5,078 research outputs found
Diamond nucleation on cleaved Si(111)
Diamond crystallites have been nucleated and grown by hot filament chemical vapor deposition at 600°C on the untreated fracture surface of a cleaved Si(111) sample. The flat surface of the cleaved crystal was inactive towards diamond nucleation while, on the terraced surface formed by the propagating crack, a high density of nuclei was found. The crystallites were nucleated in correspondence of edges between (111) terrace planes and step planes. The occurrence of edges, as determined by scanning electron microscopy (SEM) observation, is a necessary but not sufficient condition for the nucleation and this fact suggests that particular atomic arrangements are required for the diamond nucleus formation
QCD and the Chiral Critical Point
As an extension of , consider a theory with ``'' flavors, where the
current quark masses are held in a fixed ratio as the overall scale of the
quark masses is varied. At nonzero temperature and baryon density it is
expected that in the chiral limit the chiral phase transition is of first
order. Increasing the quark mass from zero, the chiral transition becomes more
weakly first order, and can end in a chiral critical point. We show that the
only massless field at the chiral critical point is a sigma meson, with the
universality class that of the Ising model. Present day lattice simulations
indicate that is (relatively) near to the chiral critical point.Comment: 7 pages + 2 figures, BNL-GGP-
Seabird Bycatch in Pelagic Longline Fisheries Is Grossly Underestimated when Using Only Haul Data
Hundreds of thousands of seabirds are killed each year as bycatch in longline fisheries. Seabirds are predominantly caught during line setting but bycatch is generally recorded during line hauling, many hours after birds are caught. Bird loss during this interval may lead to inaccurate bycatch information. In this 15 year study, seabird bycatch was recorded during both line setting and line hauling from four fishing regions: Indian Ocean, Southern Ocean, Coral Sea and central Pacific Ocean. Over 43,000 albatrosses, petrels and skuas representing over 25 species were counted during line setting of which almost 6,000 seabirds attempted to take the bait. Bait-taking interactions were placed into one of four categories. (i) The majority (57%) of bait-taking attempts were “unsuccessful” involving seabirds that did not take the bait nor get caught or hooked. (ii) One-third of attempts were “successful” with seabirds removing the bait while not getting caught. (iii) One-hundred and seventy-six seabirds (3% of attempts) were observed being “caught” during line setting, with three albatross species – Laysan (Phoebastria immutabilis), black-footed (P. nigripes) and black-browed (Thalassarche melanophrys)– dominating this category. However, of these, only 85 (48%) seabird carcasses were retrieved during line hauling. Most caught seabirds were hooked through the bill. (iv) The remainder of seabird-bait interactions (7%) was not clearly observed, but likely involved more “caught” seabirds. Bait taking attempts and percentage outcome (e.g. successful, caught) varied between seabird species and was not always related to species abundance around fishing vessels. Using only haul data to calculate seabird bycatch grossly underestimates actual bycatch levels, with the level of seabird bycatch from pelagic longline fishing possibly double what was previously thought
Heavy top quark from Fritzsch mass matrices
It is shown, contrary to common belief, that the Fritzsch ansatz for the
quark mass matrices admits a heavy top quark. With the ansatz prescribed at the
supersymmetric grand unified (GUT) scale, one finds that the top quark may be
as heavy as 145 GeV, provided that tan (the ratio of the vacuum
expectation values of the two higgs doublets) . Within a
non-supersymmetric GUT framework with two (one) light higgs doublets, the
corresponding approximate upper bound on the top mass is GeV. Our
results are based on a general one--loop renormalization group analysis of the
quark masses and mixing angles and are readily applied to alternative mass
matrix ans\"{a}tze.Comment: LaTeX, 14 figures (not included, available on request
Origins and composition of fine atmospheric carbonaceous aerosol in the Sierra Nevada Mountains, California
In this paper we report chemically resolved measurements
of organic aerosol (OA) and related tracers during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at the Blodgett Forest Research Station, California from 15 August–10 October 2007. OA contributed the majority of the mass to the fine atmospheric particles and was predominately oxygenated (OOA). The highest concentrations of OA were during sporadic wildfire influence when aged plumes were impacting the site. In
situ measurements of particle phase molecular markers were dominated by secondary compounds and along with gas phase compounds could be categorized into six factors or sources: (1) aged biomass burning emissions and oxidized urban emissions, (2) oxidized urban emissions (3) oxidation products of monoterpene emissions, (4) monoterpene emissions, (5) anthropogenic emissions and (6) local
methyl chavicol emissions and oxidation products. There were multiple biogenic components that contributed to OA at this site whose contributions varied diurnally, seasonally and in response to changing meteorological conditions, e.g. temperature and precipitation events. Concentrations of isoprene oxidation products were larger when temperatures were higher during the first half of the campaign (15 August–12 September) due to more substantial emissions of isoprene and enhanced photochemistry. The oxidation of methyl chavicol, an oxygenated terpene emitted by
