9,200 research outputs found
Nonradiative lifetimes in intermediate band materials - absence of lifetime recovery
Intermediate band photovoltaics hold the promise of being highly efficient
and cost effective photovoltaic cells. Intermediate states in the band gap,
however, are known to facilitate nonradiative recombination. Much effort has
been dedicated to producing metallic intermediate bands in hopes of producing
lifetime recovery -- an increase in carrier lifetime as doping levels increase.
We show that lifetime recovery induced by the insulator-to-metal transition
will not occur, because the metallic extended states will be localised by
phonons during the recombination process. Only trivial forms of lifetime
recovery, e.g., from an overall shift in intermediate levels, are possible.
Future work in intermediate band photovoltaics must focus on optimizing subgap
optical absorption and minimizing recombination, but not via lifetime recovery.Comment: 8 page
Scaling and localization lengths of a topologically disordered system
We consider a noninteracting disordered system designed to model particle
diffusion, relaxation in glasses, and impurity bands of semiconductors.
Disorder originates in the random spatial distribution of sites. We find strong
numerical evidence that this model displays the same universal behavior as the
standard Anderson model. We use finite-size-scaling to find the localization
length as a function of energy and density, including localized states away
from the delocalization transition. Results at many energies all fit onto the
same universal scaling curve.Comment: 5+ page
Sources and budgets for CO and O-3 in the northeastern Pacific during the spring of 2001: Results from the PHOBEA-II Experiment
Abstract. Ground and airborne measurements of CO, ozone, and aerosols were obtained in th
Seismic Tremor Reveals Spatial Organization and Temporal Changes of Subglacial Water System
©2019. American Geophysical Union. All Rights Reserved.Subglacial water ïŹow impacts glacier dynamics and shapes the subglacial environment. However, due to the challenges of observing glacier beds, the spatial organization of subglacial water systems and the time scales of conduit evolution and migration are largely unknown. To address these questions, we analyze 1.5â to 10âHz seismic tremor that we associate with subglacial water ïŹow,
that is, glaciohydraulic tremor, at Taku Glacier, Alaska, throughout the 2016 melt season. We use frequencyâdependent polarization analysis to estimate glaciohydraulic tremor propagation direction (related to the subglacial conduit location) and a degree day melt model to monitor variations in meltâwater
input. We suggest that conduit formation requires sustained water input and that multiconduit ïŹow paths can be distinguished from singleâconduit ïŹow paths. Theoretical analysis supports our seismic interpretations that subglacial discharge likely ïŹows through a singleâconduit in regions of steep hydraulic potential gradients but may be distributed among multiple conduits in regions with shallower potential gradients. Seismic tremor in regions with multiple conduits evolves through abrupt jumps between stable conïŹgurations that last 3â7 days, while tremor produced by singleâconduit ïŹow remains more stationary.
