6,643 research outputs found
Formation of Embedded Nitride Semiconductor Nanocrystals.
In this thesis, the formation, phase selection, and spatial positioning of GaAs:N
(InAs:N) nanocomposite layers produced by N-implantation, focused ion beam (FIB)
irradiation, and rapid thermal annealing (RTA) of GaAs (InAs) were investigated.
To examine nanocrystal formation mechanisms, the influence of annealing
temperature and annealing time on phase formation in GaAs:N were examined. For RTA
times of 30 s, we observed the nucleation of zincblende (ZB) GaN at temperatures of 650
to 900ÂşC. For furnace anneal (FA) times of 10 min, wurtzite (WZ) GaN nucleation was
observed for anneal temperatures as low as 650ÂşC. In the case of InAs:N, for RTA at
500-550ÂşC ZB InN nanocrystals were nucleated. However, RTA temperatures of 600ÂşC
led to the nucleation of WZ InN nanocrystals with a larger average diameter. These
results indicated the key role of annealing time and temperature on crystallite nucleation.
A TTT diagram was developed for GaN nucleation in ion-implanted GaAs. The TTT
diagram provides an annealing schedule for the selective formation of ZB and WZ GaN.
The formation mechanisms for nucleation of ZB and WZ GaN (InN) were
investigated and a thermodynamic model for the preferential nucleation of the ZB phase
was proposed. ZB nanocrystals had a higher density of low-energy surface planes, which
drove the adoption of the ZB phase for sufficiently small nuclei. We demonstrated the
first nucleation of ZB and WZ InN in InAs using ion-implantation followed by thermal
annealing.
Finally, a new process for simultaneous nanostructuring and phase selection,
termed “directed matrix seeding,” was demonstrated. Broad-area N+ implantation of
GaAs followed by rapid thermal annealing led to the formation of nanocrystals at the
depth of maximum ion damage. With additional irradiation using a Ga+ focused ion
beam, selective lateral positioning of the nanocrystals within the GaAs matrix was
observed in isolated regions of increased vacancy concentration. Following rapid thermal
annealing, the formation of zincblende GaN was observed in the regions of highest
vacancy concentration. The directed matrix seeding process offers a method for precisely
controlling the phase and spatial location of embedded nitride nanostructures in a variety
of host materials.Ph.D.PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91523/1/adamww_1.pd
NASA Low-Speed Centrifugal Compressor for Fundamental Research
A centrifugal compressor facility being built by the NASA Lewis Research Center is described; its purpose is to obtain benchmark experimental data for internal flow code verification and modeling. The facility will be heavily instrumented with standard pressure and temperature probes and have provisions for flow visualization and laser Doppler velocimetry. The facility will accommodate rotational speeds to 2400 rpm and will be rated at pressures to 1.25 atm. The initial compressor stage for testing is geometrically and dynamically representative of modern high-performance stages with the exception of Mach number levels. Design exit tip speed for the initial stage is 500 ft/sec with a pressure ratio of 1.17. The rotor exit backsweep is 55 deg from radial
A Long-Term Hydrologically-Based Data Set of Land Surface Fluxes and States for the Conterminous United States
A frequently encountered difficulty in assessing model-predicted land–atmosphere exchanges of moisture and energy is the absence of comprehensive observations to which model predictions can be compared at the spatial and temporal resolutions at which the models operate. Various methods have been used to evaluate the land surface schemes in coupled models, including comparisons of model-predicted evapotranspiration with values derived from atmospheric water balances, comparison of model-predicted energy and radiative fluxes with tower measurements during periods of intensive observations, comparison of model-predicted runoff with observed streamflow, and comparison of model predictions of soil moisture with spatial averages of point observations. While these approaches have provided useful model diagnostic information, the observation-based products used in the comparisons typically are inconsistent with the model variables with which they are compared—for example, observations are for points or areas much smaller than the model spatial resolution, comparisons are restricted to temporal averages, or the spatial scale is large compared to that resolved by the model. Furthermore, none of the datasets available at present allow an evaluation of the interaction of the water balance components over large regions for long periods. In this study, a model-derived dataset of land surface states and fluxes is presented for