2,576 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
Van der Waals Density Functional for General Geometries
A scheme within density functional theory is proposed that provides a
practical way to generalize to unrestricted geometries the method applied with
some success to layered geometries [H. Rydberg, et al., Phys. Rev. Lett. 91,
126402 (2003)]. It includes van der Waals forces in a seamless fashion. By
expansion to second order in a carefully chosen quantity contained in the long
range part of the correlation functional, the nonlocal correlations are
expressed in terms of a density-density interaction formula. It contains a
relatively simple parametrized kernel, with parameters determined by the local
density and its gradient. The proposed functional is applied to rare gas and
benzene dimers, where it is shown to give a realistic description.Comment: 4 pages, 4 figure
Flash ionization of the partially ionized wind of the progenitor of SN 1987A
The H II region created by the progenitor of SN 1987A was further heated and
ionized by the supernova flash. Prior to the flash, the temperature of the gas
was 4000 - 5000 K, and helium was neutral, while the post-flash temperature was
only slightly less than 10^5 K, with the gas being ionized to helium-like
ionization stages of C, N and O. We have followed the slow post-flash cooling
and recombination of the gas, as well as its line emission, and find that the
strongest lines are N V 1240 and O VI 1034. Both these lines are good probes
for the density of the gas, and suitable instruments to detect the lines are
STIS on HST and FUSE, respectively. Other lines which may be detectable are N
IV] 1486 and [O III] 5007, though they are expected to be substantially weaker.
The relative strength of the oxygen lines is found to be a good tracer of the
color temperature of the supernova flash. From previous observations, we put
limits on the hydrogen density, n_H, of the H II region. The early N V 1240
flux measured by IUE gives an upper limit which is n_H ~ 180 \eta^{-0.40}
cm^{-3}, where \eta is the filling factor of the gas. The recently reported
emission in [O III] 5007 at 2500 days requires n_H = (160\pm12) \eta^{-0.19}
cm^{-3}, for a supernova burst similar to that in the 500full1 model of Ensman
& Burrows (1992). For the more energetic 500full2 burst the density is n_H =
(215\pm15) \eta^{-0.19} cm^{-3}. These values are much higher than in models of
the X-ray emission from the supernova (n_H ~ 75 cm^{-3}), and it seems
plausible that the observed [O III] emission is produced primarily elsewhere
than in the H II region. We also discuss the type of progenitor consistent with
the H II region. In particular, it seems unlikely that its spectral type was
much earlier than B2 Ia.Comment: LaTeX, 23 pages including 4 figures. To appear in ApJ (Main Journal
A Higher-Accuracy van der Waals Density Functional
We propose a second version of the van der Waals density functional (vdW-DF2)
of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing a more accurate
semilocal exchange functional and the use of a large-N asymptote gradient
correction in determining the vdW kernel. The predicted binding energy,
equilibrium separation, and potential-energy curve shape are close to those of
accurate quantum chemical calculations on 22 duplexes. We anticipate the
enabling of chemically accurate calculations in sparse materials of importance
for condensed-matter, surface, chemical, and biological physics.Comment: 14 pages, 10 figure
Cognitive behavioral intervention in sport psychology: A case illustration of the exposure method with an elite athlete
One common method in Cognitive behavior therapy (CBT) to treat anxiety problems is exposure, but there are few articles examining its applicability to sport. The aim of this article is to give a background of the use of exposure in sport and present a case of how exposure can be used with athletes. The athlete was a 17-year-old female cross-country skier with high levels of performance anxiety. In the case description, common procedures in CBT such as behavioral analysis, psychoeducation, and exposure are presented, as well as how anxiety can be managed. After the intervention the athlete perceived lower levels of anxiety as well as improved behavioral repertoire (e.g., less avoidant behaviors and more functional sport-specific behaviors). This case may be used to help practitioners consider the use of exposure in competitive sports. © 2016 Association for Applied Sport Psycholog
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