2,280 research outputs found
Inelastic quantum transport in superlattices: success and failure of the Boltzmann equation
Electrical transport in semiconductor superlattices is studied within a fully
self-consistent quantum transport model based on nonequilibrium Green
functions, including phonon and impurity scattering. We compute both the drift
velocity-field relation and the momentum distribution function covering the
whole field range from linear response to negative differential conductivity.
The quantum results are compared with the respective results obtained from a
Monte Carlo solution of the Boltzmann equation. Our analysis thus sets the
limits of validity for the semiclassical theory in a nonlinear transport
situation in the presence of inelastic scattering.Comment: final version with minor changes, to appear in Physical Review
Letters, sceduled tentatively for July, 26 (1999
Effects of impurity scattering on electron-phonon resonances in semiconductor superlattice high-field transport
A non-equilibrium Green's function method is applied to model high-field
quantum transport and electron-phonon resonances in semiconductor
superlattices. The field-dependent density of states for elastic (impurity)
scattering is found non-perturbatively in an approach which can be applied to
both high and low electric fields. I-V curves, and specifically electron-phonon
resonances, are calculated by treating the inelastic (LO phonon) scattering
perturbatively. Calculations show how strong impurity scattering suppresses the
electron-phonon resonance peaks in I-V curves, and their detailed sensitivity
to the size, strength and concentration of impurities.Comment: 7 figures, 1 tabl
New Sensitivity to Solar WIMP Annihilation using Low-Energy Neutrinos
Dark matter particles captured by the Sun through scattering may annihilate
and produce neutrinos, which escape. Current searches are for the few
high-energy neutrinos produced in the prompt decays of some final states. We
show that interactions in the solar medium lead to a large number of pions for
nearly all final states. Positive pions and muons decay at rest, producing
low-energy neutrinos with known spectra, including nuebar through neutrino
mixing. We demonstrate that Super-Kamiokande can thereby provide a new probe of
the spin-dependent WIMP-proton cross section. Compared to other methods, the
sensitivity is competitive and the uncertainties are complementary.Comment: 5 pages, 4 figure
An examination of the precipitation delivery mechanisms for Dolleman Island, eastern Antarctic Peninsula
Copyright @ 2004 Wiley-BlackwellThe variability of size and source of significant precipitation events were studied at an Antarctic ice core drilling site: Dolleman Island (DI), located on the eastern coast of the Antarctic Peninsula. Significant precipitation events that occur at DI were temporally located in the European Centre for Medium-Range Weather Forecasting (ECMWF) reanalysis data set, ERA-40. The annual and summer precipitation totals from ERA-40 at DI both show significant increases over the reanalysis period. Three-dimensional backwards air parcel trajectories were then run for 5 d using the ECMWF ERA-15 wind fields. Cluster analyses were performed on two sets of these backwards trajectories: all days in the range 1979–1992 (the climatological time-scale) and a subset of days when a significant precipitation event occurred. The principal air mass sources and delivery mechanisms were found to be the Weddell Sea via lee cyclogenesis, the South Atlantic when there was a weak circumpolar trough (CPT) and the South Pacific when the CPT was deep. The occurrence of precipitation bearing air masses arriving via a strong CPT was found to have a significant correlation with the southern annular mode (SAM); however, the arrival of air masses from the same region over the climatological time-scale showed no such correlation. Despite the dominance in both groups of back trajectories of the westerly circulation around Antarctica, some other key patterns were identified. Most notably there was a higher frequency of lee cyclogenesis events in the significant precipitation trajectories compared to the climatological time-scale. There was also a tendency for precipitation trajectories to come from more northerly latitudes, mostly from 50–70°S. The El Niño Southern Oscillation (ENSO) was found to have a strong influence on the mechanism by which the precipitation was delivered; the frequency of occurrence of precipitation from the east (west) of DI increased during El Niño (La Niña) events
Evolution of surface velocities and ice discharge of Larsen B outlet glaciers from 1995 to 2013
We use repeat-pass SAR data to produce detailed maps of surface motion covering the glaciers draining into the former Larsen B ice shelf, Antarctic Peninsula, for different epochs between 1995 and 2013. We combine the velocity maps with estimates of ice thickness to analyze fluctuations of ice discharge. The collapse of the central and northern sections of the ice shelf in 2002 led to a near-immediate acceleration of tributary glaciers as well as of the remnant ice shelf in Scar Inlet. Velocities of the glaciers discharging directly into the ocean remain to date well above the velocities of the pre-collapse period. The response of individual glaciers differs and velocities show significant temporal fluctuations, implying major variations in ice discharge and mass balance as well. Due to reduced velocity and ice thickness the ice discharge of Crane Glacier decreased from 5.02 Gt a−1 in 2007 to 1.72 Gt a−1 in 2013, whereas Hektoria and Green glaciers continue to show large temporal fluctuations in response to successive stages of frontal retreat. The velocity on Scar Inlet ice shelf increased two- to three fold since 1995, with the largest increase in the first years after the break-up of the main section of Larsen B. Flask and Leppard glaciers, the largest tributaries to Scar Inlet ice shelf, accelerated. In 2013 their discharge was 38% and 46%, higher than in 1995
Production and optical properties of liquid scintillator for the JSNS experiment
The JSNS (J-PARC Sterile Neutrino Search at J-PARC Spallation Neutron
Source) experiment will search for neutrino oscillations over a 24 m short
baseline at J-PARC. The JSNS inner detector will be filled with 17 tons
of gadolinium-loaded liquid scintillator (LS) with an additional 31 tons of
unloaded LS in the intermediate -catcher and outer veto volumes.
