444 research outputs found
A Unified Measure of Audio System Fidelity
A new technique to qualitatively measure distortion in dynamically controlled audio systems using non-stationary noise sequences is explored and compared to traditional methods based upon stationary test signals. This technique can easily be adapted to give a qualitative measure of distortion as a function of the perceived Sound Pressure Level (SPL)
A Unified Measure of Audio System Fidelity
A new technique to qualitatively measure distortion in dynamically controlled audio systems using non-stationary noise sequences is explored and compared to traditional methods based upon stationary test signals. This technique can easily be adapted to give a qualitative measure of distortion as a function of the perceived Sound Pressure Level (SPL)
Polarization selection rules for inter-Landau level transitions in epitaxial graphene revealed by infrared optical Hall effect
We report on polarization selection rules of inter-Landau level transitions
using reflection-type optical Hall effect measurements from 600 to 4000 cm-1 on
epitaxial graphene grown by thermal decomposition of silicon carbide. We
observe symmetric and anti-symmetric signatures in our data due to polarization
preserving and polarization mixing inter-Landau level transitions,
respectively. From field-dependent measurements we identify that transitions in
decoupled graphene mono-layers are governed by polarization mixing selection
rules, whereas transitions in coupled graphene mono-layers are governed by
polarization preserving selection rules. The selection rules may find
explanation by different coupling mechanisms of inter-Landau level transitions
with free charge carrier magneto-optic plasma oscillations
Graphene formation on SiC substrates
Graphene layers were created on both C and Si faces of semi-insulating,
on-axis, 4H- and 6H-SiC substrates. The process was performed under high vacuum
(<10-4 mbar) in a commercial chemical vapor deposition SiC reactor. A method
for H2 etching the on-axis sub-strates was developed to produce surface steps
with heights of 0.5 nm on the Si-face and 1.0 to 1.5 nm on the C-face for each
polytype. A process was developed to form graphene on the substrates
immediately after H2 etching and Raman spectroscopy of these samples confirmed
the formation of graphene. The morphology of the graphene is described. For
both faces, the underlying substrate morphology was significantly modified
during graphene formation; sur-face steps were up to 15 nm high and the uniform
step morphology was sometimes lost. Mo-bilities and sheet carrier
concentrations derived from Hall Effect measurements on large area (16 mm
square) and small area (2 and 10 um square) samples are presented and shown to
compare favorably to recent reports.Comment: European Conference on Silicon Carbide and Related Materials 2008
(ECSCRM '08), 4 pages, 4 figure
Evolving Spatially Aggregated Features from Satellite Imagery for Regional Modeling
Satellite imagery and remote sensing provide explanatory variables at
relatively high resolutions for modeling geospatial phenomena, yet regional
summaries are often desirable for analysis and actionable insight. In this
paper, we propose a novel method of inducing spatial aggregations as a
component of the machine learning process, yielding regional model features
whose construction is driven by model prediction performance rather than prior
assumptions. Our results demonstrate that Genetic Programming is particularly
well suited to this type of feature construction because it can automatically
synthesize appropriate aggregations, as well as better incorporate them into
predictive models compared to other regression methods we tested. In our
experiments we consider a specific problem instance and real-world dataset
relevant to predicting snow properties in high-mountain Asia
Improvement of Morphology and Free Carrier Mobility through Argon-Assisted Growth of Epitaxial Graphene on Silicon Carbide
Graphene was epitaxially grown on both the C- and Si-faces of 4H- and
6H-SiC(0001) under an argon atmosphere and under high vacuum conditions.
Following growth, samples were imaged with Nomarski interference contrast and
atomic force microscopies and it was found that growth under argon led to
improved morphologies on the C-face films but the Si-face films were not
significantly affected. Free carrier transport studies were conducted through
Hall effect measurements, and carrier mobilities were found to increase and
sheet carrier densities were found to decrease for those films grown under
argon as compared to high vacuum conditions. The improved mobilities and
concurrent decreases in sheet carrier densities suggest a decrease in
scattering in the films grown under argon.Comment: 215th Meeting of the Electrochemical Society (ECS 215), 14 pages, 6
figure
Hall Effect Mobility of Epitaxial Graphene Grown on Silicon Carbide
Epitaxial graphene films were grown in vacuo by silicon sublimation from the
(0001) and (000-1) faces of 4H- and 6H-SiC. Hall effect mobilities and sheet
carrier densities of the films were measured at 300 K and 77 K and the data
depended on the growth face. About 40% of the samples exhibited holes as the
dominant carrier, independent of face. Generally, mobilities increased with
decreasing carrier density, independent of carrier type and substrate polytype.
The contributions of scattering mechanisms to the conductivities of the films
are discussed. The results suggest that for near-intrinsic carrier densities at
300 K epitaxial graphene mobilities will be ~150,000 cm2V-1s-1 on the (000-1)
face and ~5,800 cm2V-1s-1 on the (0001) face.Comment: Accepted for publication in Applied Physics Letters, 10 pages, 2
figure
Morphology Characterization of Argon-Mediated Epitaxial Graphene on C-face SiC
Epitaxial graphene layers were grown on the C-face of 4H- and 6H-SiC using an
argon-mediated growth process. Variations in growth temperature and pressure
were found to dramatically affect the morphological properties of the layers.
The presence of argon during growth slowed the rate of graphene formation on
the C-face and led to the observation of islanding. The similarity in the
morphology of the islands and continuous films indicated that island nucleation
and coalescence is the growth mechanism for C-face graphene.Comment: 12 pages, 4 figure
"Plantar pro gasto": a importância do autoconsumo entre famÃlias de agricultores do Rio Grande do Sul.
O artigo discute a valoração e importância da produção para o autoconsumo na reprodução social das unidades familiares e caracteriza os alimentos autoconsumidos. Vale-se da pesquisa "Agricultura Familiar, Desenvolvimento Local e Pluriatividade" (UFRGS/UFPel/ CNPq-2003) que propiciou a formação de um banco de dados com informações sobre a dinâmica da agricultura familiar em quatro regiões distintas da geografia gaúcha, suas fontes e tipos de renda, entre estas o autoconsumo. Trazer este debate significa retomar um tema pouco discutido até então, e que, embora marginalizado ou considerado sem importância, desenvolve importante papel como renda não monetária, fortalece a segurança alimentar e adentra esferas da sociabilidade e identidade social. Além da introdução, apresenta-se o papel do autoconsumo na agricultura familiar, o cálculo da produção para o autoconsumo, discussão dos objetivos e resultados, e considerações finais. Os resultados demonstram que a produção para o autoconsumo é uma estratégia recorrente pelas unidades familiares e se diferencia de acordo com a dinâmica da agricultura familiar. Diferença esta expressa em valores relativos (%) e no número de estabelecimentos pertencentes a estratos diferenciados de autoconsumo, e pouco nos tipos de alimentos produzidos para este fim, observando-se uma homogeneidade dos hábitos alimentares
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