the conterminous United States and portions of Canada and Mexico. The dataset spans the period 1950–2000, and is at a 3-h time step with a spatial resolution of ⅛ degree. The data are distinct from reanalysis products in that precipitation is a gridded product derived directly from observations, and both the land surface water and energy budgets balance at every time step. The surface forcings include precipitation and air temperature (both gridded from observations), and derived downward solar and longwave radiation, vapor pressure deficit, and wind. Simulated runoff is shown to match observations quite well over large river basins. On this basis, and given the physically based model parameterizations, it is argued that other terms in the surface water balance (e.g., soil moisture and evapotranspiration) are well represented, at least for the purposes of diagnostic studies such as those in which atmospheric model reanalysis products have been widely used. These characteristics make this dataset useful for a variety of studies, especially where ground observations are lacking
Characterizing Undetected Stellar Companions with Combined Datasets
Binaries play a critical role in the formation, evolution, and fundamental
properties of planets, stars, and stellar associations. Observational studies
in these areas often include a mix of observations aimed at detecting or ruling
out the presence of stellar companions. Rarely can non-detections rule out all
possible binary configurations. Here we present MOLUSC, our framework for
constraining the range of properties of unseen companions using astrometric,
imaging, and velocity information. We showcase the use of MOLUSC on a number of
systems, ruling out stellar false positives in the signals of HIP67522b, and DS
Tuc Ab. We also demonstrate how MOLUSC could be used to predict the number of
missing companions in a stellar sample using the ZEIT sample of young planet
hosts. Although our results are not significant, with a larger sample MOLUSC
could be used to see if close-in planets are less common in young binary
systems, as is seen for their older counterparts.Comment: 19 pages, 10 figures, 4 tables, Accepted to A
New Integrable Sectors in Skyrme and 4-dimensional CP^n Model
The application of a weak integrability concept to the Skyrme and
models in 4 dimensions is investigated. A new integrable subsystem of the
Skyrme model, allowing also for non-holomorphic solutions, is derived. This
procedure can be applied to the massive Skyrme model, as well. Moreover, an
example of a family of chiral Lagrangians providing exact, finite energy
Skyrme-like solitons with arbitrary value of the topological charge, is given.
In the case of models a tower of integrable subsystems is obtained. In
particular, in (2+1) dimensions a one-to-one correspondence between the
standard integrable submodel and the BPS sector is proved. Additionally, it is
shown that weak integrable submodels allow also for non-BPS solutions.
Geometric as well as algebraic interpretations of the integrability conditions
are also given.Comment: 23 page
CfAIR2: Near Infrared Light Curves of 94 Type Ia Supernovae
CfAIR2 is a large homogeneously reduced set of near-infrared (NIR) light
curves for Type Ia supernovae (SN Ia) obtained with the 1.3m Peters Automated
InfraRed Imaging TELescope (PAIRITEL). This data set includes 4607 measurements
of 94 SN Ia and 4 additional SN Iax observed from 2005-2011 at the Fred
Lawrence Whipple Observatory on Mount Hopkins, Arizona. CfAIR2 includes JHKs
photometric measurements for 88 normal and 6 spectroscopically peculiar SN Ia
in the nearby universe, with a median redshift of z~0.021 for the normal SN Ia.
CfAIR2 data span the range from -13 days to +127 days from B-band maximum. More
than half of the light curves begin before the time of maximum and the coverage
typically contains ~13-18 epochs of observation, depending on the filter. We
present extensive tests that verify the fidelity of the CfAIR2 data pipeline,
including comparison to the excellent data of the Carnegie Supernova Project.
CfAIR2 contributes to a firm local anchor for supernova cosmology studies in
the NIR. Because SN Ia are more nearly standard candles in the NIR and are less
vulnerable to the vexing problems of extinction by dust, CfAIR2 will help the
supernova cosmology community develop more precise and accurate extragalactic
distance probes to improve our knowledge of cosmological parameters, including
dark energy and its potential time variation.Comment: 31 pages, 15 figures, 10 tables. Accepted to ApJS. v2 modified to
more closely match journal versio
Baseline White Matter Hyperintensities and Hippocampal Volume are Associated With Conversion From Normal Cognition to Mild Cognitive Impairment in the Framingham Offspring Study.
