allows us to
measure the Fermi wavevector of 2D holes via commensurability oscillations as a
function of strain. Our results show that a small amount of in-plane strain, on
the order of , can induce significant Fermi wavevector anisotropy as
large as 3.3, equivalent to a mass anisotropy of 11 in a parabolic band. Our
method to tune the anisotropy \textit{in situ} provides a platform to study the
role of anisotropy on phenomena such as the fractional quantum Hall effect and
composite fermions in interacting 2D systems.Comment: Accepted to Applied Physics Letter
Spin interference in silicon three-terminal one-dimensional rings
We present the first findings of the spin transistor effect in the Rashba
gate-controlled ring embedded in the p-type self-assembled silicon quantum well
that is prepared on the n-type Si (100) surface. The coherence and phase
sensitivity of the spin-dependent transport of holes are studied by varying the
value of the external magnetic field and the bias voltage that are applied
perpendicularly to the plane of the double-slit ring. Firstly, the amplitude
and phase sensitivity of the 0.7(2e^2/h) feature of the hole quantum
conductance staircase revealed by the quantum point contact inserted in the one
of the arms of the double-slit ring are found to result from the interplay of
the spontaneous spin polarization and the Rashba spin-orbit interaction.
Secondly, the quantum scatterers connected to two one-dimensional leads and the
quantum point contact inserted are shown to define the amplitude and the phase
of the Aharonov-Bohm and the Aharonov-Casher conductance oscillations.Comment: 8 pages, 5 figure
Power dependence of pure spin current injection by quantum interference
We investigate the power dependence of pure spin current injection in GaAs
bulk and quantum-well samples by a quantum interference and control technique.
Spin separation is measured as a function of the relative strength of the two
transition pathways driven by two laser pulses. By keeping the relaxation time
of the current unchanged, we are able to relate the spin separation to the
injected average velocity. We find that the average velocity is determined by
the relative strength of the two transitions in the same way as in classical
interference. Based on this, we conclude that the density of injected pure spin
current increases monotonically with the excitation laser intensities. The
experimental results are consistent with theoretical calculations based on
Fermi's golden rule.Comment: 6 pages, 4 figure
COMPTEL measurements of MeV gamma-ray burst spectra
We present results from the on-going spectral analysis of gamma-ray bursts measured by the COMPTEL instrument in its main Compton “Telescope” observing mode (0.75–30 MeV). Thus far, 18 bursts have been analyzed from three years (April 1991–April 1994) of observations. The time-averaged spectra of these events above 1 MeV are all consistent with a simple power law model with spectral index in the range 1.5–3.5. Exponential, thermal bremsstrahlung and thermal synchrotron models are statistically inconsistent with the burst sample, although they can adequately describe some of the individual burst spectra. We find good agreement between burst spectra measured simultaneously by BATSE, COMPTEL and EGRET, which typically show a spectral transition or “break” in the BATSE energy range around a few hundred keV followed by simple power law emission extending to hundreds of MeV. However, the temporal relation between MeV and GeV (e.g., as measured by EGRET) burst emission is still unclear. Measurement of rapid variability at MeV energies in the stronger bursts provides evidence that either the sources are nearby (within the Galaxy) or the gamma-ray emission is relativistically beamed
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