1,076 research outputs found
Non- Mn-driven ferroelectricity in antiferromagnetic BaMnO
Using first-principles density functional theory we predict a ferroelectric
ground state -- driven by off-centering of the magnetic Mn ion -- in
perovskite-structure BaMnO.
Our finding is surprising, since the competition between energy-lowering
covalent bond formation, and energy-raising
Coulombic repulsions usually only favors off-centering on the perovskite
-site for non-magnetic ions.
We explain this tendency for ferroelectric off-centering by analyzing the
changes in electronic structure between the centrosymmetric and polar states,
and by calculating the Born effective charges; we find anomalously large values
for Mn and O consistent with our calculated polarization of 12.8 C/cm.
Finally, we suggest possible routes by which the perovskite phase may be
stabilized over the usual hexagonal phase, to enable a practical realization of
a single-phase multiferroic.Comment: 6 pages, 3 figure
The Theory of the Interleaving Distance on Multidimensional Persistence Modules
In 2009, Chazal et al. introduced -interleavings of persistence
modules. -interleavings induce a pseudometric on (isomorphism
classes of) persistence modules, the interleaving distance. The definitions of
-interleavings and generalize readily to multidimensional
persistence modules. In this paper, we develop the theory of multidimensional
interleavings, with a view towards applications to topological data analysis.
We present four main results. First, we show that on 1-D persistence modules,
is equal to the bottleneck distance . This result, which first
appeared in an earlier preprint of this paper, has since appeared in several
other places, and is now known as the isometry theorem. Second, we present a
characterization of the -interleaving relation on multidimensional
persistence modules. This expresses transparently the sense in which two
-interleaved modules are algebraically similar. Third, using this
characterization, we show that when we define our persistence modules over a
prime field, satisfies a universality property. This universality result
is the central result of the paper. It says that satisfies a stability
property generalizing one which is known to satisfy, and that in
addition, if is any other pseudometric on multidimensional persistence
modules satisfying the same stability property, then . We also show
that a variant of this universality result holds for , over arbitrary
fields. Finally, we show that restricts to a metric on isomorphism
classes of finitely presented multidimensional persistence modules.Comment: Major revision; exposition improved throughout. To appear in
Foundations of Computational Mathematics. 36 page
Clinical, Pathological, and Surgical Outcomes for Adult Pineoblastomas
Introduction
Pineoblastomas are uncommon primitive neuroectodermal tumors that occur mostly in children; they are exceedingly rare in adults. Few published reports have compared the various aspects of these tumors between adults and children.
Methods
The authors report a series of 12 pineoblastomas in adults from 2 institutions over 24 years. The clinical, radiologic, and pathologic features and clinical outcomes were compared with previously reported cases in children and adults.
Results
Patient age ranged from 24 to 81 years, and all but 1 patient exhibited symptoms of obstructive hydrocephalus. Three patients underwent gross total resection, and subtotal resection was performed in 3 patients. Diagnostic biopsy specimens were obtained in an additional 6 patients. Pathologically, the tumors had the classical morphologic and immunohistochemical features of pineoblastomas. Postoperatively, 10 patients received radiotherapy, and 5 patients received chemotherapy. Compared with previously reported cases, several differences were noted in clinical outcomes. Of the 12 patients, only 5 (42%) died of their disease (average length of survival, 118 months); 5 patients (42%) are alive with no evidence of disease (average length of follow-up, 92 months). One patient died of unrelated causes, and one was lost to follow-up. Patients with subtotal resections or diagnostic biopsies did not suffer a worse prognosis. Of the 9 patients with biopsy or subtotal resection, 4 are alive, 4 died of their disease, and 1 died of an unrelated hemorrhagic cerebral infarction.
