3,161 research outputs found
Search for a simultaneous signal from small transient events in the Pierre Auger Observatory and the Tupi muon telescopes
We present results of a search for a possible signal from small scale solar
transient events (such as flares and interplanetary shocks) as well as possible
counterparts to Gamma-Ray Burst (GRB) observed simultaneously by the Tupi muon
telescope Niteroi-Brazil, 22.90S, 43.20W, 3 m above sea level) and the Pierre
Auger Observatory surface detectors (Malargue-Argentina, 69.30S, 35.30W,
altitude 1400 m). Both cosmic ray experiments are located inside the South
Atlantic Anomaly (SAA) region. Our analysis of several examples shows
similarities in the behavior of the counting rate of low energy (above 100 MeV)
particles in association with the solar activity (solar flares and
interplanetary shocks). We also report an observation by the Tupi experiment of
the enhancement of muons at ground level with a significance higher than 8
sigma in the 1-sec binning counting rate (raw data) in close time coincidence
(T-184 sec) with the Swift-BAT GRB110928B (trigger=504307). The GRB 110928B
coordinates are in the field of view of the vertical Tupi telescope, and the
burst was close to the MAXI source J1836-194. The 5-min muon counting rate in
the vertical Tupi telescope as well as publicly available data from Auger (15
minutes averages of the scaler rates) show small peaks above the background
fluctuations at the time following the Swift-BAT GRB 110928B trigger. In
accordance with the long duration trigger, this signal can possibly suggest a
long GRB, with a precursor narrow peak at T-184 sec.Comment: 9 pages, 13 figure
Coulomb Drag near the metal-insulator transition in two-dimensions
We studied the drag resistivity between dilute two-dimensional hole systems,
near the apparent metal-insulator transition. We find the deviations from the
dependence of the drag to be independent of layer spacing and
correlated with the metalliclike behavior in the single layer resistivity,
suggesting they both arise from the same origin. In addition, layer spacing
dependence measurements suggest that while the screening properties of the
system remain relatively independent of temperature, they weaken significantly
as the carrier density is reduced. Finally, we demonstrate that the drag itself
significantly enhances the metallic dependence in the single layer
resistivity.Comment: 6 pages, 5 figures; revisions to text, to appear in Phys. Rev.
Zener Tunneling Between Landau Orbits in a High-Mobility Two-Dimensional Electron Gas
Magnetotransport in a laterally confined two-dimensional electron gas (2DEG)
can exhibit modified scattering channels owing to a tilted Hall potential.
Transitions of electrons between Landau levels with shifted guiding centers can
be accomplished through a Zener tunneling mechanism, and make a significant
contribution to the magnetoresistance. A remarkable oscillation effect in weak
field magnetoresistance has been observed in high-mobility 2DEGs in
GaAs-AlGaAs heterostructures, and can be well explained by the
Zener mechanism.Comment: 5 pages, 4 figures. Text slightly shortened, figures resize
Field-induced breakdown of the quantum Hall effect
A numerical analysis is made of the breakdown of the quantum Hall effect
caused by the Hall electric field in competition with disorder. It turns out
that in the regime of dense impurities, in particular, the number of localized
states decreases exponentially with the Hall field, with its dependence on the
magnetic and electric field summarized in a simple scaling law. The physical
picture underlying the scaling law is clarified. This intra-subband process,
the competition of the Hall field with disorder, leads to critical breakdown
fields of magnitude of a few hundred V/cm, consistent with observations, and
accounts for their magnetic-field dependence \propto B^{3/2} observed
experimentally. Some testable consequences of the scaling law are discussed.Comment: 7 pages, Revtex, 3 figures, to appear in Phys. Rev.
Universality at integer quantum Hall transitions
We report in this paper results of experimental and theoretical studies of
transitions between different integer quantum Hall phases, as well as
transition between the insulating phase and quantum Hall phases at high
magnetic fields. We focus mainly on universal properties of the transitions. We
demonstrate that properly defined conductivity tensor is universal at the
transitions. We also present numerical results of a non-interacting electron
model, which suggest that the Thouless conductance is universal at integer
quantum Hall transitions, just like the conductivity tensor. Finite temperature
and system size effects near the transition point are also studied.Comment: 20 pages, 15 figure
Reorientation of Anisotropy in a Square Well Quantum Hall Sample
We have measured magnetotransport at half-filled high Landau levels in a
quantum well with two occupied electric subbands. We find resistivities that
are {\em isotropic} in perpendicular magnetic field but become strongly {\em
anisotropic} at = 9/2 and 11/2 on tilting the field. The anisotropy
appears at an in-plane field, 2.5T, with the easy-current
direction {\em parallel} to but rotates by 90 at 10T and points now in the same direction as in single-subband samples.
This complex behavior is in quantitative agreement with theoretical
calculations based on a unidirectional charge density wave state model.Comment: 4 pages, 4 figure
Transition from an electron solid to the sequence of fractional quantum Hall states at very low Landau level filling factor
At low Landau level filling of a two-dimensional electron system, typically
associated with the formation of an electron crystal, we observe local minima
in Rxx at filling factors nu=2/11, 3/17, 3/19, 2/13, 1/7, 2/15, 2/17, and 1/9.
Each of these developing fractional quantum Hall (FQHE) states appears only
above a filling factor-specific temperature. This can be interpreted as the
melting of an electron crystal and subsequent FQHE liquid formation. The
observed sequence of FQHE states follow the series of composite fermion states
emanating from nu=1/6 and nu=1/8
Fractional Quantum Hall States in Narrow Channels
A model system is considered where two dimensional electrons are confined by
a harmonic potential in one direction, and are free in the other direction.
Ground state in strong magnetic fields is investigated through numerical
diagonalization of the Hamiltonian. It is shown that the fractional quantum
Hall states are realized even in the presence of the external potential under
suitable conditions, and a phase diagram is obtained.Comment: 8 pages, 2 figures (not included
A Fermi Fluid Description of the Half-Filled Landau Level
We present a many-body approach to calculate the ground state properties of a
system of electrons in a half-filled Landau level. Our starting point is a
simplified version of the recently proposed trial wave function where one
includes the antisymmetrization operator to the bosonic Laughlin state. Using
the classical plasma analogy, we calculate the pair-correlation function, the
static structure function and the ground state energy in the thermodynamic
limit. These results are in good agreement with the expected behavior at
.Comment: 4 pages, REVTEX, and 4 .ps file
Theory of Incompressible States in a Narrow Channel
We report on the properties of a system of interacting electrons in a narrow
channel in the quantum Hall effect regime. It is shown that an increase in the
strength of the Coulomb interaction causes abrupt changes in the width of the
charge-density profile of translationally invariant states. We derive a phase
diagram which includes many of the stable odd-denominator states as well as a
novel fractional quantum Hall state at lowest half-filled Landau level. The
collective mode evaluated at the half-filled case is strikingly similar to that
for an odd-denominator fractional quantum Hall state.Comment: 4 pages, REVTEX, and 4 .ps file
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