ponderosa pine trees, contributed similarly to OA throughout the campaign. In contrast, the abundances of monoterpene oxidation products in the particle phase were greater during the cooler conditions in the latter half of the campaign (13 September–10 October), even though emissions of the precursors were lower, although the mechanism is not known. OA was correlated with the anthropogenic tracers 2-propyl nitrate and carbon monoxide (CO), consistent with previous observations, while being comprised of mostly non-fossil carbon (>75%). The correlation between OA and an anthropogenic tracer does not necessarily identify the source of the carbon as being anthropogenic but instead suggests a coupling between the anthropogenic and biogenic components in the air mass that might be related to the source of the oxidant and/or the aerosol sulfate. Observations of organosulfates of isoprene and α-pinene provided evidence for the likely importance of aerosol sulfate in spite of neutralized aerosol although acidic plumes might have played a role upwind of the site. This is in contrast to laboratory studies where strongly acidic seed aerosols were needed in order to form these compounds. These compounds together represented only a minor fraction (<1%) of the total OA mass, which may be the result of the neutralized aerosol at the site or because only a small number of organosulfates were quantified. The low contribution of organosulfates to total OA suggests that other mechanisms, e.g. NO_x enhancement of oxidant levels, are likely responsible for the majority of the anthropogenic enhancement of biogenic secondary organic aerosol observed at this site
Electric-magnetic duality and the conditions of inflationary magnetogenesis
The magnetogenesis scenarios triggered by the early variation of the gauge
coupling are critically analyzed. In the absence of sources, it is shown that
the electric and magnetic power spectra can be explicitly computed by means of
electric-magnetic duality transformations. The remnants of a pre-inflationary
expansion and the reheating process break explicitly electric-magnetic duality
by inducing Ohmic currents. The generation of large-scale magnetic field and
the physical distinction between electric and magnetic observables stems, in
this class of models, from the final value reached by the conductivity of the
plasma right after inflation. Specific numerical examples are given. The
physical requirements of viable magnetogenesis scenarios are spelled out.Comment: 25 pages, 9 figure
Isospin Breaking Effects in the Extraction of Isoscalar and Isovector Spectral Functions From
We investigate the problem of the extraction of the isovector and isoscalar
spectral functions from data on , in the presence of
non-zero isospin breaking. It is shown that the conventional approach to
extracting the isovector spectral function in the resonance region, in
which only the isoscalar contribution associated with is
subtracted, fails to fully remove the effects of the isoscalar component of the
electromagnetic current. The additional subtractions required to extract the
pure isovector and isoscalar spectral functions are estimated using results
from QCD sum rules. It is shown that the corrections are small () in
the isovector case (though relevant to precision tests of CVC), but very large
() in the case of the contribution to the isoscalar spectral
function. The reason such a large effect is natural in the isoscalar channel is
explained, and implications for other applications, such as the extraction of
the sixth order chiral low-energy constant, , are discussed.Comment: minor changes to introduction, section 2. In Press Phys. Rev.
Third Generation Effects on Fermion Mass Predictions in Supersymmetric Grand Unified Theories
Relations among fermion masses and mixing angles at the scale of grand
unification are modified at lower energies by renormalization group running
induced by gauge and Yukawa couplings. In supersymmetric theories, the
quark and lepton Yukawa couplings, as well as the quark coupling,
may cause significant running if , the ratio of Higgs field
expectation values, is large. We present approximate analytic expressions for
the scaling factors for fermion masses and CKM matrix elements induced by all
three third generation Yukawa couplings. We then determine how running caused
by the third generation of fermions affects the predictions arising from three
possible forms for the Yukawa coupling matrices at the GUT scale: the
Georgi-Jarlskog, Giudice, and Fritzsch textures.Comment: phyzzx, 26 pp., 6 figures not included, e-mailable upon request,
JHU-TIPAC-93000
Current helicity of active regions as a tracer of large-scale solar magnetic helicity
We demonstrate that the current helicity observed in solar active regions
traces the magnetic helicity of the large-scale dynamo generated field. We use
an advanced 2D mean-field dynamo model with dynamo saturation based on the
evolution of the magnetic helicity and algebraic quenching. For comparison, we
also studied a more basic 2D mean-field dynamo model with simple algebraic
alpha quenching only. Using these numerical models we obtained butterfly
diagrams both for the small-scale current helicity and also for the large-scale
magnetic helicity, and compared them with the butterfly diagram for the current
helicity in active regions obtained from observations. This comparison shows
that the current helicity of active regions, as estimated by
evaluated at the depth from which the active region arises, resembles the
observational data much better than the small-scale current helicity calculated
directly from the helicity evolution equation. Here and are
respectively the dynamo generated mean magnetic field and its vector potential.
A theoretical interpretation of these results is given.Comment: 11 pages, 5 figures, revised versio
- …