14,616 research outputs found
Quantum Hall Phase Diagram of Second Landau-level Half-filled Bilayers: Abelian versus Non-Abelian States
The quantum Hall phase diagram of the half-filled bilayer system in the
second Landau level is studied as a function of tunneling and layer separation
using exact diagonalization. We make the striking prediction that bilayer
structures would manifest two distinct branches of incompressible fractional
quantum Hall effect (FQHE) corresponding to the Abelian 331 state (at moderate
to low tunneling and large layer separation) and the non-Abelian Pfaffian state
(at large tunneling and small layer separation). The observation of these two
FQHE branches and the quantum phase transition between them will be compelling
evidence supporting the existence of the non-Abelian Pfaffian state in the
second Landau level.Comment: 4 pages, 3 figure
Orbital Landau level dependence of the fractional quantum Hall effect in quasi-two dimensional electron layers: finite-thickness effects
The fractional quantum Hall effect (FQHE) in the second orbital Landau level
at filling factor 5/2 remains enigmatic and motivates our work. We consider the
effect of the quasi-2D nature of the experimental FQH system on a number of FQH
states (fillings 1/3, 1/5, 1/2) in the lowest, second, and third Landau levels
(LLL, SLL, TLL,) by calculating the overlap, as a function of quasi-2D layer
thickness, between the exact ground state of a model Hamiltonian and the
consensus variational wavefunctions (Laughlin wavefunction for 1/3 and 1/5 and
the Moore-Read Pfaffian wavefunction for 1/2). Using large overlap as a
stability, or FQHE robustness, criterion we find the FQHE does not occur in the
TLL (for any thickness), is the most robust for zero thickness in the LLL for
1/3 and 1/5 and for 11/5 in the SLL, and is most robust at finite-thickness
(4-5 magnetic lengths) in the SLL for the mysterious 5/2 state and the 7/3
state. No FQHE is found at 1/2 in the LLL for any thickness. We examine the
orbital effects of an in-plane (parallel) magnetic field finding its
application effectively reduces the thickness and could destroy the FQHE at 5/2
and 7/3, while enhancing it at 11/5 as well as for LLL FQHE states. The
in-plane field effects could thus be qualitatively different in the LLL and the
SLL by virtue of magneto-orbital coupling through the finite thickness effect.
In the torus geometry, we show the appearance of the threefold topological
degeneracy expected for the Pfaffian state which is enhanced by thickness
corroborating our findings from overlap calculations. Our results have
ramifications for wavefunction engineering--the possibility of creating an
optimal experimental system where the 5/2 FQHE state is more likely described
by the Pfaffian state with applications to topological quantum computing.Comment: 27 pages, 20 figures, revised version (with additional author) as
accepted for publication in Physical Review
Subband Engineering Even-Denominator Quantum Hall States
Proposed even-denominator fractional quantum Hall effect (FQHE) states
suggest the possibility of excitations with non-Abelian braid statistics.
