13,383 research outputs found
From Point Defects in Graphene to Two-Dimensional Amorphous Carbon
While crystalline two-dimensional materials have become an experimental
reality during the past few years, an amorphous 2-D material has not been
reported before. Here, using electron irradiation we create an sp2-hybridized
one-atom-thick flat carbon membrane with a random arrangement of polygons,
including four-membered carbon rings. We show how the transformation occurs
step-by-step by nucleation and growth of low-energy multi-vacancy structures
constructed of rotated hexagons and other polygons. Our observations, along
with first-principles calculations, provide new insights to the bonding
behavior of carbon and dynamics of defects in graphene. The created domains
possess a band gap, which may open new possibilities for engineering
graphene-based electronic devices.Comment: 10 pages, 10 figures including supplementary informatio
The X-ray Fundamental Plane and Relation of Clusters of Galaxies
We analyze the relations among central gas density, core radius, and
temperature of X-ray clusters by plotting the observational data in the
three-dimensional (, , and ) space and find that
the data lie on a 'fundamental plane'. Its existence implies that the clusters
form a two-parameter family. The data on the plane still has a correlation and
form a band on the plane. The observed relation turns
out to be the cross section of the band perpendicular to the major axis, while
the major axis is found to describe the virial density. We discuss implications
of this two-parameter family nature of X-ray clusters.Comment: 7 pages, 2 figures. To be published in ApJ Letter
Continuous-flow IRMS technique for determining the 17O excess of CO2 using complete oxygen isotope exchange with cerium oxide
This paper presents an analytical system for analysis of all single
substituted isotopologues (<sup>12</sup>C<sup>16</sup>O<sup>17</sup>O,
<sup>12</sup>C<sup>16</sup>O<sup>18</sup>O, <sup>13</sup>C<sup>16</sup>O<sup>16</sup>O) in nanomolar quantities
of CO<sub>2</sub> extracted from stratospheric air samples. CO<sub>2</sub> is
separated from bulk air by gas chromatography and CO<sub>2</sub> isotope ratio
measurements (ion masses 45 / 44 and 46 / 44) are performed using isotope ratio
mass spectrometry (IRMS). The <sup>17</sup>O excess (Î<sup>17</sup>O) is
derived from isotope measurements on two different CO<sub>2</sub> aliquots:
unmodified CO<sub>2</sub> and CO<sub>2</sub> after complete oxygen isotope exchange with
cerium oxide (CeO<sub>2</sub>) at 700 °C. Thus, a single measurement of
Î<sup>17</sup>O requires two injections of 1 mL of air with a CO<sub>2</sub>
mole fraction of 390 Îźmol mol<sup>â1</sup> at 293 K and 1 bar pressure
(corresponding to 16 nmol CO<sub>2</sub> each). The required sample size
(including flushing) is 2.7 mL of air. A single analysis (one pair of
injections) takes 15 minutes. The analytical system is fully automated for
unattended measurements over several days. The standard deviation of the
<sup>17</sup>O excess analysis is 1.7‰. Multiple
measurements on an air sample reduce the measurement uncertainty, as
expected for the statistical standard error. Thus, the uncertainty for a
group of 10 measurements is 0.58‰ for Δ
<sup>17</sup>O in 2.5 h of analysis. 100 repeat analyses of one air sample
decrease the standard error to 0.20‰. The instrument
performance was demonstrated by measuring CO<sub>2</sub> on stratospheric air
samples obtained during the EU project RECONCILE with the high-altitude
aircraft Geophysica. The precision for RECONCILE data is 0.03‰ (1σ) for δ<sup>13</sup>C, 0.07‰ (1σ) for δ<sup>18</sup>O and 0.55‰ (1σ) for δ<sup>17</sup>O for a sample of 10
measurements. This is sufficient to examine stratospheric enrichments, which
at altitude 33 km go up to 12‰ for δ<sup>17</sup>O
and up to 8‰ for δ<sup>18</sup>O with respect to
tropospheric CO<sub>2</sub> : δ<sup>17</sup>O ~
21‰ Vienna Standard Mean Ocean Water (VSMOW), δ<sup>18</sup>O ~
41‰ VSMOW (Lämmerzahl et al., 2002). The samples
measured with our analytical technique agree with available data for
stratospheric CO<sub>2</sub>
Direct measurements of the polarization of terrestrial kilometric radiation from Voyagers 1 and 2
Terrestrial radiation measurements obtained with planetary radio astronomy experiments on Voyager-1 and 2 during the early portions of each flight show the signals to be predominantly left-hand circularly polarized. Since these emissions were most probably generated above the Northern Hemisphere auroral zone, it is concluded that the radiation is emitted primarily in the extraordinary mode
Feasibility of Experimental Realization of Entangled Bose-Einstein Condensation
We examine the practical feasibility of the experimental realization of the
so-called entangled Bose-Einstein condensation (BEC), occurring in an entangled
state of two atoms of different species. We demonstrate that if the energy gap
remains vanishing, the entangled BEC persists as the ground state of the
concerned model in a wide parameter regime. We establish the experimental
accessibility of the isotropic point of the effective parameters, in which the
entangled BEC is the exact ground state, as well as the consistency with the
generalized Gross-Pitaevskii equations. The transition temperature is
estimated. Possible experimental implementations are discussed in detail.Comment: 6 pages, published versio
Probing Dark Energy with Baryonic Acoustic Oscillations from Future Large Galaxy Redshift Surveys
We show that the measurement of the baryonic acoustic oscillations in large
high redshift galaxy surveys offers a precision route to the measurement of
dark energy. The cosmic microwave background provides the scale of the
oscillations as a standard ruler that can be measured in the clustering of
galaxies, thereby yielding the Hubble parameter and angular diameter distance
as a function of redshift. This, in turn, enables one to probe dark energy. We
use a Fisher matrix formalism to study the statistical errors for redshift
surveys up to z=3 and report errors on cosmography while marginalizing over a
large number of cosmological parameters including a time-dependent equation of
state. With redshifts surveys combined with cosmic microwave background
satellite data, we achieve errors of 0.037 on Omega_x, 0.10 on w(z=0.8), and
0.28 on dw(z)/dz for cosmological constant model. Models with less negative
w(z) permit tighter constraints. We test and discuss the dependence of
performance on redshift, survey conditions, and fiducial model. We find results
that are competitive with the performance of future supernovae Ia surveys. We
conclude that redshift surveys offer a promising independent route to the
measurement of dark energy.Comment: submitted to ApJ, 24 pages, LaTe
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