6,669 research outputs found
Bilayer Quantum Hall Systems at nuT = 1: Coulomb Drag and the Transition from Weak to Strong Interlayer Coupling
Measurements revealing anomalously large frictional drag at the transition between the weakly and strongly coupled regimes of a bilayer two-dimensional electron system at total Landau level filling factor nuT = 1 are reported. This result suggests the existence of fluctuations, either static or dynamic, near the phase boundary separating the quantized Hall state at small layer separations from the compressible state at larger separations. Interestingly, the anomalies in drag seem to persist to larger layer separations than does interlayer phase coherence as detected in tunneling
Double layer two-dimensional electron systems: Probing the transition from weak to strong coupling with Coulomb drag
Frictional drag measurements revealing anomalously large dissipation at the
transition between the weakly- and strongly-coupled regimes of a bilayer
two-dimensional electron system at total Landau level filling factor
are reported. This result suggests the existence of fluctuations, either static
or dynamic, near the phase boundary separating the quantized Hall state at
small layer separations from the compressible state at larger separations.
Interestingly, the anomalies in drag seem to persist to larger layer
separations than does interlayer phase coherence as detected in tunneling.Comment: 4 pages, 4 figure
A compact high-flux cold atom beam source
We report on an efficient and compact high-flux Cs atom beam source based on
a retro-reflected two-dimensional magneto-optical trap (2D MOT). We realize an
effective pushing field component by tilting the 2D MOT collimators towards a
separate three-dimensional magneto-optical trap (3D MOT) in ultra-high vacuum.
This technique significantly improved 3D MOT loading rates to greater than atoms/s using only 20 mW of total laser power for the source. When
operating below saturation, we achieve a maximum efficiency of atoms/s/W
Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System
At total Landau level filling factor a double layer
two-dimensional electron system with small interlayer separation supports a
collective state possessing spontaneous interlayer phase coherence. This state
exhibits the quantized Hall effect when equal electrical currents flow in
parallel through the two layers. In contrast, if the currents in the two layers
are equal, but oppositely directed, both the longitudinal and Hall resistances
of each layer vanish in the low temperature limit. This finding supports the
prediction that the ground state at is an excitonic superfluid.Comment: 4 pages, 4 figure
Onset of Interlayer Phase Coherence in a Bilayer Two-Dimensional Electron System: Effect of Layer Density Imbalance
Tunneling and Coulomb drag are sensitive probes of spontaneous interlayer
phase coherence in bilayer two-dimensional electron systems at total Landau
level filling factor . We find that the phase boundary between the
interlayer phase coherent state and the weakly-coupled compressible phase moves
to larger layer separations as the electron density distribution in the bilayer
is imbalanced. The critical layer separation increases quadratically with layer
density difference.Comment: 4 pages, 3 figure
Swift Observations of Hard X-ray Emitting White Dwarfs in Symbiotic Stars
The X-ray emission from most accreting white dwarfs (WDs) in symbiotic binary
stars is quite soft. Several symbiotic WDs, however, produce strong X-ray
emission at energies greater than ~20 keV. The Swift BAT instrument has
detected hard X-ray emission from 4 such accreting WDs in symbiotic stars: RT
Cru, T CrB, CD -57 3057, and CH Cyg. In one case (RT Cru), Swift detected
X-rays out to greater than 50 keV at a > 5 sigma confidence level. Combining
data from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RT
Cru, T CrB, and CD -57 3057 are well described by emission from a
single-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9
keV Fe line complex. The X-ray spectrum of CH Cyg contains an additional bright
soft component. For all 4 systems, the spectra suffer high levels of absorption
from material that both fully and partially covers the source of hard X-rays.
The XRT data did not show any of the rapid, periodic variations that one would
expect if the X-ray emission were due to accretion onto a rotating, highly
magnetized WD. The X-rays were thus more likely from the accretion-disk
boundary layer around a massive, non-magnetic WD in each binary. The X-ray
emission from RT Cru varied on timescales of a few days. This variability is
consistent with being due to changes in the absorber that partially covers the
source, suggesting localized absorption from a clumpy medium moving into the
line of sight. The X-ray emission from CD -57 3057 and T CrB also varied during
the 9 months of Swift observations, in a manner that was also consistent with
variable absorption.Comment: Accepted for publication in ApJ. 9 pages, 6 figure
Spacecraft charging and ion wake formation in the near-Sun environment
A three-dimensional (3-D), self-consistent code is employed to solve for the
static potential structure surrounding a spacecraft in a high photoelectron
environment. The numerical solutions show that, under certain conditions, a
spacecraft can take on a negative potential in spite of strong photoelectron
currents. The negative potential is due to an electrostatic barrier near the
surface of the spacecraft that can reflect a large fraction of the
photoelectron flux back to the spacecraft. This electrostatic barrier forms if
(1) the photoelectron density at the surface of the spacecraft greatly exceeds
the ambient plasma density, (2) the spacecraft size is significantly larger
than local Debye length of the photoelectrons, and (3) the thermal electron
energy is much larger than the characteristic energy of the escaping
photoelectrons. All of these conditions are present near the Sun. The numerical
solutions also show that the spacecraft's negative potential can be amplified
by an ion wake. The negative potential of the ion wake prevents secondary
electrons from escaping the part of spacecraft in contact with the wake. These
findings may be important for future spacecraft missions that go nearer to the
Sun, such as Solar Orbiter and Solar Probe Plus.Comment: 25 pages, 7 figures, accepted for publication in Physics of Plasma
Stability of the Excitonic Phase in Bilayer Quantum Hall Systems at Total Filling One -- Effects of Finite Well Width and Pseudopotentials --
The ground state of a bilayer quantum Hall system at with
model pseudopotential is investigated by the DMRG method. Firstly,
pseudopotential parameters appropriate for the system with finite layer
thickness are derived, and it is found that the finite thickness makes the
excitonic phase more stable. Secondly, a model, where only a few
pseudopotentials with small relative angular momentum have finite values, is
studied, and it is clarified how the excitonic phase is destroyed as
intra-layer pseudopotential becomes larger. The importance of the intra-layer
repulsive interaction at distance twice of the magnetic length for the
destruction of the excitonic phase is found.Comment: 7 pages, 7 figure
Magnetic Surfaces in Stationary Axisymmetric General Relativity
In this paper a new method is derived for constructing electromagnetic
surface sources for stationary axisymmetric electrovac spacetimes endowed with
non-smooth or even discontinuous
Ernst potentials. This can be viewed as a generalization of some classical
potential theory results, since lack of continuity of the potential is related
to dipole density and lack of smoothness, to monopole density. In particular
this approach is useful for constructing the dipole source for the magnetic
field. This formalism involves solving a linear elliptic differential equation
with boundary conditions at infinity. As an example, two different models of
surface densities for the Kerr-Newman electrovac spacetime are derived.Comment: 15 page
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