2,003 research outputs found
Nonstoichiometric doping and Bi antisite defect in single crystal Bi2Se3
We studied the defects of Bi2Se3 generated from Bridgman growth of
stoichiometric and nonstoichiometric self-fluxes. Growth habit, lattice size,
and transport properties are strongly affected by the types of defect
generated. Major defect types of Bi_Se antisite and partial Bi_2-layer
intercalation are identified through combined studies of direct atomic-scale
imaging with scanning transmission electron microscopy (STEM) in conjunction
with energy-dispersive X-ray spectroscopy (STEM-EDX), X-ray diffraction, and
Hall effect measurements. We propose a consistent explanation to the origin of
defect type, growth morphology, and transport property.Comment: 5 pages, 5 figure
Virtual-pion and two-photon production in pp scattering
Two-photon production in pp scattering is proposed as a means of studying
virtual-pion emission. Such a process is complementary to real-pion emission in
pp scattering. The virtual-pion signal is embedded in a background of
double-photon bremsstrahlung. We have developed a model to describe this
background process and show that in certain parts of phase space the
virtual-pion signal gives significant contribution. In addition, through
interference with the two-photon bremsstrahlung background, one can determine
the relative phase of the virtual-pion process
Statistical Estimation of Orbital Debris Populations with a Spectrum of Object Size
Orbital debris is a real concern for the safe operations of satellites. In general, the hazard of debris impact is a function of the size and spatial distributions of the debris populations. To describe and characterize the debris environment as reliably as possible, the current NASA Orbital Debris Engineering Model (ORDEM2000) is being upgraded to a new version based on new and better quality data. The data-driven ORDEM model covers a wide range of object sizes from 10 microns to greater than 1 meter. This paper reviews the statistical process for the estimation of the debris populations in the new ORDEM upgrade, and discusses the representation of large-size (greater than or equal to 1 m and greater than or equal to 10 cm) populations by SSN catalog objects and the validation of the statistical approach. Also, it presents results for the populations with sizes of greater than or equal to 3.3 cm, greater than or equal to 1 cm, greater than or equal to 100 micrometers, and greater than or equal to 10 micrometers. The orbital debris populations used in the new version of ORDEM are inferred from data based upon appropriate reference (or benchmark) populations instead of the binning of the multi-dimensional orbital-element space. This paper describes all of the major steps used in the population-inference procedure for each size-range. Detailed discussions on data analysis, parameter definition, the correlation between parameters and data, and uncertainty assessment are included
A global 3-D CTM evaluation of black carbon in the Tibetan Plateau
We systematically evaluate the black carbon (BC) simulations for 2006 over
the Tibetan Plateau by a global 3-D chemical transport model (CTM)
(GEOS-Chem) driven by GEOS-5 assimilated meteorological fields, using in situ
measurements of BC in surface air, BC in snow, and BC absorption aerosol
optical depth (AAOD). Using improved anthropogenic BC emission inventories
for Asia that account for rapid technology renewal and energy consumption
growth (Zhang et al., 2009; Lu et al., 2011) and improved global biomass
burning emission inventories that account for small fires (van der Werf et
al., 2010; Randerson et al., 2012), we find that model results of both BC in
surface air and in snow are statistically in good agreement with
observations (biases < 15%) away from urban centers. Model
results capture the seasonal variations of the surface BC concentrations at
rural sites in the Indo-Gangetic Plain, but the observed elevated values in
winter are absent. Modeled surface-BC concentrations are within a factor of 2 of the observations at remote sites. Part of the discrepancy is
explained by the deficiencies of the meteorological fields over the complex
Tibetan terrain. We find that BC concentrations in snow computed from
modeled BC deposition and GEOS-5 precipitation are spatiotemporally
consistent with observations (<i>r</i> = 0.85). The computed BC concentrations in
snow are a factor of 2–4 higher than the observations at several Himalayan
sites because of excessive BC deposition. The BC concentrations in snow are
biased low by a factor of 2 in the central plateau, which we attribute to
the absence of snow aging in the CTM and strong local emissions unaccounted
for in the emission inventories. Modeled BC AAOD is more than a factor of
2 lower than observations at most sites, particularly to the northwest of
the plateau and along the southern slopes of the Himalayas in winter and
spring, which is attributable in large part to underestimated emissions and
the assumption of external mixing of BC aerosols in the model. We find that
assuming a 50% increase of BC absorption associated with internal mixing
reduces the bias in modeled BC AAOD by 57% in the Indo-Gangetic Plain and
the northeastern plateau and to the northeast of the plateau, and by 16%
along the southern slopes of the Himalayas and to the northwest of the
plateau. Both surface BC concentration and AAOD are strongly sensitive to
anthropogenic emissions (from China and India), while BC concentration in
snow is especially responsive to the treatment of BC aerosol aging. We find
that a finer model resolution (0.5° × 0.667° nested over Asia) reduces the bias in modeled surface-BC concentration from
15 to 2%. The large range and non-homogeneity of discrepancies
between model results and observations of BC across the Tibetan Plateau
undoubtedly undermine current assessments of the climatic and hydrological
impact of BC in the region and thus warrant imperative needs for more extensive
measurements of BC, including its concentration in surface air and snow,
AAOD, vertical profile and deposition
Spitzer/IRAC Limits to Planetary Companions of Fomalhaut and epsilon Eridani
Fomalhaut and epsilon Eridani are two young, nearby stars that possess
extended debris disks whose structures suggest the presence of perturbing
planetary objects. With its high sensitivity and stable point spread function,
Spitzer/IRAC is uniquely capable of detecting cool, Jupiter-like planetary
companions whose peak emission is predicted to occur near 4.5 um. We report on
deep IRAC imaging of these two stars, taken at 3.6 and 4.5 um using subarray
mode and in all four channels in wider-field full array mode. Observations
acquired at two different telescope roll angles allowed faint surrounding
objects to be separated from the stellar diffraction pattern. No companion
candidates were detected at the reported position of Fomalhaut b with 3 sigma
model-dependent mass upper limits of 3 MJ (for an age of 200 Myr). Around
epsilon Eridani we instead set a limit of 4 and <1 MJ (1 Gyr model age) at the
inner and outer edge of the sub-millimeter debris ring, respectively. These
results are consistent with non-detections in recent near-infrared imaging
searches, and set the strongest limits to date on the presence of planets
outside epsilon Eridani sub-millimeter ring.Comment: Accepted by The Astrophysical Journal. Request electronic-only plates
to M. Marengo ([email protected]
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