14,005 research outputs found
Electronic Structure of Hyperkagome Na4Ir3O8
We investigate the electronic structure of the frustrated magnet Na4Ir3O8
using density functional theory. Due to strong spin-orbit coupling, the
hyperkagome lattice is characterized by a half-filled complex of states, making
it a cubic iridium analogue of the high temperature superconducting cuprates.
The implications of our results for this unique material are discussed.Comment: expanded discussion with extra figures - 6 pages, 10 figure
Spin Hamiltonian of Hyperkagome Na4Ir3O8
We derive the spin Hamiltonian for the quantum spin liquid Na4Ir3O8, and then
estimate the direct and superexchange contributions between near neighbor
iridium ions using a tight binding parametrization of the electronic structure.
We find a magnitude of the exchange interaction comparable to experiment for a
reasonable value of the on-site Coulomb repulsion. For one of the two tight
binding parametrizations we have studied, the direct exchange term, which is
isotropic, dominates the total exchange. This provides support for those
theories proposed to describe this novel quantum spin liquid that assume an
isotropic Heisenberg model.Comment: 9 pages, 4 figure
Why Limits on Contributions to Super PACs Should Survive \u3ci\u3eCitizens United\u3c/i\u3e
Soon after the Supreme Court decided Citizens United v. FEC, the D.C. Circuit held all limits on contributions to super PACs unconstitutional. Its decision in SpeechNow.org v. FEC created a regime in which contributions to candidates are limited but in which contributions to less responsible groups urging votes for these candidates are unbounded. No legislator voted for this system of campaign financing, and the judgment that the Constitution requires it is astonishing. Forty-two years ago, Buckley v. Valeo held that Congress could limit contributions to candidates because these contributions are corrupting or create an appearance of corruption. According to the D.C. Circuit, however, Congress may not prohibit multi-million-dollar contributions to satellite campaigns because these contributions do not create even an appearance of corruption. The D.C. Circuit said that a single sentence of the Citizens United opinion compelled its result. It wrote, “In light of the Court’s holding as a matter of law that independent expenditures do not corrupt or create the appearance of corruption, contributions to groups that make only independent expenditures also cannot corrupt or create the appearance of corruption.” This Article contends that, contrary to the D.C. Circuit’s reasoning, contributions to super PACs can corrupt even when expenditures by these groups do not. Moreover, the statement that the D.C. Circuit took as its premise was dictum, and the Supreme Court did not mean this statement to be taken in the way the D.C. Circuit took it. The Supreme Court’s long-standing distinction between contribution limits and expenditure limits does not rest on the untenable proposition thatcandidates cannot be corrupted by funds paid to and spent on their behalf by others. Rather, Buckley noted five differences between contributions and expenditures. A review of these differences makes clear that contributions to super PACs cannot be distinguished from the contributions to candidates whose limitation the Court upheld. The ultimate question posed by Buckley is whether super PAC contributions create a sufficient appearance of corruption to justify their limitation. This Article reviews the statements of candidates of both parties in the 2016 presidential election, the views of Washington insiders, and public opinion polls. It shows that SpeechNow has sharpened class divisions and helped to tear America apart. The Justice Department did not seek Supreme Court review of the SpeechNow decision. In a statement that belongs on a historic list of wrong predictions, Attorney General Holder explained that the decision would “affect only a small subset of federally regulated contributions.” Although eight years have passed since SpeechNow, the Supreme Court has not decided whether the Constitution guarantees the right to give unlimited funds to super PACs. A final section of this Article describes the efforts of members of Congress and candidates for Congress to bring that question before the Court. The Federal Election Commission is opposing their efforts, offering arguments that, if accepted, would be likely to keep the Court from ever deciding the issue
Population III star formation in a Lambda CDM universe, II: Effects of a photodissociating background
We examine aspects of primordial star formation in the presence of a
molecular hydrogen-dissociating ultraviolet background. We compare a set of AMR
hydrodynamic cosmological simulations using a single cosmological realization
but with a range of ultraviolet background strengths in the Lyman-Werner band.
