5,652 research outputs found
Measurement of the electronic compressibility of bilayer graphene
We present measurements of the electronic compressibility, , of bilayer
graphene in both zero and finite magnetic fields up to 14 T, and as a function
of both the carrier density and electric field perpendicular to the graphene
sheet. The low energy hyperbolic band structure of bilayer graphene is clearly
revealed in the data, as well as a sizable asymmetry between the conduction and
valence bands. A sharp increase in near zero density is observed for
increasing electric field strength, signaling the controlled opening of a gap
between these bands. At high magnetic fields, broad Landau level (LL)
oscillations are observed, directly revealing the doubled degeneracy of the
lowest LL and allowing for a determination of the disorder broadening of the
levels.Comment: 5 pages, 3 figures; final version for publicatio
The Baryonic Fraction in Groups of Galaxies from X-Ray Measurements
The recent {\sl ROSAT \/} X-ray detections of hot intergalactic gas in three
groups of galaxies are reviewed and the resulting baryonic fraction in these
groups is reevaluated. We show that the baryonic fraction obtained, assuming
hydrostatic equilibrium, should depend, perhaps sensitively, on the radius out
to which the X-rays are detected, and the temperature profile of the gas. We
find that the NGC 2300 group has a baryonic fraction out to of at least
20\%, thus over five times higher than in the original analysis of Mulchaey
\etal\ (1993), and also much higher than one would obtain from big-bang
nucleosynthesis, but similar to the other two groups as well as rich clusters.
With this baryonic fraction, groups would be fair tracers of the distribution
of baryons in the Universe if . A baryonic fraction that
increases with radius is consistent with the X-ray data from all three groups.
However, a detailed analysis of the NGC 2300 group shows that the dependence of
baryonic fraction on radius is not well constrained by the data, in part
because of uncertainties in the estimated background.Comment: ApJ Lett 421 (Feb 1 1994), in press. 4 pages of uuencoded compressed
Postscript (extract on UNIX with 'csh' after removing header: 1st line should
be '#/bin/csh -f') with 3 figures. POP-DAEC-9306
Diffuse Non-thermal X-ray Emission: Evidence for Cosmic-ray Acceleration at the Shock Front in IC1262
We report the first localization of diffuse, non-thermal, X-ray emission from
a nearby galaxy cluster. Using Chandra data, we have isolated a diffuse
non-thermal X-ray component with a photon index, Gamma_ X = 2.21 +0.14 -0.15
and a flux of 9.5 +1.1 -2.5 x 10^-5 photons cm^-2 s^-1 keV^-1 at 1 keV, that
extends from ~1'.5 to ~2'.5 to the south of the X-ray flux peak. Comparison to
simulations implies that the diffuse non-thermal emission is produced by
primary electrons, accelerated at shocks to relativistic velocities. Using
these results and the flux and hardness maps produced with data from the
Chandra Advanced CCD Imaging Spectrometer, we conclude that a smaller subclump
coming from the north merged with IC1262. The offset of the cD galaxy from the
X-ray peak and large peculiar velocity indicate that the subclump's impact
parameter was to the west and on the near side of IC1262.Comment: 5 pages, 4 figures, 2 tables. Accepted by ApJ
Non-isothermal X-ray Emitting Gas in Clusters of Galaxies
We have analyzed X-ray spectra from six galaxy clusters which contain cooling
flows: A85, A478, A1795, A2142, A2147, & A2199. The X-ray spectra were taken
with the HEAO1-A2 Medium and High Energy Detectors and the Einstein Solid State
Spectrometer. For each cluster, we simultaneously fit the spectra from these
three detectors with models incorporating one or more emission components
comprised of either thermal or cooling flow models. Five of the clusters (all
but A2142) are better fit by a multi-component model (a cooling flow plus one
or two thermal components or a two thermal component model) than by isothermal
models. In four of the clusters (A85, A1795, A2147, & A2199), we find evidence
for cool gas outside of the canonical cooling flow region. These latter four
clusters can be characterized by three temperature components: a temperature
inversion in the central region, a hotter region with an emission-weighted
temperature which is higher than that of an isothermal model fit to the entire
cluster, and a cooler region with an emission-weighted temperature of ~1 keV.
The cool component outside the cooling flow region has a large minimum emission
measure which we attribute, in part, to diffuse cool gas in the outer cluster
atmosphere. If at least some of the cool exterior gas is virialized, this would
imply a radially decreasing temperature profile. Together with the density
profiles we have found, this leads to a baryon fraction in gas which increases
with radius and is larger than that for an isothermal cluster atmosphere.
Consequently, if clusters of galaxies trace the mass distribution in the
Universe, the gas mass fraction we have calculated for an isothermal gas (which
is ~15%) together with the nominal galaxy contribution (~5%) gives a baryon
fraction of ~20%. Using the upper limit to the baryon density derived from BigComment: gzipped tar file of 26 PostScript pages, including 2 figures, 7
tables. Also available at
http://www.astr.ua.edu/preprints/white/INDEX_READ_ME_1st.htm
Spectral constraints on models of gas in clusters of galaxies
The HEAO 1A2 spectra of clusters of galaxies are used to determine the temperature profile which characterizes the X-ray emitting gas. Strong evidence of nonisothermality is found for the Coma, A85, and A1795 clusters. Properties of the cluster potential which binds the gas are calculated for a range of model parameters. The typical binding mass, if the gas is adiabatic, is 2-4E14 solar masses and is quite centrally concentrated. In addition, the Fe abundance in Coma is .26 + or - .06 solar, less than the typical value (.5) found for rich clusters. The results for the gas in Coma may imply a physical description of the cluster which is quite different from what was previously believed
Quantum Hall Effect and Semimetallic Behavior of Dual-Gated ABA-Stacked Trilayer Graphene
The electronic structure of multilayer graphenes depends strongly on the
number of layers as well as the stacking order. Here we explore the electronic
transport of purely ABA-stacked trilayer graphenes in a dual-gated field-effect
device configuration. We find that both the zero-magnetic-field transport and
the quantum Hall effect at high magnetic fields are distinctly different from
the monolayer and bilayer graphenes, and that they show electron-hole
asymmetries that are strongly suggestive of a semimetallic band overlap. When
the ABA trilayers are subjected to an electric field perpendicular to the
sheet, Landau level splittings due to a lifting of the valley degeneracy are
clearly observed.Comment: 5 figure
Possible mechanism for changes in glycogen metabolism in unloaded soleus muscle
Carbohydrate metabolism has been shown to be affected in a number of ways by different models of hypokinesia. In vivo glycogen levels in the soleus muscle are known to be increased by short-term denervation and harness suspension. In addition, exposure to 7 days of hypogravity also caused a dramatic increase in glycogen concentration in this muscle. The biochemical alterations caused by unloading that may bring about these increases in glycogen storage in the soleus were sought
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