We also ïŹnd that polarized glaciohydraulic tremor wave types are potentially linked to the distance from source to station and that multiple peak frequencies propagate from a similar direction. Tremor appears undetectable at distances beyond 2â6 km from the source. This new understanding of the spatial organization and temporal development of subglacial conduits informs our understanding of dynamism within the subglacial hydrologic system.Raw seismic data described in this paper are available through the Incorporated Research Institutions for Seismology Data Management Center (http://ds.iris.edu/mda/ZQ? timewindow=2015â2016; Amundson et al., 2015). The raw weather data used in this paper can be found through the Arctic Data Center (https://doi.org/ 10.18739/A2H98ZC7V; Bartholomaus & Walter, 2018). Python code developed to carry out the analyses presented here is available at https://github.com/ voremargot/SeismicâTremorâRevealsâ SpatialâOrganizationâandâTemporalâ Changesâof SubglacialâWaterâSystem and https://github.com/ tbartholomaus/med_spec. This study was made possible with support from the University of Texas Institute for Geophysics and the University of Idaho. We thank Ginny Catania for the loan of weather stations. J. P. W.'s and J. M. A.'s contributions to this work were supported by the U.S. National Science Foundation (OPPâ1337548 and OPPâ 1303895). T. C. B. thanks Dylan Mikesell for an early conversation, which inspired the analysis presented here.Ye
Seismic Tremor Reveals Spatial Organization and Temporal Changes of Subglacial Water System
©2019. American Geophysical Union. All Rights Reserved.Subglacial water ïŹow impacts glacier dynamics and shapes the subglacial environment. However, due to the challenges of observing glacier beds, the spatial organization of subglacial water systems and the time scales of conduit evolution and migration are largely unknown. To address these questions, we analyze 1.5â to 10âHz seismic tremor that we associate with subglacial water ïŹow,
that is, glaciohydraulic tremor, at Taku Glacier, Alaska, throughout the 2016 melt season. We use frequencyâdependent polarization analysis to estimate glaciohydraulic tremor propagation direction (related to the subglacial conduit location) and a degree day melt model to monitor variations in meltâwater
input. We suggest that conduit formation requires sustained water input and that multiconduit ïŹow paths can be distinguished from singleâconduit ïŹow paths. Theoretical analysis supports our seismic interpretations that subglacial discharge likely ïŹows through a singleâconduit in regions of steep hydraulic potential gradients but may be distributed among multiple conduits in regions with shallower potential gradients. Seismic tremor in regions with multiple conduits evolves through abrupt jumps between stable conïŹgurations that last 3â7 days, while tremor produced by singleâconduit ïŹow remains more stationary.
We also ïŹnd that polarized glaciohydraulic tremor wave types are potentially linked to the distance from source to station and that multiple peak frequencies propagate from a similar direction. Tremor appears undetectable at distances beyond 2â6 km from the source. This new understanding of the spatial organization and temporal development of subglacial conduits informs our understanding of dynamism within the subglacial hydrologic system.Raw seismic data described in this paper are available through the Incorporated Research Institutions for Seismology Data Management Center (http://ds.iris.edu/mda/ZQ? timewindow=2015â2016; Amundson et al., 2015). The raw weather data used in this paper can be found through the Arctic Data Center (https://doi.org/ 10.18739/A2H98ZC7V; Bartholomaus & Walter, 2018). Python code developed to carry out the analyses presented here is available at https://github.com/ voremargot/SeismicâTremorâRevealsâ SpatialâOrganizationâandâTemporalâ Changesâof SubglacialâWaterâSystem and https://github.com/ tbartholomaus/med_spec. This study was made possible with support from the University of Texas Institute for Geophysics and the University of Idaho. We thank Ginny Catania for the loan of weather stations. J. P. W.'s and J. M. A.'s contributions to this work were supported by the U.S. National Science Foundation (OPPâ1337548 and OPPâ 1303895). T. C. B. thanks Dylan Mikesell for an early conversation, which inspired the analysis presented here.Ye
A photometrically and spectroscopically confirmed population of passive spiral galaxies
We have identified a population of passive spiral galaxies from photometry and integral field spectroscopy. We selected z < 0.035 spiral galaxies that have WISE colours consistent with little mid-infrared emission from warm dust. Matched aperture photometry of 51 spiral galaxies in ultraviolet, optical and mid-infrared show these galaxies have colours consistent with passive galaxies. Six galaxies form a spectroscopic pilot study and were observed using the Wide-Field Spectrograph to check for signs of nebular emission from star formation. We see no evidence of substantial nebular emission found in previous red spiral samples. These six galaxies possess absorption-line spectra with 4000âĂ