INTRODUCTION: We examined associations between magnetic resonance imaging (MRI) markers of cerebrovascular disease and neurodegeneration with mild cognitive impairment (MCI) diagnosis at baseline and conversion from normal cognition to MCI at follow-up.
METHODS: Framingham Offspring participants underwent brain MRI and neuropsychological assessment at baseline (n=1049) and follow-up (n=561). Participants were classified at baseline and at follow-up as cognitively normal or MCI using sensitive neuropsychological criteria. White matter hyperintensity (WMH) volume, covert brain infarcts, hippocampal volume, and total cerebral brain volume were quantified.
RESULTS: Baseline measures of WMH and hippocampal volume were associated with MCI status cross-sectionally and also with conversion from normal cognition to MCI at 6.5-year follow-up. Annualized change rates in total cerebral brain volume and hippocampal volume were associated with conversion from normal cognition to MCI to follow-up.
DISCUSSION: Baseline WMH and hippocampal volume are markers that are both associated with conversion from normal cognition to MCI, highlighting the role of both vascular lesions and neurodegeneration in MCI
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Regional climate impacts of a possible future grand solar minimum.
This is the final published version. It first appeared at http://www.nature.com/ncomms/2015/150623/ncomms8535/full/ncomms8535.html.Any reduction in global mean near-surface temperature due to a future decline in solar activity is likely to be a small fraction of projected anthropogenic warming. However, variability in ultraviolet solar irradiance is linked to modulation of the Arctic and North Atlantic Oscillations, suggesting the potential for larger regional surface climate effects. Here, we explore possible impacts through two experiments designed to bracket uncertainty in ultraviolet irradiance in a scenario in which future solar activity decreases to Maunder Minimum-like conditions by 2050. Both experiments show regional structure in the wintertime response, resembling the North Atlantic Oscillation, with enhanced relative cooling over northern Eurasia and the eastern United States. For a high-end decline in solar ultraviolet irradiance, the impact on winter northern European surface temperatures over the late twenty-first century could be a significant fraction of the difference in climate change between plausible AR5 scenarios of greenhouse gas concentrations.This work was supported by the Joint DECC/Defra Met Office Hadley Centre Climate
Programme (GA01101) and also by the EU project SPECS funded by the European
Commission’s Seventh Framework Research Programme under the grant agreement
308378 (Met Office Hadley Centre authors), by the NERC National Centre for
Atmospheric Science (NCAS) Climate directorate (L.J.G. and A.C.M.), an ERC ACCI
grant (A.C.M) and an AXA Postdoctoral Fellowship (A.C.M.)
A Lithium Depletion Age for the Carina Association
The dispersed remnants of stellar nurseries, stellar associations provide
unparalleled samples of coeval stars critical for studies of stellar and
planetary formation and evolution. The Carina Stellar Association is one of the
closest stellar associations to Earth, and yet measurements of its age have
varied from 13 to 45 Myr. We aim to update the age of Carina using the Lithium
Depletion Boundary method. We obtain new measurements of the Li 6708 Angstrom,
absorption feature in likely members using optical spectra from the Goodman HTS
on SOAR and NRES on LCO. We detect the depletion boundary at M_K ~= 6.8 (M5),
which corresponds to an age of 41(+3,-5) Myr. The age is consistent within
uncertainties across six different models, including those that account for
magnetic fields and spots. We also estimate the age through analysis of the
group's overall variability, and by comparing the association members' CMD to
stellar evolutionary models using a Gaussian Mixture Model, recovering ages
consistent with the LDB. The resulting age agrees with the older end of
previous age measurements and is consistent with the lithium depletion age for
the neighboring Tucana-Horologium Moving Group.Comment: 9 pages, 6 figures, accepted to AJ on 10/17/202
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