Recent experiments on wide square quantum wells observe even-denominator FQHE
even under electrostatic tilt. We theoretically analyze these structures and
develop a procedure to accurately test proposed quantum Hall wavefunctions. We
find that tilted wells favor partial subband polarization to yield Abelian
even-denominator states. Our results show that tilting quantum wells
effectively engineers different interaction potentials allowing exploration of
a wide variety of even-denominator states
Fractional quantum Hall effects in bilayers in the presence of inter-layer tunneling and charge imbalance
Two-component fractional quantum Hall systems are providing a major
motivation for a large section of the physics community. Here we study
two-component fractional quantum Hall systems in the spin-polarized half-filled
lowest Landau level (filling factor 1/2) and second Landau level (filling
factor 5/2) with exact diagonalization utilizing both the spherical and torus
geometries. The two distinct two-component systems we consider are the true
bilayer and effective bilayers (wide-quantum-well). In each model (bilayer and
wide-quantum-well) we completely take into account inter-layer tunneling and
charge imbalancing terms. We find that in the half-filled lowest Landau level,
the FQHE is described by the two-component Abelian Halperin 331 state which is
remarkably robust to charge imbalancing. In the half-filled second Landau, we
find that the FQHE is likely described by the non-Abelian Moore-Read Pfaffian
state which is also quite robust to charge imbalancing. Furthermore, we suggest
the possibility of experimentally tuning from an Abelian to non-Abelian FQHE
state in the second Landau level, and comment on recent experimental studies of
FQHE in wide quantum well structures.Comment: 25 pages, 27 figure
Brownian dynamics around the core of self-gravitating systems
We derive the non-Maxwellian distribution of self-gravitating -body
systems around the core by a model based on the random process with the
additive and the multiplicative noise. The number density can be obtained
through the steady state solution of the Fokker-Planck equation corresponding
to the random process. We exhibit that the number density becomes equal to that
of the King model around the core by adjusting the friction coefficient and the
intensity of the multiplicative noise. We also show that our model can be
applied in the system which has a heavier particle. Moreover, we confirm the
validity of our model by comparing with our numerical simulation.Comment: 11 pages, 4 figure
First limits on the 3-200 keV X-ray spectrum of the quiet Sun using RHESSI
We present the first results using the Reuven Ramaty High-Energy Solar
Spectroscopic Imager, RHESSI, to observe solar X-ray emission not associated
with active regions, sunspots or flares (the quiet Sun). Using a newly
developed chopping technique (fan-beam modulation) during seven periods of
offpointing between June 2005 to October 2006, we obtained upper limits over
3-200 keV for the quietest times when the GOES12 1-8A flux fell below
Wm. These values are smaller than previous limits in the 17-120 keV
range and extend them to both lower and higher energies. The limit in 3-6 keV
is consistent with a coronal temperature MK. For quiet Sun periods
when the GOES12 1-8A background flux was between Wm and
Wm, the RHESSI 3-6 keV flux correlates to this as a power-law,
with an index of . The power-law correlation for microflares has
a steeper index of . We also discuss the possibility of
observing quiet Sun X-rays due to solar axions and use the RHESSI quiet Sun
limits to estimate the axion-to-photon coupling constant for two different
axion emission scenarios.Comment: 4 pages, 3 figures, Accepted by ApJ letter
Reverberation Mapping and the Physics of Active Galactic Nuclei
Reverberation-mapping campaigns have revolutionized our understanding of AGN.
They have allowed the direct determination of the broad-line region size,
enabled mapping of the gas distribution around the central black hole, and are
starting to resolve the continuum source structure. This review describes the
recent and successful campaigns of the International AGN Watch consortium,
outlines the theoretical background of reverberation mapping and the
calculation of transfer functions, and addresses the fundamental difficulties
of such experiments. It shows that such large-scale experiments have resulted
in a ``new BLR'' which is considerably different from the one we knew just ten
years ago. We discuss in some detail the more important new results, including
the luminosity-size-mass relationship for AGN, and suggest ways to proceed in
the near future.Comment: Review article to appear in Astronomical Time Series, Proceedings of
the Wise Observatory 25th Ann. Symposium. 24 pages including 7 figure
A performance comparison of the contiguous allocation strategies in 3D mesh connected multicomputers
The performance of contiguous allocation strategies can be significantly affected by the distribution of job execution times. In this paper, the performance of the existing contiguous allocation strategies for 3D mesh multicomputers is re-visited in the context of heavy-tailed distributions (e.g., a Bounded Pareto distribution). The strategies are evaluated and compared using simulation experiments for both First-Come-First-Served (FCFS) and Shortest-Service-Demand (SSD) scheduling strategies under a variety of system loads and system sizes. The results show that the performance of the allocation strategies degrades considerably when job execution times follow a heavy-tailed distribution. Moreover, SSD copes much better than FCFS scheduling strategy in the presence of heavy-tailed job execution times. The results also show that the strategies that depend on a list of allocated sub-meshes for both allocation and deallocation have lower allocation overhead and deliver good system performance in terms of average turnaround time and mean system utilization
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