1,879 research outputs found
Effect of uniaxial strain on plasmon excitations in graphene
Uniaxial strain is known to modify significantly the electronic properties of
graphene, a carbon single layer of atomic width. Here, we study the effect of
applied strain on the composite excitations arising from the coupling of charge
carriers and plasmons in graphene, i.e. the plasmarons. Specifically, we
predict that the plasmaron energy dispersion, which has been recently observed
experimentally in unstrained graphene, is shifted and broadened by applied
uniaxial strain. Thus, strain constitutes an additional parameter which may be
useful to tune graphene properties in plasmaronic devices.Comment: Invited oral lecture at the 23rd AIRAPT International Conference on
"High Pressure Science and Technology", Mumbai (India), September 25-30,
2011. To be published in J. Phys.: Conf. Series (2012
van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions
The theoretical description of sparse matter attracts much interest, in
particular for those ground-state properties that can be described by density
functional theory (DFT). One proposed approach, the van der Waals density
functional (vdW-DF) method, rests on strong physical foundations and offers
simple yet accurate and robust functionals. A very recent functional within
this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89,
035412] stands out in its attempt to use an exchange energy derived from the
same plasmon-based theory from which the nonlocal correlation energy was
derived. Encouraged by its good performance for solids, layered materials, and
aromatic molecules, we apply it to several systems that are characterized by
competing interactions. These include the ferroelectric response in PbTiO,
the adsorption of small molecules within metal-organic frameworks (MOFs), the
graphite/diamond phase transition, and the adsorption of an aromatic-molecule
on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well
suited to tackle these challenging systems. In addition to being a competitive
density functional for sparse matter, the vdW-DF-cx construction presents a
more robust general purpose functional that could be applied to a range of
materials problems with a variety of competing interactions
Atrial high-rate episodes: prevalence, stroke risk, implications for management, and clinical gaps in evidence
Self-terminating atrial arrhythmias are commonly detected on continuous rhythm monitoring, e.g. by pacemakers or defibrillators. It is unclear whether the presence of these arrhythmias has therapeutic consequences. We sought to summarize evidence on the prevalence of atrial high-rate episodes (AHREs) and their impact on risk of stroke. We performed a comprehensive, tabulated review of published literature on the prevalence of AHRE. In patients with AHRE, but without atrial fibrillation (AF), we reviewed the stroke risk and the potential risk/benefit of oral anticoagulation. Atrial high-rate episodes are found in 10-30% of AF-free patients. Presence of AHRE slightly increases stroke risk (0.8% to 1%/year) compared with patients without AHRE. Atrial high-rate episode of longer duration (e.g. those >24 h) could be associated with a higher stroke risk. Oral anticoagulation has the potential to reduce stroke risk in patients with AHRE but is associated with a rate of major bleeding of 2%/year. Oral anticoagulation is not effective in patients with heart failure or survivors of a stroke without AF. It remains unclear whether anticoagulation is effective and safe in patients with AHRE. Atrial high-rate episodes are common and confer a slight increase in stroke risk. There is true equipoise on the best way to reduce stroke risk in patients with AHRE. Two ongoing trials (NOAH-AFNET 6 and ARTESiA) will provide much-needed information on the effectiveness and safety of oral anticoagulation using non-vitamin K antagonist oral anticoagulants in patients with AHRE.info:eu-repo/semantics/publishedVersio
No trace of a single-degenerate companion in late spectra of SNe 2011fe and 2014J
Left-over, ablated material from a possible non-degenerate companion can
reveal itself after about one year in spectra of Type Ia SNe (SNe Ia). We have
searched for such material in spectra of SN 2011fe (at 294 days after the
explosion) and for SN 2014J (315 days past explosion). The observations are
compared with numerical models simulating the expected line emission. The
spectral lines sought for are H-alpha, [O I] 6300 and [Ca II] 7291,7324, and
the expected width of these lines is about 1000 km/s. No signs of these lines
can be traced in any of the two supernovae. When systematic uncertainties are
included, the limits on hydrogen-rich ablated gas in SNe 2011fe and 2014J are
0.003 M_sun and 0.0085 M_sun, respectively, where the limit for SN 2014J is the
second lowest ever, and the limit for SN 2011fe is a revision of a previous
limit. Limits are also put on helium-rich ablated gas. These limits are used,
in conjunction with other data, to argue that these supernovae can stem from
double-degenerate systems, or from single-degenerate systems with a spun
up/spun down super-Chandrasekhar white dwarf. For SN 2011fe, other types of
hydrogen-rich donors can likely be ruled out, whereas for SN 2014J a
main-sequence donor system with large intrinsic separation is still possible.
Helium-rich donor systems cannot be ruled out for any of the two supernovae,
but the expected short delay time for such progenitors makes this possibility
less likely, especially for SN 2011fe. The broad [Ni II] 7378 emission in SN
2014J is redshifted by about +1300 km/s, as opposed to the known blueshift of
roughly -1100 km/s for SN 2011fe. [Fe II] 7155 is also redshifted in SN 2014J.
SN 2014J belongs to a minority of SNe Ia that both have a nebular redshift of
[Fe II] 7155 and [Ni II] 7378, and a slow decline of the Si II 6355 absorption
trough just after B-band maximum.Comment: 13 pages, submitted to A&
Towards a working density-functional theory for polymers: First-principles determination of the polyethylene crystal structure
Equilibrium polyethylene crystal structure, cohesive energy, and elastic
constants are calculated by density-functional theory applied with a recently
proposed density functional (vdW-DF) for general geometries [Phys. Rev. Lett.
