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

Wigner crystallization in quantum electron bilayers

The phase diagram of quantum electron bilayers in zero magnetic field is obtained using density functional theory. For large electron densities the system is in the liquid phase, while for smaller densities the liquid may freeze (Wigner crystallization) into four different crystalline phases; the lattice symmetry and the critical density depend on the the inter-layer distance. The phase boundaries between different Wigner crystals consist of both first and second order transitions, depending on the phases involved, and join the freezing curve at three different triple points.Comment: To appear in Europhys. Lett. (11 pages in REVTEX + 2 figures in postscript

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$_3$, 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

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

Electron self-energy in A3C60 (A=K, Rb): Effects of t1u plasmon in GW approximation

The electron self-energy of the t1u states in A3C60 (A=K, Rb) is calculated using the so-called GW approximation. The calculation is performed within a model which considers the t1u charge carrier plasmon at 0.5 eV and takes into account scattering of the electrons within the t1u band. A moderate reduction (35 %) of the t1u band width is obtained.Comment: 4 pages, revtex, 1 figure more information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene

Managers' and employees' experiences of how managers' wellbeing impacts their leadership behaviours in Swedish small businesses

BACKGROUND: There is a growing interest in managers' wellbeing due to the observed associations between their wellbeing and leadership behaviours, and between leadership behaviours and employees' wellbeing. However, it is still unclear how managers' wellbeing influences their practiced leadership across different workplace contexts, which specific behaviours are affected, and how this varies across time. OBJECTIVE: The purpose of this study was therefore to explore managers' and employees' experiences and perceptions regarding the consequences of managers' wellbeing for their leadership behaviours in small businesses. METHODS: Semi-structured interviews were conducted with 39 participants (20 managers and 19 employees) working at 12 Swedish small firms, and analysed using content analysis. RESULTS: The findings showthat managers were more constructive when they felt well, and more passively destructive when unwell. Variations in managers' wellbeing influenced their mood, energy level, and performance, as well as the company's working climate. However, these destructive leadership variations did not have a substantial impact, because several protective factors were present. CONCLUSION: This study shows that the wellbeing of managers in small businesses has perceptible consequences for their leadership behaviours. The study also shows that sustained leadership behaviours may coexist with temporary variations of these behaviours on a constructive-destructive continuum depending on the leader's wellbeing. Overall, the findings contribute to a more nuanced and dynamic understanding of how the interaction between managers' wellbeing and their behaviours unfolds in the particular context of small companies

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

Recent X-Ray Observations of SN1986J with ASCA and ROSAT

We present ASCA and ROSAT observations of SN 1986J covering the period 1991 August to 1996 January. From observations with the ROSAT HRI and PSPC, we find that the 0.5-2.5 keV flux decreased proportional to $t^{-2}$ during this period; the ASCA data are consistent with this result and extend it to the 2-10 keV band. ASCA spectra from 1994 January and 1996 January are consistent with thermal emission from a solar metallicity plasma at an equilibrium temperature kT = 5-7.5 keV, somewhat hotter than that observed from other X-ray supernovae. These spectra also show a clear Fe K emission line at 6.7 keV with FWHM < 20,000 km/s (90% confidence). This limit on the line width is consistent with the reverse shock model of Chevalier & Fransson (1994), but does not rule out the clumpy wind model of Chugai (1993).Comment: 20 pages, 9 postscript figures, latex, uses aastex4.0, submitted to The Astrophysical Journa

Human observers and automated assessment of dynamic emotional facial expressions: KDEF-dyn database validation

Most experimental studies of facial expression processing have used static stimuli (photographs), yet facial expressions in daily life are generally dynamic. In its original photographic format, the Karolinska Directed Emotional Faces (KDEF) has been frequently utilized. In the current study, we validate a dynamic version of this database, the KDEF-dyn. To this end, we applied animation between neutral and emotional expressions (happy, sad, angry, fearful, disgusted, and surprised; 1,033-ms unfolding) to 40 KDEF models, with morphing software. Ninety-six human observers categorized the expressions of the resulting 240 video-clip stimuli, and automated face analysis assessed the evidence for 6 expressions and 20 facial action units (AUs) at 31 intensities. Low-level image properties (luminance, signal-to-noise ratio, etc.) and other purely perceptual factors (e.g., size, unfolding speed) were controlled. Human recognition performance (accuracy, efficiency, and confusions) patterns were consistent with prior research using static and other dynamic expressions. Automated assessment of expressions and AUs was sensitive to intensity manipulations. Significant correlations emerged between human observers' categorization and automated classification. The KDEF-dyn database aims to provide a balance between experimental control and ecological validity for research on emotional facial expression processing. The stimuli and the validation data are available to the scientific community