breaks consistent with an average luminosity-weighted age of 2.3 Gyr. Our photometric and integral field spectroscopic observations confirm the existence of a population of local passive spiral galaxies, implying that transformation into early-type morphologies is not required for the quenching of star formation
Potential of observations from the Tropospheric Emission Spectrometer to constrain continental sources of carbon monoxide
Mapping isoprene emissions over North America using formaldehyde column observations from space
We present a methodology for deriving emissions of volatile organic compounds (VOC) using space-based column observations of formaldehyde (HCHO) and apply it to data from the Global Ozone Monitoring Experiment (GOME) satellite instrument over North America during July 1996. The HCHO column is related to local VOC emissions, with a spatial smearing that increases with the VOC lifetime. Isoprene is the dominant HCHO precursor over North America in summer, and its lifetime (â1 hour) is sufficiently short that the smearing can be neglected. We use the Goddard Earth Observing System global 3-D model of tropospheric chemistry (GEOS-CHEM) to derive the relationship between isoprene emissions and HCHO columns over North America and use these relationships to convert the GOME HCHO columns to isoprene emissions. We also use the GEOS-CHEM model as an intermediary to validate the GOME HCHO column measurements by comparison with in situ observations. The GEOS-CHEM model including the Global Emissions Inventory Activity (GEIA) isoprene emission inventory provides a good simulation of both the GOME data (r2 = 0.69, n = 756, bias = +11%) and the in situ summertime HCHO measurements over North America (r2 = 0.47, n = 10, bias = â3%). The GOME observations show high values over regions of known high isoprene emissions and a day-to-day variability that is consistent with the temperature dependence of isoprene emission. Isoprene emissions inferred from the GOME data are 20% less than GEIA on average over North America and twice those from the U.S. EPA Biogenic Emissions Inventory System (BEIS2) inventory. The GOME isoprene inventory when implemented in the GEOS-CHEM model provides a better simulation of the HCHO in situ measurements than either GEIA or BEIS2 (r2 = 0.71, n = 10, bias = â10%)
Spatio-selection in Expanding Bacterial Colonies
Segregation of populations is a key question in evolution theory. One
important aspect is the relation between spatial organization and the
population's composition. Here we study a specific example -- sectors in
expanding bacterial colonies. Such sectors are spatially segregated
sub-populations of mutants. The sectors can be seen both in disk-shaped
colonies and in branching colonies. We study the sectors using two models we
have used in the past to study bacterial colonies -- a continuous
reaction-diffusion model with non-linear diffusion and a discrete
``Communicating Walkers'' model. We find that in expanding colonies, and
especially in branching colonies, segregation processes are more likely than in
a spatially static population. One such process is the establishment of stable
sub- population having neutral mutation. Another example is the maintenance of
wild-type population along side with sub-population of advantageous mutants.
Understanding such processes in bacterial colonies is an important subject by
itself, as well as a model system for similar processes in other spreading
populations
A Self-consistent DFT+DMFT scheme in the Projector Augmented Wave : Applications to Cerium, Ce2O3 and Pu2O3 with the Hubbard I solver and comparison to DFT+U
An implementation of full self-consistency over the electronic density in the
DFT+DMFT framework on the basis of a plane wave-projector augmented wave (PAW)
DFT code is presented. It allows for an accurate calculation of the total
energy in DFT+DMFT within a plane wave approach. In contrast to frameworks
based on the maximally localized Wannier function, the method is easily applied
to f electron systems, such as cerium, cerium oxide (Ce2O3) and plutonium oxide
(Pu2O3). In order to have a correct and physical calculation of the energy
terms, we find that the calculation of the self-consistent density is
mandatory. The formalism is general and does not depend on the method used to
solve the impurity model. Calculations are carried out within the Hubbard I
approximation, which is fast to solve, and gives a good description of strongly
correlated insulators. We compare the DFT+DMFT and DFT+U solutions, and
underline the qualitative differences of their converged densities. We
emphasize that in contrast to DFT+U, DFT+DMFT does not break the spin and
orbital symmetry. As a consequence, DFT+DMFT implies, on top of a better
physical description of correlated metals and insulators, a reduced occurrence
of unphysical metastable solutions in correlated insulators in comparison to
DFT+U.Comment: 19 pages, 9 figures. This is an author-created, un-copyedited version
of an article accepted for publication in Journal of Physics: Condensed
Matter. IOP Publishing Ltd is not responsible for any errors or omissions in
this version of the manuscript or any version derived from it. The Version of
Record is available online at doi: 10.1088/0953-8984/24/7/07560
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