This allows us to study the effects of Lyman-Werner radiation on suppressing H2
cooling at low densities as well as the high-density evolution of the
collapsing core in a self-consistent cosmological framework. We find that the
addition of a photodissociating background results in a delay of the collapse
of high density gas at the center of the most massive halo in the simulation
and, as a result, an increase in the virial mass of this halo at the onset of
baryon collapse. We find that, contrary to previous results, Population III
star formation is not suppressed for J, but occurs even with
backgrounds as high as J. We find that H2 cooling leads to collapse
despite the depressed core molecular hydrogen fractions due to the elevated H2
cooling rates at K. We observe a relationship between the
strength of the photodissociating background and the rate of accretion onto the
evolving protostellar cloud core, with higher LW background fluxes resulting in
higher accretion rates. Finally, we find that the collapsing halo cores in our
simulations do not fragment at densities below cm
regardless of the strength of the LW background, suggesting that Population III
stars forming in halos with T K may still form in isolation.Comment: 46 pages, 14 figures (9 color). Accepted by the Astrophysical
Journal, some minor revision
Late Pop III Star Formation During the Epoch of Reionization: Results from the Renaissance Simulations
We present results on the formation of Pop III stars at redshift 7.6 from the
Renaissance Simulations, a suite of extremely high-resolution and physics-rich
radiation transport hydrodynamics cosmological adaptive-mesh refinement
simulations of high redshift galaxy formation performed on the Blue Waters
supercomputer. In a survey volume of about 220 comoving Mpc, we found 14
Pop III galaxies with recent star formation. The surprisingly late formation of
Pop III stars is possible due to two factors: (i) the metal enrichment process
is local and slow, leaving plenty of pristine gas to exist in the vast volume;
and (ii) strong Lyman-Werner radiation from vigorous metal-enriched star
formation in early galaxies suppresses Pop III formation in ("not so") small
primordial halos with mass less than 3 10 M. We
quantify the properties of these Pop III galaxies and their Pop III star
formation environments. We look for analogues to the recently discovered
luminous Ly emitter CR7 (Sobral et al. 2015), which has been
interpreted as a Pop III star cluster within or near a metal-enriched star
forming galaxy. We find and discuss a system similar to this in some respects,
however the Pop III star cluster is far less massive and luminous than CR7 is
inferred to be.Comment: 8 pages, 4 figures, 3 tables. Accepted by Ap
Probing The Ultraviolet Luminosity Function of the Earliest Galaxies with the Renaissance Simulations
In this paper, we present the first results from the Renaissance Simulations,
a suite of extremely high-resolution and physics-rich AMR calculations of high
redshift galaxy formation performed on the Blue Waters supercomputer. These
simulations contain hundreds of well-resolved galaxies at , and
make several novel, testable predictions. Most critically, we show that the
ultraviolet luminosity function of our simulated galaxies is consistent with
observations of high-z galaxy populations at the bright end of the luminosity
function (M), but at lower luminosities is essentially flat
rather than rising steeply, as has been inferred by Schechter function fits to
high-z observations, and has a clearly-defined lower limit in UV luminosity.
This behavior of the luminosity function is due to two factors: (i) the strong
dependence of the star formation rate on halo virial mass in our simulated
galaxy population, with lower-mass halos having systematically lower star
formation rates and thus lower UV luminosities; and (ii) the fact that halos
with virial masses below M do not universally
contain stars, with the fraction of halos containing stars dropping to zero at
M. Finally, we show that the brightest of our
simulated galaxies may be visible to current and future ultra-deep space-based
surveys, particularly if lensed regions are chosen for observation.Comment: 7 pages, 4 figures, accepted by The Astrophysical Journal Letter
A study of jet impingment on curved surfaces followed by oblique introduction into a freestream flow
Technology of thrust reversers with particular application to STOL aircraf
Qualitative and quantitative analysis of mixtures of compounds containing both hydrogen and deuterium
Method allows qualitative and quantitative analysis of mixtures of partially deuterated compounds. Nuclear magnetic resonance spectroscopy determines location and amount of deuterium in organic compounds but not fully deuterated compounds. Mass spectroscopy can detect fully deuterated species but not the location
Nonlinear response of a blade-stiffened graphite-epoxy panel with a discontinuous stiffener: Work in progress
The problem of calculating detailed stress distributions around discontinuities in buckled, composite structural components for use with the various analytical failure prediction techniques has not been thoroughly explored. The purpose here is the application of computational methods to the detailed stress analysis problem which is the focus of this session of the workshop. One approach to uncovering the difficulties of this type of analysis and to providing specific directions for future research in this area is a direct attack on the problem using currently available analysis tools. A candidate problem has been selected and experiences from calculating its structural response are described
Scaling Relations for Galaxies Prior to Reionization
The first galaxies in the Universe are the building blocks of all observed
galaxies. We present scaling relations for galaxies forming at redshifts when reionization is just beginning. We utilize the ``Rarepeak'
cosmological radiation hydrodynamics simulation that captures the complete star
formation history in over 3,300 galaxies, starting with massive Population III
stars that form in dark matter halos as small as ~. We make
various correlations between the bulk halo quantities, such as virial, gas, and
stellar masses and metallicities and their respective accretion rates,
quantifying a variety of properties of the first galaxies up to halo masses of
. Galaxy formation is not solely relegated to atomic cooling
halos with virial temperatures greater than K, where we find a dichotomy
in galaxy properties between halos above and below this critical mass scale.
Halos below the atomic cooling limit have a stellar mass -- halo mass
relationship .
We find a non-monotonic relationship between metallicity and halo mass for the
smallest galaxies. Their initial star formation events enrich the interstellar
medium and subsequent star formation to a median of and
, respectively, in halos of total mass that
is then diluted by metal-poor inflows, well beyond Population III
pre-enrichment levels of . The scaling relations presented
here can be employed in models of reionization, galaxy formation and chemical
evolution in order to consider these galaxies forming prior to reionization.Comment: 10 pages, 10 figures. Accepted to Ap
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