JSNS has chosen Linear Alkyl Benzene (LAB) as an organic solvent because
of its chemical properties. The unloaded LS was produced at a refurbished
facility, originally used for scintillator production by the RENO experiment.
JSNS plans to use ISO tanks for the storage and transportation of the LS.
In this paper, we describe the LS production, and present measurements of its
optical properties and long term stability. Our measurements show that storing
the LS in ISO tanks does not result in degradation of its optical properties.Comment: 7 pages, 4 figures
Causally-interpretable meta-analysis: clearly-defined causal effects and two case studies
Meta-analysis is commonly used to combine results from multiple clinical
trials, but traditional meta-analysis methods do not refer explicitly to a
population of individuals to whom the results apply and it is not clear how to
use their results to assess a treatment's effect for a population of interest.
We describe recently-introduced causally-interpretable meta-analysis methods
and apply their treatment effect estimators to two individual-participant data
sets. These estimators transport estimated treatment effects from studies in
the meta-analysis to a specified target population using individuals'
potentially effect-modifying covariates. We consider different regression and
weighting methods within this approach and compare the results to traditional
aggregated-data meta-analysis methods. In our applications, certain versions of
the causally-interpretable methods performed somewhat better than the
traditional methods, but the latter generally did well. The
causally-interpretable methods offer the most promise when covariates modify
treatment effects and our results suggest that traditional methods work well
when there is little effect heterogeneity. The causally-interpretable approach
gives meta-analysis an appealing theoretical framework by relating an estimator
directly to a specific population and lays a solid foundation for future
developments.Comment: 31 pages, 2 figures Submitted to Research Synthesis Method
WiSeBE: Window-based Sentence Boundary Evaluation
Sentence Boundary Detection (SBD) has been a major research topic since
Automatic Speech Recognition transcripts have been used for further Natural
Language Processing tasks like Part of Speech Tagging, Question Answering or
Automatic Summarization. But what about evaluation? Do standard evaluation
metrics like precision, recall, F-score or classification error; and more
important, evaluating an automatic system against a unique reference is enough
to conclude how well a SBD system is performing given the final application of
the transcript? In this paper we propose Window-based Sentence Boundary
Evaluation (WiSeBE), a semi-supervised metric for evaluating Sentence Boundary
Detection systems based on multi-reference (dis)agreement. We evaluate and
compare the performance of different SBD systems over a set of Youtube
transcripts using WiSeBE and standard metrics. This double evaluation gives an
understanding of how WiSeBE is a more reliable metric for the SBD task.Comment: In proceedings of the 17th Mexican International Conference on
Artificial Intelligence (MICAI), 201
Металографічний аналіз потенційних ділянок зародження руйнування зони термічного вливу низьковуглицевих сталей після лазерного зварювання
The work investigates the detection of dangerous areas of the zones of thermal impact on the basis of the research of welding joint structure formation after laser welding of low-carbon steels 09Mn2Si, 10CrNiCu, 10Mn2VNb. The complex of studies carried out showed that the welded joint is structurally divided into several zones. At the same time, metallographic analysis showed that in the zones of the welded joint there is a conglomerate of structures, which contains components characteristic both for metal in the cast state and for metal after accelerated cooling.Досліджено виявлення небезпечних ділянок зони термічного впливу на підставі дослідження формування структури зварного з'єднання після лазерного зварювання низьковуглецевих сталей 09Г2С, 10ХСНД, 10Г2ФБ. Проведений комплекс досліджень показав, що зварене з'єднання структурно розділити на кілька зон. При цьому, металографічний аналіз показав, що в зонах зварного з'єднання присутні конгломерат структур, який містить складові характерні як для металу в литому стані так і для металу після прискореного охолодження.Исследовано выявление опасных участков зоны термического влияния на основании исследования формирования структуры сварного соединения после лазерной сварки низкоуглеродистых сталей 09Г2С, 10ХСНД, 10Г2ФБ. Проведенный комплекс исследований показал, что сварное соединение структурно разделить на несколько зон. При этом, металлографический анализ показал, что в зонах сварного соединения присутствуют конгломерат структур, который содержит составляющие характерные как для металла в литом состоянии так и для металла после ускоренного охлаждения
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