5,628 research outputs found
Do ultrafast exciton-polaron decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells?
All-organic-based photovoltaic solar cells have attracted considerable
attention because of their low-cost processing and short energy payback time.
In such systems the primary dissociation of an optical excitation into a pair
of photocarriers has been recently shown to be extremely rapid and efficient,
but the physical reason for this remains unclear. Here, two-dimensional
photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy,
is used to probe the ultrafast coherent decay of photoexcitations into
charge-producing states in a polymer:fullerene based solar cell. The
two-dimensional photocurrent spectra are interpreted by introducing a
theoretical model for the description of the coupling of the electronic states
of the system to an external environment and to the applied laser fields. The
experimental data show no cross-peaks in the two-dimensional photocurrent
spectra, as predicted by the model for coherence times between the exciton and
the photocurrent producing states of 20\,fs or less
Women over 40, foreigners of color, and other missing persons in globalizing mediascapes: understanding marketing images as mirrors of intersectionality
Media diversity studies regularly invoke the notion of marketing images as mirrors of racism and sexism. This article develops a higher-order concept of marketing images as “mirrors of intersectionality.” Drawing on a seven-dimensional study of coverperson diversity in a globalizing mediascape, the emergent concept highlights that marketing images reflect not just racism and sexism, but all categorical forms of marginalization, including ableism, ageism, colorism, fatism, and heterosexism, as well as intersectional forms of marginalization, such as sexist ageism and racist multiculturalism. Fueled by the legacies of history, aspirational marketing logics, and an industry-wide distribution of discriminatory work, marketing images help to perpetuate multiple, cumulative, and enduring advantages for privileged groups and disadvantages for marginalized groups. In this sense, marketing images, as mirrors of intersectionality, are complicit agents in the structuration of inequitable societies
A Gemini/GMOS Study of Intermediate Luminosity Early-Type Virgo Cluster Galaxies. I. Globular Cluster and Stellar Kinematics
We present a kinematic analysis of the globular cluster systems and diffuse
stellar light of four intermediate luminosity (sub-) early-type
galaxies in the Virgo cluster based on Gemini/GMOS data. Our galaxy sample is
fainter () than most previous studies, nearly doubling the
number of galaxies in this magnitude range that now have GC kinematics. The
data for the diffuse light extends to , and the data for the globular
clusters reaches 8--. We find that the kinematics in these outer regions
are all different despite the fact that these four galaxies have similar
photometric properties, and are uniformly classified as "fast rotators" from
their stellar kinematics within . The globular cluster systems exhibit a
wide range of kinematic morphology. The rotation axis and amplitude can change
between the inner and outer regions, including a case of counter-rotation. This
difference shows the importance of wide-field kinematic studies, and shows that
stellar and GC kinematics can change significantly as one moves beyond the
inner regions of galaxies. Moreover, the kinematics of the globular cluster
systems can differ from that of the stars, suggesting that the formation of the
two populations are also distinct.Comment: 24 pages, 21 figures, 9 table, ApJ in pres
Scattering Theory of Kondo Mirages and Observation of Single Kondo Atom Phase Shift
We explain the origin of the Kondo mirage seen in recent quantum corral
Scanning Tunneling Microscope (STM) experiments with a scattering theory of
electrons on the surfaces of metals. Our theory combined with experimental data
provides the first direct observation of a single Kondo atom phase shift. The
Kondo mirage at the empty focus of an elliptical quantum corral is shown to
arise from multiple electron bounces off the walls of the corral in a manner
analagous to the formation of a real image in optics. We demonstrate our theory
with direct quantitive comparision to experimental data.Comment: 13 pages; significant clarifications of metho
Optical cooper pair breaking spectroscopy of cuprate superconductors
The photon energy dependence of the optical Cooper pair breaking rate (CPBR) is studied for compressibly strained La1.85Sr0.15CuO4 (LSCO) films and YBa2Cu2.92Zn0.08O7-δ (YBCZO) thin films, and compared to that in YBa2Cu3O7-δ (YBCO). Unlike YBCO, the CPBR for LSCO does not show an obvious photon energy dependence. In YBCZO, the CPBR shows a strong energy dependence similar to YBCO, but with a redshift in the peak position. Analysis of these results strongly favors a physical picture based on electronic phase separation in high-Tc superconductivity