Conclusions
Although this series is small, the data suggest that pineoblastomas in adults have a less aggressive clinical course than in children
Optical Spectropolarimetry of the GRB 020813 Afterglow
The optical afterglow of gamma-ray burst 020813 was observed for 3 hours with
the LRIS spectropolarimeter at the Keck-I telescope, beginning 4.7 hours after
the burst was detected by HETE-2. The spectrum reveals numerous metal
absorption lines that we identify with two systems at z=1.223 and z=1.255. We
also detect an O II 3727 emission line at z=1.255 and we identify this galaxy
as the likely host of the GRB. After a correction for Galactic interstellar
polarization, the optical afterglow has a linear polarization of 1.8-2.4%
during 4.7-7.9 hours after the burst. A measurement of p = 0.80% +/- 0.16% on
the following night by Covino et al. demonstrates significant polarization
variability over the next 14 hours. The lack of strong variability in the
position angle of linear polarization indicates that the magnetic field in the
jet is likely to be globally ordered rather than composed of a number of
randomly oriented cells. Within the framework of afterglow models with
collimated flows, the relatively low observed polarization suggests that the
magnetic field components perpendicular and parallel to the shock front are
only different by about 20%.Comment: To appear in ApJ Letters. 6 pages including 2 figure
Surface and lightning sources of nitrogen oxides over the United States: Magnitudes, chemical evolution, and outflow
We use observations from two aircraft during the ICARTT campaign over the eastern United States and North Atlantic during summer 2004, interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem) to test current understanding of regional sources, chemical evolution, and export of NOx. The boundary layer NOx data provide top-down verification of a 50% decrease in power plant and industry NOx emissions over the eastern United States between 1999 and 2004. Observed NOx concentrations at 8–12 km altitude were 0.55 ± 0.36 ppbv, much larger than in previous U.S. aircraft campaigns (ELCHEM, SUCCESS, SONEX) though consistent with data from the NOXAR program aboard commercial aircraft. We show that regional lightning is the dominant source of this upper tropospheric NOx and increases upper tropospheric ozone by 10 ppbv. Simulating ICARTT upper tropospheric NOx observations with GEOS-Chem requires a factor of 4 increase in modeled NOx yield per flash (to 500 mol/ flash). Observed OH concentrations were a factor of 2 lower than can be explained from current photochemical models, for reasons that are unclear. A NOy-CO correlation analysis of the fraction f of North American NOx emissions vented to the free troposphere as NOy (sum of NOx and its oxidation products) shows observed f = 16 ± 10% and modeled f = 14 ± 9%, consistent with previous studies. Export to the lower free troposphere is mostly HNO3 but at higher altitudes is mostly PAN. The model successfully simulates NOy export efficiency and speciation, supporting previous model estimates of a large U.S. anthropogenic contribution to global tropospheric ozone through PAN export
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Health, Wealth, and Air Pollution: Advancing Theory and Methods
The effects of both ambient air pollution and socioeconomic position (SEP) on health are well documented. A limited number of recent studies suggest that SEP may itself play a role in the epidemiology of disease and death associated with exposure to air pollution. Together with evidence that poor and working-class communities are often more exposed to air pollution, these studies have stimulated discussion among scientists, policy makers, and the public about the differential distribution of the health impacts from air pollution. Science and public policy would benefit from additional research that integrates the theory and practice from both air pollution and social epidemiologies to gain a better understanding of this issue. In this article we aim to promote such research by introducing readers to methodologic and conceptual approaches in the fields of air pollution and social epidemiology; by proposing theories and hypotheses about how air pollution and socioeconomic factors may interact to influence health, drawing on studies conducted worldwide; by discussing methodologic issues in the design and analysis of studies to determine whether health effects of exposure to ambient air pollution are modified by SEP; and by proposing specific steps that will advance knowledge in this field, fill information gaps, and apply research results to improve public health in collaboration with affected communities
Automating Deductive Verification for Weak-Memory Programs
Writing correct programs for weak memory models such as the C11 memory model
is challenging because of the weak consistency guarantees these models provide.
The first program logics for the verification of such programs have recently
been proposed, but their usage has been limited thus far to manual proofs.
Automating proofs in these logics via first-order solvers is non-trivial, due
to reasoning features such as higher-order assertions, modalities and rich
permission resources. In this paper, we provide the first implementation of a
weak memory program logic using existing deductive verification tools. We
tackle three recent program logics: Relaxed Separation Logic and two forms of
Fenced Separation Logic, and show how these can be encoded using the Viper
verification infrastructure. In doing so, we illustrate several novel encoding
techniques which could be employed for other logics. Our work is implemented,
and has been evaluated on examples from existing papers as well as the Facebook
open-source Folly library.Comment: Extended version of TACAS 2018 publicatio
Not Even Decoupling Can Save Minimal Supersymmetric SU(5)
We make explicit the statement that Minimal Supersymmetric SU(5) has been
excluded by the Super-Kamiokande search for the process . This exclusion is made by first placing limits on the colored
Higgs triplet mass, by forcing the gauge couplings to unify. We also show that
taking the superpartners of the first two generations to be very heavy in order
to avoid flavor changing neutral currents, the so-called ``decoupling'' idea,
is insufficient to resurrect the Minimal SUSY SU(5). We comment on various
mechanisms to further suppress proton decay in SUSY SU(5). Finally, we address
the contributions to proton decay from gauge boson exchange in the Minimal SUSY
SU(5) and flipped SU(5) models.Comment: 8 pages, 4 figure
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