3,907 research outputs found
Dominant particle-hole contributions to the phonon dynamics in the spinless one-dimensional Holstein model
In the spinless Holstein model at half-filling the coupling of electrons to
phonons is responsible for a phase transition from a metallic state at small
coupling to a Peierls distorted insulated state when the electron-phonon
coupling exceeds a critical value. For the adiabatic case of small phonon
frequencies, the transition is accompanied by a phonon softening at the
Brillouin zone boundary whereas a hardening of the phonon mode occurs in the
anti-adiabatic case. The phonon dynamics studied in this letter do not only
reveal the expected renormalization of the phonon modes but also show
remarkable additional contributions due to electronic particle-hole
excitations.Comment: 7 pages, 4 figures and 1 table included; v2: discussion of Luttinger
liquid parameters adde
Linear response within the projection-based renormalization method: Many-body corrections beyond the random phase approximation
The explicit evaluation of linear response coefficients for interacting
many-particle systems still poses a considerable challenge to theoreticians. In
this work we use a novel many-particle renormalization technique, the so-called
projector-based renormalization method, to show how such coefficients can
systematically be evaluated. To demonstrate the prospects and power of our
approach we consider the dynamical wave-vector dependent spin susceptibility of
the two-dimensional Hubbard model and also determine the subsequent magnetic
phase diagram close to half-filling. We show that the superior treatment of
(Coulomb) correlation and fluctuation effects within the projector-based
renormalization method significantly improves the standard random phase
approximation results.Comment: 17 pages, 7 figures, revised versio
Safety of intravenous thrombolysis for acute ischemic stroke in patients receiving antiplatelet therapy at stroke onset
<p><b>Background and Purpose:</b> Antiplatelets (APs) may increase the risk of symptomatic intracerebral hemorrhage (ICH) following intravenous thrombolysis after ischemic stroke.</p>
<p><b>Methods:</b> We assessed the safety of thrombolysis under APs in 11 865 patients compliant with the European license criteria and recorded between 2002 and 2007 in the Safe Implementation of Treatments in Stroke (SITS) International Stroke Thrombolysis Register (SITS-ISTR). Outcome measures of univariable and multivariable analyses included symptomatic ICH (SICH) per SITS Monitoring Study (SITS-MOST [deterioration in National Institutes of Health Stroke Scale >= 4 plus ICH type 2 within 24 hours]), per European Cooperative Acute Stroke Study II (ECASS II [deterioration in National Institutes of Health Stroke Scale >= 4 plus any ICH]), functional outcome at 3 months and mortality.</p>
<p><b>Results:</b> A total of 3782 (31.9%) patients had received 1 or 2 AP drugs at baseline: 3016 (25.4%) acetylsalicylic acid (ASA), 243 (2.0%) clopidogrel, 175 (1.5%) ASA and dipyridamole, 151 (1.3%) ASA and clopidogrel, and 197 (1.7%) others. Patients receiving APs were 5 years older and had more risk factors than AP nave patients. Incidences of SICH per SITS-MOST (ECASS II respectively) were as follows: 1.1% (4.1%) AP naive, 2.5% (6.2%) any AP, 2.5% (5.9%) ASA, 1.7% (4.2%) clopidogrel, 2.3% (5.9%) ASA and dipyridamole, and 4.1% (13.4%) ASA and clopidogrel. In multivariable analyses, the combination of ASA and clopidogrel was associated with increased risk for SICH per ECASS II (odds ratio, 2.11; 95% CI, 1.29 to 3.45; P = 0.003). However, we found no significant increase in the risk for mortality or poor functional outcome, irrespective of the AP subgroup or SICH definition.</p>
<p><b>Conclusion:</b> The absolute excess of SICH of 1.4% (2.1%) in the pooled AP group is small compared with the benefit of thrombolysis seen in randomized trials. Although caution is warranted in patients receiving the combination of ASA and clopidogrel, AP treatment should not be considered a contraindication to thrombolysis.</p>
Modelling magnetic flux emergence in the solar convection zone
[Abridged] Bipolar magnetic regions are formed when loops of magnetic flux
emerge at the solar photosphere. Our aim is to investigate the flux emergence
process in a simulation of granular convection. In particular we aim to
determine the circumstances under which magnetic buoyancy enhances the flux
emergence rate (which is otherwise driven solely by the convective upflows). We
use three-dimensional numerical simulations, solving the equations of
compressible magnetohydrodynamics in a horizontally-periodic Cartesian domain.
A horizontal magnetic flux tube is inserted into fully developed hydrodynamic
convection. We systematically vary the initial field strength, the tube
thickness, the initial entropy distribution along the tube axis and the
magnetic Reynolds number. Focusing upon the low magnetic Prandtl number regime
(Pm<1) at moderate magnetic Reynolds number, we find that the flux tube is
always susceptible to convective disruption to some extent. However, stronger
flux tubes tend to maintain their structure more effectively than weaker ones.
