15,833 research outputs found
2007 Disability Status Reports: United States
The Annual Disability Status Reports provide policy makers, disability advocates, reporters, and the public with a summary of the most recent demographic and economic statistics on the working-age (ages 21-64) population with disabilities. They contain information on the population size, prevalence, employment, earnings, poverty, household income, home ownership, and activity limitations of working-age people with disabilities, as well as the composition of this population by age, race, gender, and educational attainment
Criteria for formation of metallic glasses: The role of atomic size ratio
We consider metallic alloys of Cu*, Cu, and Cu** in which the atoms differ only in their atomic radii and examine how the size ratio affects the local orders in the alloy systems. These studies use molecular dynamics simulations in which the atomic interactions are modeled with a Sutton–Chen many-body potential. Considering rapid cooling of these binary and ternary alloys from the melt, we find three regimes defined by the magnitude of atomic size ratio lambda (lambda<=1.0): with (i) large size ratios of 0.95<lambda<=1.0, crystallization occurs; (ii) with moderate size ratios of 0.60<=lambda<=0.95, a glass phase forms; and (iii) with small size ratios of lambda<0.60, the alloy phase separates into pure phases and crystallize. From analyzing the structures of these binary and ternary alloys, we find that the liquid phase is characterized by local structures in which bonded atoms have local fivefold symmetry, which becomes more prominent as the glass phase forms. For phases that crystallize this local fivefold symmetry disappears as the long-range order of the crystalline phase dominates. The fivefold symmetry in the glass phase is mainly due to the icosahedral cluster formation. Energetically, the formation of icosahedral cluster is favored at the atomic size ratio of lambda~0.85, which is close to the lambda at which our analyses shows the maximum in the fivefold symmetry and the number of icosahedral clusters. As lambda decreases further, the phase separation is observed. The fivefold symmetry character and the number of icosahedral cluster shows the local minimum at this onset of the phase separation
High-frequency QPOs as a problem in physics: non-linear resonance
The presence of a kHz frequency in LMXBs has been expected from scaling laws,
by analogy with the QPO phenomenon in HMXB X-ray pulsars. Interpretation of the
two kHz frequencies, observed in accreting neutron stars, in terms of
non-linear resonance in strong-field gravity led to the prediction of twin QPOs
in black hole systems, in a definite frequency ratio (such as 2/3). The imprint
of a subharmonic of the 401 Hz rotation rate in the frequencies of the QPOs
detected in the accreting millisecond pulsar is at once a signature of
non-linear resonance and of coupling between accretion disk modes and the
neutron star spin.Comment: presented at X-ray Timing 2003: Rossi and Beyond, Boston, November
200
The Best for Last: The Timing of U.S. Supreme Court Decisions
This Article investigates the hypothesis that the most important and, often, controversial and divisive cases—so called big cases—are disproportionately decided at the end of June. We define a big case in one of four ways: front-page coverage in the New York Times; front-page and other coverage in four national newspapers (the New York Times, Los Angeles Times, Washington Post, and Chicago Tribune); the number of amicus curiae briefs filed in a case; and the number of subsequent citations by the Supreme Court to its decision in a case. We find a statistically significant association between each measure of a big case and end-of-term decisions even after controlling for the month of oral argument (cases argued later in the term are more likely to be decided near the end of the term) and case attributes (e.g., dissents and concurrences) that increase the time it takes to decide a case. We also speculate on why big cases cluster at the end of the term. One possibility is legacy and reputational concerns: when writing what they think will be a major decision, the Justices and their law clerks take more time polishing until the last minute with the hope of promoting their reputations. Another is that the end-of-term clustering of the most important cases may tend to diffuse media coverage of and other commentary regarding any particular case, and thus spare the Justices unwanted criticism just before they leave Washington for their summer recess
Can disorder enhance incoherent exciton diffusion?
Recent experiments aimed at probing the dynamics of excitons have revealed
that semiconducting films composed of disordered molecular subunits, unlike
expectations for their perfectly ordered counterparts, can exhibit a
time-dependent diffusivity in which the effective early time diffusion constant
is larger than that of the steady state. This observation has led to
speculation about what role, if any, microscopic disorder may play in enhancing
exciton transport properties. In this article, we present the results of a
model study aimed at addressing this point. Specifically, we present a general
model, based upon F\"orster theory, for incoherent exciton diffusion in a
material composed of independent molecular subunits with static energetic
disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule
transition rates which we demonstrate has two important consequences related to
exciton transport. First, the distribution of local site-specific diffusivity
is broadened in a manner that results in a decrease in average exciton
diffusivity relative to that in a perfectly ordered film. Second, since
excitons prefer to make transitions that are downhill in energy, the steady
state distribution of exciton energies is biased towards low energy molecular
subunits, those that exhibit reduced diffusivity relative to a perfectly
ordered film. These effects combine to reduce the net diffusivity in a manner
that is time dependent and grows more pronounced as disorder is increased.