The Next Generation Virgo Cluster Survey. VI. The Kinematics of Ultra-compact Dwarfs and Globular Clusters in M87
The origin of ultra-compact dwarfs (UCDs)--objects larger and more massive
than typical globular clusters (GCs), but more compact than typical dwarf
galaxies--has been hotly debated in the 15 years since their discovery. Even
whether UCDs should be considered galactic in origin, or simply the most
extreme GCs, is not yet settled. We present the dynamical properties of 97
spectroscopically confirmed UCDs (rh >~10 pc) and 911 GCs associated with
central cD galaxy of the Virgo cluster, M87. Our UCDs, of which 89% have M_star
> ~2X10^6 M_sun and 92% are as blue as the classic blue GCs, nearly triple the
sample of previous confirmed Virgo UCDs, providing by far the best opportunity
for studying the global dynamics of a UCD system. We found that (1) UCDs have a
surface number density profile that is shallower than that of the blue GCs in
the inner ~ 70 kpc and as steep as that of the red GCs at larger radii; (2)
UCDs exhibit a significantly stronger rotation than the GCs, and the blue GCs
seem to have a velocity field that is more consistent with that of the
surrounding dwarf ellipticals than with that of UCDs; (3) UCDs have a radially
increasing orbital anisotropy profile, and are tangentially-biased at radii < ~
40 kpc and radially-biased further out. In contrast, the blue GCs become more
tangentially-biased at larger radii beyond ~ 40 kpc; (4) GCs with M_star >
2X10^6 M_sun have rotational properties indistinguishable from the less massive
ones, suggesting that it is the size, instead of mass, that differentiates UCDs
from GCs as kinematically distinct populations. We conclude that most UCDs in
M87 are not consistent with being merely the most luminous and extended
examples of otherwise normal GCs. The radially-biased orbital structure of UCDs
at large radii is in general agreement with the "tidally threshed dwarf galaxy"
scenario.Comment: 27 pages, 21 figures. To appear in The Astrophysical Journa
Large-scale Graphitic Thin Films Synthesized on Ni and Transferred to Insulators: Structural and Electronic Properties
We present a comprehensive study of the structural and electronic properties
of ultrathin films containing graphene layers synthesized by chemical vapor
deposition (CVD) based surface segregation on polycrystalline Ni foils then
transferred onto insulating SiO2/Si substrates. Films of size up to several
mm's have been synthesized. Structural characterizations by atomic force
microscopy (AFM), scanning tunneling microscopy (STM), cross-sectional
transmission electron microscopy (XTEM) and Raman spectroscopy confirm that
such large scale graphitic thin films (GTF) contain both thick graphite regions
and thin regions of few layer graphene. The films also contain many wrinkles,
with sharply-bent tips and dislocations revealed by XTEM, yielding insights on
the growth and buckling processes of the GTF. Measurements on mm-scale
back-gated transistor devices fabricated from the transferred GTF show
ambipolar field effect with resistance modulation ~50% and carrier mobilities
reaching ~2000 cm^2/Vs. We also demonstrate quantum transport of carriers with
phase coherence length over 0.2 m from the observation of 2D weak
localization in low temperature magneto-transport measurements. Our results
show that despite the non-uniformity and surface roughness, such large-scale,
flexible thin films can have electronic properties promising for device
applications.Comment: This version (as published) contains additional data, such as cross
sectional TEM image
High-Dimensional Feature Selection by Feature-Wise Kernelized Lasso
The goal of supervised feature selection is to find a subset of input
features that are responsible for predicting output values. The least absolute
shrinkage and selection operator (Lasso) allows computationally efficient
feature selection based on linear dependency between input features and output
values. In this paper, we consider a feature-wise kernelized Lasso for
capturing non-linear input-output dependency. We first show that, with
particular choices of kernel functions, non-redundant features with strong
statistical dependence on output values can be found in terms of kernel-based
independence measures. We then show that the globally optimal solution can be
efficiently computed; this makes the approach scalable to high-dimensional
problems. The effectiveness of the proposed method is demonstrated through
feature selection experiments with thousands of features.Comment: 18 page
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