92, 246401 (2004)] and with a pseudopotential-planewave scheme. The vdW-DF with
its account for the long-ranged van der Waals interactions gives not only a
stabilized crystal structure but also values of the calculated lattice
parameters and elastic constants in quite good agreement with experimental
data, giving promise for successful application to a wider range of polymers.Comment: 4 pages, 3 figure
Charge carrier interaction with a purely electronic collective mode: Plasmarons and the infrared response of elemental bismuth
We present a detailed optical study of single crystal bismuth using infrared
reflectivity and ellipsometry. Colossal changes in the plasmon frequency are
observed as a function of temperature due to charge transfer between hole and
electron Fermi pockets. In the optical conductivity, an anomalous temperature
dependent mid-infrared absorption feature is observed. An extended Drude model
analysis reveals that it can be connected to a sharp upturn in the scattering
rate, the frequency of which exactly tracks the temperature dependent plasmon
frequency. We interpret this absorption and increased scattering as the first
direct optical evidence for a charge carrier interaction with a collective mode
of purely electronic origin; here electron-plasmon scattering. The observation
of a \emph{plasmaron} as such is made possible only by the unique coincidence
of various energy scales and exceptional properties of semi-metal bismuth.Comment: 4 pages, 4 figure
Spectral evolution and polarization of variable structures in the pulsar wind nebula of PSR B0540-69.3
We present high spatial resolution optical imaging and polarization
observations of the PSR B0540-69.3 and its highly dynamical pulsar wind nebula
(PWN) performed with HST, and compare them with X-ray data obtained with the
Chandra X-ray Observatory. We have studied the bright region southwest of the
pulsar where a bright "blob" is seen in 1999. We show that it may be a result
of local energy deposition around 1999, and that the emission from this then
faded away. Polarization data from 2007 show that the polarization properties
show dramatic spatial variations at the 1999 blob position arguing for a local
process. Several other positions along the pulsar-"blob" orientation show
similar changes in polarization, indicating previous recent local energy
depositions. In X-rays, the spectrum steepens away from the "blob" position,
faster orthogonal to the pulsar-"blob" direction than along this axis of
orientation. This could indicate that the pulsar-"blob" orientation is an axis
along where energy in the PWN is mainly injected, and that this is then
mediated to the filaments in the PWN by shocks. We highlight this by
constructing an [S II]-to-[O III]-ratio map. We argue, through modeling, that
the high [S II]/[O III] ratio is not due to time-dependent photoionization
caused by possible rapid Xray emission variations in the "blob" region. We have
also created a multiwavelength energy spectrum for the "blob" position showing
that one can, to within 2sigma, connect the optical and X-ray emission by a
single power law. We obtain best power-law fits for the X-ray spectrum if we
include "extra" oxygen, in addition to the oxygen column density in the
interstellar gas of the Large Magellanic Cloud and the Milky Way. This oxygen
is most naturally explained by the oxygen-rich ejecta of the supernova remnant.
The oxygen needed likely places the progenitor mass in the 20 - 25 Msun range.Comment: Accepted by MNRAS on December 6th 2010, 18 pages, 15 figures. The
article with full resolution figures is available here
ftp://ftp.astro.su.se/pub/peter/papers/pwn0540_2010_corrected.pd
Late-Time Optical and UV Spectra of SN 1979C and SN 1980K
A low-dispersion Keck I spectrum of SN 1980K taken in August 1995 (t = 14.8
yr after explosion) and a November 1997 MDM spectrum (t = 17.0 yr) show broad
5500 km s^{-1} emission lines of H\alpha, [O I] 6300,6364 A, and [O II]
7319,7330 A. Weaker but similarly broad lines detected include [Fe II] 7155 A,
[S II] 4068,4072 A, and a blend of [Fe II] lines at 5050--5400 A. The presence
of strong [S II] 4068,4072 A emission but a lack of [S II] 6716,6731 A emission
suggests electron densities of 10^{5-6} cm^{-3}. From the 1997 spectra, we
estimate an H\alpha flux of 1.3 \pm 0.2 \times 10^{-15} erg cm^{-2} s^{-1}
indicating a 25% decline from 1987--1992 levels during the period 1994 to 1997,
possibly related to a reported decrease in its nonthermal radio emission.Comment: 21 pages, 8 figures, submitted to the Astronomical Journa
Optical identification of the 3C 58 pulsar wind nebula
We have performed a deep optical imaging of 3C 58 SNR with the NOT in the B
and V bands to detect the optical counterpart of the associated pulsar
J0295+6449 and its torus-like wind nebula visible in X-rays. We analyzed our
data together with the archival data obtained with the Chandra in X-rays and
with the Spitzer in the mid-IR. We detect a faint extended elliptical object
with B=24.06 and V=23.11 whose peak brightness and center position are
consistent at the sub-arcsecond level with the position of the pulsar. Its
morphology and orientation are in excellent agreement with the torus-like
pulsar nebula, seen almost edge on in X-rays although its extension is only
about a half of that in X-rays. In the optical we likely see only the brightest
central part of the torus with the pulsar. The object is identical to the
counterpart of the torus recently detected in the mid-IR. The estimated pulsar
contribution to the optical flux is less than 10%. Combinig the optical/mid-IR
fluxes and X-ray power-law spectrum extracted from the spatial region
constrained by the optical/IR source extent we compile a tentative
multi-wavelength spectrum of the central part of the nebula. Within
uncertainties of the interstellar extinction it is reminiscent of either the
Crab or B0540-69 pulsar wind nebula spectra. The properties of the object
strongly suggest it to be the optical counterpart of the 3C 58 pulsar + its
wind nebula system, making 3C 58 the third member of such a class of the
torus-like systems identified in the optical and mid-IR.Comment: 12 pages including 7 figures, submitted for publication in A&A. For
high resolution images, see http://www.ioffe.ru/astro/NSG/obs/3C58
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