2,737 research outputs found
Abell 754: A Non-Head On Collision of Subclusters
We have analyzed spatially resolved spectra of A754 obtained with ASCA. The
combination of spectral and imaging capabilities of ASCA has set unprecedented
constraints on the hydrodynamical effects of a cluster merger. We find
significant gas temperature variations over the cluster face, indicating shock
heating of the atmosphere during the merger. The hottest region, >12 keV, is
located in the region of the Northwest galaxy clump though the entire region
along the cluster axis appears to be hotter than the mean cluster temperature
(~9 keV). The cool, ~5 keV gas originally found with the HEAO1-A2 experiment,
resides in the exterior of the cluster atmosphere and in plume of gas we
identify with a stripped cool atmosphere of the infalling subcluster. We have
also attempted to reconstruct an iron abundance map of this merging system.
Though poorly constrained, no significant deviations of abundance from the mean
value are apparent in the individual regions.
A754 is the only cluster so far which shows the significant temperature
pattern expected in a subcluster merger, in both the ROSAT (Henry & Briel 1995)
and ASCA data, providing the first possibility to compare it with theoretical
predictions. The observed temperature and surface brightness maps suggest that
the two colliding subunits have missed each other by about 1 Mpc, and are now
moving perpendicular to the cluster axis in the image plane (as, e.g., in the
simulations by Evrard etal 1996).Comment: Latex, 10 pages, 3 figures incl. color plate, uses aaspp4.sty,
flushrt.sty and pstricks.sty. Submitted to ApJ Letter
Disorder mediated splitting of the cyclotron resonance in two-dimensional electron systems
We perform a direct study of the magnitude of the anomalous splitting in the
cyclotron resonance (CR) of a two-dimensional electron system (2DES) as a
function of sample disorder. In a series of AlGaAs/GaAs quantum wells,
identical except for a range of carbon doping in the well, we find the CR
splitting to vanish at high sample mobilities but to increase dramatically with
increasing impurity density and electron scattering rates. This observation
lends strong support to the conjecture that the non-zero wavevector, roton-like
minimum in the dispersion of 2D magnetoplasmons comes into resonance with the
CR, with the two modes being coupled via disorder.Comment: accepted to PRB Rapid Com
Acoustic phonon scattering in a low density, high mobility AlGaN/GaN field effect transistor
We report on the temperature dependence of the mobility, , of the
two-dimensional electron gas in a variable density AlGaN/GaN field effect
transistor, with carrier densities ranging from 0.4 cm to
3.0 cm and a peak mobility of 80,000 cm/Vs. Between
20 K and 50 K we observe a linear dependence T
indicating that acoustic phonon scattering dominates the temperature dependence
of the mobility, with being a monotonically increasing function of
decreasing 2D electron density. This behavior is contrary to predictions of
scattering in a degenerate electron gas, but consistent with calculations which
account for thermal broadening and the temperature dependence of the electron
screening. Our data imply a deformation potential D = 12-15 eV.Comment: 3 pages, 2 figures, RevTeX. Submitted to Appl Phys Let
Interaction-induced shift of the cyclotron resonance of graphene using infrared spectroscopy
We report a study of the cyclotron resonance (CR) transitions to and from the
unusual Landau level (LL) in monolayer graphene. Unexpectedly, we find
the CR transition energy exhibits large (up to 10%) and non-monotonic shifts as
a function of the LL filling factor, with the energy being largest at
half-filling of the level. The magnitude of these shifts, and their
magnetic field dependence, suggests that an interaction-enhanced energy gap
opens in the level at high magnetic fields. Such interaction effects
normally have limited impact on the CR due to Kohn's theorem [W. Kohn, Phys.
