3,324 research outputs found
Theory of light-enhanced phonon-mediated superconductivity
We investigate the dynamics of a phonon-mediated superconductor driven out of
equilibrium. The electronic hopping amplitude is ramped down in time, resulting
in an increased electronic density of states. The dynamics of the coupled
electron-phonon model is investigated by solving Migdal-Eliashberg equations
for the double-time Keldysh Green's functions. The increase of the density of
states near the Fermi level leads to an enhancement of superconductivity when
the system thermalizes to the new state at the same temperature. We provide a
time- and momentum-resolved view on this thermalization process, and show that
it involves fast processes associated with single-particle scattering and much
slower dynamics associated with the superconducting order parameter. The
importance of electron-phonon coupling for the rapid enhancement and the
efficient thermalization of superconductivity is demonstrated, and the results
are compared to a BCS time-dependent mean-field approximation.Comment: 12 pages, 8 figure
Modelling the alumina abundance of oxygen-rich evolved stars in the Large Magellanic Cloud
In order to determine the composition of the dust in the circumstellar
envelopes of oxygen-rich asymptotic giant branch (AGB) stars we have computed a
grid of modust radiative-transfer models for a range of dust compositions,
mass-loss rates, dust shell inner radii and stellar parameters. We compare the
resulting colours with the observed oxygen-rich AGB stars from the SAGE-Spec
Large Magellanic Cloud (LMC) sample, finding good overall agreement for stars
with a mid-infrared excess. We use these models to fit a sample of 37 O-rich
AGB stars in the LMC with optically thin circumstellar envelopes, for which
535-m Spitzer infrared spectrograph (IRS) spectra and broadband
photometry from the optical to the mid-infrared are available. From the
modelling, we find mass-loss rates in the range to
M, and we show that a grain
mixture consisting primarily of amorphous silicates, with contributions from
amorphous alumina and metallic iron provides a good fit to the observed
spectra. Furthermore, we show from dust models that the AKARI [11][15]
versus [3.2][7] colour-colour diagram, is able to determine the fractional
abundance of alumina in O-rich AGB stars.Comment: 22 pages, 17 figures, accepted MNRA
Campylobacter spp., Enterococcus spp., Escherichia coli, Salmonella spp., Yersinia spp., and Cryptosporidium oocysts in semi-domesticated reindeer (Rangifer tarandus tarandus) in Northern Finland and Norway
The specific aim of this study was to assess the faecal shedding of zoonotic enteropathogens by semi-domesticated reindeer (Rangifer tarandus tarandus) to deduce the potential risk to human health through modern reindeer herding. In total, 2,243 faecal samples of reindeer from northern regions of Finland and Norway were examined for potentially enteropathogenic bacteria (Campylobacter species, Enterococcus species, Escherichia coli, Salmonella species and Yersinia species) and parasites (Cryptosporidium species) in accordance with standard procedures. Escherichia coli were isolated in 94.7%, Enterococcus species in 92.9%, Yersinia species in 4.8% of the samples and Campylobacter species in one sample only (0.04%). Analysis for virulence factors in E. coli and Yersinia species revealed no pathogenic strains. Neither Salmonella species nor Cryptosporidium oocysts were detected. The public health risk due to reindeer husbandry concerning zoonotic diseases included in this study has to be considered as very low at present but a putative epidemiological threat may arise when herding conditions are changed with respect to intensification and crowding
Ultrafast Dynamics of Vibrational Symmetry Breaking in a Charge-ordered Nickelate
The ability to probe symmetry breaking transitions on their natural time
scales is one of the key challenges in nonequilibrium physics. Stripe ordering
represents an intriguing type of broken symmetry, where complex interactions
result in atomic-scale lines of charge and spin density. Although phonon
anomalies and periodic distortions attest the importance of electron-phonon
coupling in the formation of stripe phases, a direct time-domain view of
vibrational symmetry breaking is lacking. We report experiments that track the
transient multi-THz response of the model stripe compound
LaSrNiO, yielding novel insight into its electronic and
structural dynamics following an ultrafast optical quench. We find that
although electronic carriers are immediately delocalized, the crystal symmetry
remains initially frozen - as witnessed by time-delayed suppression of
zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry.
Longitudinal and transverse vibrations react with different speeds, indicating
a strong directionality and an important role of polar interactions. The hidden
complexity of electronic and structural coupling during stripe melting and
formation, captured here within a single terahertz spectrum, opens new paths to
understanding symmetry breaking dynamics in solids.Comment: 21 pages, 4 figures; updated version with journal re
On the metallicity dependence of crystalline silicates in oxygen-rich asymptotic giant branch stars and red supergiants
We investigate the occurrence of crystalline silicates in oxygen-rich evolved
stars across a range of metallicities and mass-loss rates. It has been
suggested that the crystalline silicate feature strength increases with
increasing mass-loss rate, implying a correlation between lattice structure and
wind density. To test this, we analyse Spitzer IRS and Infrared Space
Observatory SWS spectra of 217 oxygen-rich asymptotic giant branch stars and 98
red supergiants in the Milky Way, the Large and Small Magellanic Clouds and
Galactic globular clusters. These encompass a range of spectral morphologies
from the spectrally-rich which exhibit a wealth of crystalline and amorphous
silicate features to 'naked' (dust-free) stars. We combine spectroscopic and
photometric observations with the GRAMS grid of radiative transfer models to
derive (dust) mass-loss rates and temperature. We then measure the strength of
the crystalline silicate bands at 23, 28 and 33 microns. We detect crystalline
silicates in stars with dust mass-loss rates which span over 3 dex, down to
rates of ~10^-9 solar masses/year. Detections of crystalline silicates are more
prevalent in higher mass-loss rate objects, though the highest mass-loss rate
objects do not show the 23-micron feature, possibly due to the low temperature
of the forsterite grains or it may indicate that the 23-micron band is going
into absorption due to high column density. Furthermore, we detect a change in
the crystalline silicate mineralogy with metallicity, with enstatite seen
increasingly at low metallicity.Comment: Accepted for publication in MNRAS, 24 pages, 16 figure
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