Notably, however, we demonstrate that the presence of energetic disorder can
give rise to a population of molecular subunits with exciton transfer rates
exceeding that of subunits in an energetically uniform material. Such
enhancements may play an important role in processes that are sensitive to
molecular-scale fluctuations in exciton density field.Comment: 15 pages, 3 figure
Nonequilibrium dynamics of localized and delocalized excitons in colloidal quantum dot solids
Self-assembled quantum dot (QD) solids are a highly tunable class of
materials with a wide range of applications in solid-state electronics and
optoelectronic devices. In this perspective, we highlight how the presence of
microscopic disorder in these materials can influence their macroscopic
optoelectronic properties. Specifically, we consider the dynamics of excitons
in energetically disordered QD solids using a theoretical model framework for
both localized and delocalized excitonic regimes. In both cases, we emphasize
the tendency of energetic disorder to promote nonequilibrium relaxation
dynamics and discuss how the signatures of these nonequilibrium effects
manifest in time-dependent spectral measurements. Moreover, we describe the
connection between the microscopic dynamics of excitons within the material and
the measurement of material specific parameters, such as emission linewidth
broadening and energetic dissipation rate.Comment: 4 figure
Muon Anomaly and Dark Parity Violation
The muon anomalous magnetic moment exhibits a 3.6 \sigma discrepancy between
experiment and theory. One explanation requires the existence of a light vector
boson, Z_d (the dark Z), with mass 10 - 500 MeV that couples weakly to the
electromagnetic current through kinetic mixing. Support for such a solution
also comes from astrophysics conjectures regarding the utility of a U(1)_d
gauge symmetry in the dark matter sector. In that scenario, we show that mass
mixing between the Z_d and ordinary Z boson introduces a new source of "dark"
parity violation which is potentially observable in atomic and polarized
electron scattering experiments. Restrictive bounds on the mixing (m_{Z_d} /
m_Z) \delta are found from existing atomic parity violation results, \delta^2 <
2 x 10^{-5}. Combined with future planned and proposed polarized electron
scattering experiments, a sensitivity of \delta^2 ~ 10^{-6} is expected to be
reached, thereby complementing direct searches for the Z_d boson.Comment: Version to appear in PR
The U.S. system for measuring cross-border investment in securities: a primer with a discussion of recent developments
The tremendous growth in cross-border securities investment in recent years has called attention to the systems used by the United States and other countries to measure international securities flows and holdings. Ideally, the data gathered by the United States could tell us the extent to which foreign investors hold U.S. securities, the types of securities held, and the countries in which the securities are held, for example, and could identify trends in investment. This article looks at how well the data shed light on these topics. Special attention is given to the system's design and the implications of the design for data analysis. Also discussed are anticipated changes to the system and international efforts to improve data collection systems worldwide.International finance ; Securities
Roles of replication fork-interacting and Chk1-activating domains from claspin in a DNA replication checkpoint response
Claspin is essential for the ATR-dependent activation of Chk1 in Xenopus egg extracts containing incompletely replicated DNA. Claspin associates with replication forks upon origin unwinding. We show that Claspin contains a replication fork-interacting domain (RFID, residues 265–605) that associates with Cdc45, DNA polymerase ε, replication protein A, and two replication factor C complexes on chromatin. The RFID contains two basic patches (BP1 and BP2) at amino acids 265–331 and 470–600, respectively. Deletion of either BP1 or BP2 compromises optimal binding of Claspin to chromatin. Absence of BP1 has no effect on the ability of Claspin to mediate activation of Chk1. By contrast, removal of BP2 causes a large reduction in the Chk1-activating potency of Claspin. We also find that Claspin contains a small Chk1-activating domain (residues 776–905) that does not bind stably to chromatin, but it is fully effective at high concentrations for mediating activation of Chk1. These results indicate that stable retention of Claspin on chromatin is not necessary for activation of Chk1. Instead, our findings suggest that only transient interaction of Claspin with replication forks potentiates its Chk1-activating function. Another implication of this work is that stable binding of Claspin to chromatin may play a role in other functions besides the activation of Chk1
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