Rev. {\bf 123}, 1242 (1961)], which does not apply in graphene as a consequence
of the underlying linear band structure.Comment: 4 pages, 4 figures. Version 2, edited for publication. Includes a
number of edits for clarity; also added a paragraph contrasting our work w/
previous CR expts. in 2D Si and GaA
The final fate of spherical inhomogeneous dust collapse II: Initial data and causal structure of singularity
Further to results in [9], pointing out the role of initial density and
velocity distributions towards determining the final outcome of spherical dust
collapse, the causal structure of singularity is examined here in terms of
evolution of the apparent horizon. We also bring out several related features
which throw some useful light towards understanding the nature of this
singularity, including the behaviour of geodesic families coming out and some
aspects related to the stability of singularity.Comment: Latex file, uses epsf.sty, 15 pages and 3 eps figures. Paragraph on
role of smooth functions rewritten. Four references added. To appear in
Classical & Quantum Gravit
On Universal Halos and the Radial Orbit Instability
The radial orbit instability drives dark matter halos toward a universal
structure. This conclusion, first noted by Huss, Jain, and Steinmetz, is
explored in detail through a series of numerical experiments involving the
collapse of an isolated halo into the non-linear regime. The role played by the
radial orbit instability in generating the density profile, shape, and orbit
structure is carefully analyzed and, in all cases, the instability leads to
universality independent of initial conditions. New insights into the
underlying physics of the radial orbit instability are presented.Comment: 31 pages, 11 figures, submitted to the Astrophysical Journa
Electrostatically confined Quantum Rings in bilayer Graphene
We propose a new system where electron and hole states are electrostatically
confined into a quantum ring in bilayer graphene. These structures can be
created by tuning the gap of the graphene bilayer using nanostructured gates or
by position-dependent doping. The energy levels have a magnetic field ()
dependence that is strikingly distinct from that of usual semiconductor quantum
rings. In particular, the eigenvalues are not invariant under a
transformation and, for a fixed total angular momentum index , their field
dependence is not parabolic, but displays two minima separated by a saddle
point. The spectra also display several anti-crossings, which arise due to the
overlap of gate-confined and magnetically-confined states.Comment: 5 pages, 6 figures, to appear in Nano Letter
Cyclotron motion in graphene
We investigate cyclotron motion in graphene monolayers considering both the
full quantum dynamics and its semiclassical limit reached at high carrier
energies. Effects of zitterbewegung due to the two dispersion branches of the
spectrum dominate the irregular quantum motion at low energies and are obtained
as a systematic correction to the semiclassical case. Recent experiments are
shown to operate in the semiclassical regime.Comment: 6 pages, 1 figure include
Lignocellulose Conversion via Catalytic Transformations Yields Methoxyterephthalic Acid Directly from Sawdust
Poly(ethylene terephthalate) polyester represents the most common class of thermoplastic polymers widely used in the textile, bottling, and packaging industries. Terephthalic acid and ethylene glycol, both of petrochemical origin, are polymerized to yield the polyester. However, an earlier report suggests that polymerization of methoxyterephthalic acid with ethylene glycol provides a methoxy-polyester with similar properties. Currently, there are no established biobased synthetic routes toward the methoxyterephthalic acid monomer. Here, we show a viable route to the dicarboxylic acid from various tree species involving three catalytic steps. We demonstrate that sawdust can be converted to valuable aryl nitrile intermediates through hydrogenolysis, followed by an efficient fluorosulfation–catalytic cyanation sequence (>90%) and then converted to methoxyterephthalic acid by hydrolysis and oxidation. A preliminary polymerization result indicates a methoxy-polyester with acceptable thermal properties
Tracing the Mass during Low-Mass Star Formation. II. Modelling the Submillimeter Emission from Pre-Protostellar Cores
We have modeled the emission from dust in pre-protostellar cores, including a
self-consistent calculation of the temperature distribution for each input
density distribution. Model density distributions include Bonnor-Ebert spheres
and power laws. The Bonnor-Ebert spheres fit the data well for all three cores
we have modeled. The dust temperatures decline to very low values (\Td \sim 7
K) in the centers of these cores, strongly affecting the dust emission.
Compared to earlier models that assume constant dust temperatures, our models
indicate higher central densities and smaller regions of relatively constant
density. Indeed, for L1544, a power-law density distribution, similar to that
of a singular, isothermal sphere, cannot be ruled out. For the three sources
modeled herein, there seems to be a sequence of increasing central
condensation, from L1512 to L1689B to L1544. The two denser cores, L1689B and
L1544, have spectroscopic evidence for contraction, suggesting an evolutionary
sequence for pre-protostellar cores.Comment: 22 pages, 9 figures, Ap. J. accepted, uses emulateapj5.st
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