Magnetic buoyancy does enhance the flux emergence rates in the strongest
initial field cases, and this enhancement becomes more pronounced when we
increase the width of the flux tube. This is also the case at higher magnetic
Reynolds numbers, although the flux emergence rates are generally lower in
these less dissipative simulations because the convective disruption of the
flux tube is much more effective in these cases. These simulations seem to be
relatively insensitive to the precise choice of initial conditions: for a given
flow, the evolution of the flux tube is determined primarily by the initial
magnetic field distribution and the magnetic Reynolds number.Comment: 12 pages, 15 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
Solar Flux Emergence Simulations
We simulate the rise through the upper convection zone and emergence through
the solar surface of initially uniform, untwisted, horizontal magnetic flux
with the same entropy as the non-magnetic plasma that is advected into a domain
48 Mm wide from from 20 Mm deep. The magnetic field is advected upward by the
diverging upflows and pulled down in the downdrafts, which produces a hierarchy
of loop like structures of increasingly smaller scale as the surface is
approached. There are significant differences between the behavior of fields of
10 kG and 20 or 40 kG strength at 20 Mm depth. The 10 kG fields have little
effect on the convective flows and show little magnetic buoyancy effects,
reaching the surface in the typical fluid rise time from 20 Mm depth of 32
hours. 20 and 40 kG fields significantly modify the convective flows, leading
to long thin cells of ascending fluid aligned with the magnetic field and their
magnetic buoyancy makes them rise to the surface faster than the fluid rise
time. The 20 kG field produces a large scale magnetic loop that as it emerges
through the surface leads to the formation of a bipolar pore-like structure.Comment: Solar Physics (in press), 12 pages, 13 figur
Detection of supersonic downflows and associated heating events in the transition region above sunspots
IRIS data allow us to study the solar transition region (TR) with an
unprecedented spatial resolution of 0.33 arcsec. On 2013 August 30, we observed
bursts of high Doppler shifts suggesting strong supersonic downflows of up to
200 km/s and weaker, slightly slower upflows in the spectral lines Mg II h and
k, C II 1336 \AA, Si IV 1394 \AA, and 1403 \AA, that are correlated with
brightenings in the slitjaw images (SJIs). The bursty behavior lasts throughout
the 2 hr observation, with average burst durations of about 20 s. The locations
of these short-lived events appear to be the umbral and penumbral footpoints of
EUV loops. Fast apparent downflows are observed along these loops in the SJIs
and in AIA, suggesting that the loops are thermally unstable. We interpret the
observations as cool material falling from coronal heights, and especially
coronal rain produced along the thermally unstable loops, which leads to an
increase of intensity at the loop footpoints, probably indicating an increase
of density and temperature in the TR. The rain speeds are on the higher end of
previously reported speeds for this phenomenon, and possibly higher than the
free-fall velocity along the loops. On other observing days, similar bright
dots are sometimes aligned into ribbons, resembling small flare ribbons. These
observations provide a first insight into small-scale heating events in
sunspots in the TR.Comment: accepted by ApJ
An Interface Region Imaging Spectrograph first view on Solar Spicules
Solar spicules have eluded modelers and observers for decades. Since the
discovery of the more energetic type II, spicules have become a heated topic
but their contribution to the energy balance of the low solar atmosphere
remains unknown. Here we give a first glimpse of what quiet Sun spicules look
like when observed with NASA's recently launched Interface Region Imaging
Spectrograph (IRIS). Using IRIS spectra and filtergrams that sample the
chromosphere and transition region we compare the properties and evolution of
spicules as observed in a coordinated campaign with Hinode and the Atmospheric
Imaging Assembly. Our IRIS observations allow us to follow the thermal
evolution of type II spicules and finally confirm that the fading of Ca II H
spicules appears to be caused by rapid heating to higher temperatures. The IRIS
spicules do not fade but continue evolving, reaching higher and falling back
down after 500-800 s. Ca II H type II spicules are thus the initial stages of
violent and hotter events that mostly remain invisible in Ca II H filtergrams.
These events have very different properties from type I spicules, which show
lower velocities and no fading from chromospheric passbands. The IRIS spectra
of spicules show the same signature as their proposed disk counterparts,
reinforcing earlier work. Spectroheliograms from spectral rasters also confirm
that quiet Sun spicules originate in bushes from the magnetic network. Our
results suggest that type II spicules are indeed the site of vigorous heating
(to at least transition region temperatures) along extensive parts of the
upward moving spicular plasma.Comment: 6 pages, 4 figures, accepted for publication in ApJ Letters. For
associated movies, see http://folk.uio.no/tiago/iris_spic
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