155 research outputs found
Electronic structure of triangular, hexagonal and round graphene flakes near the Fermi level
The electronic shell structure of triangular, hexagonal and round graphene
quantum dots (flakes) near the Fermi level has been studied using a
tight-binding method. The results show that close to the Fermi level the shell
structure of a triangular flake is that of free massless particles, and that
triangles with an armchair edge show an additional sequence of levels ("ghost
states"). These levels result from the graphene band structure and the plane
wave solution of the wave equation, and they are absent for triangles with an
zigzag edge. All zigzag triangles exhibit a prominent edge state at the Fermi
level, and few low-energy conduction electron states occur both in triangular
and hexagonal flakes due to symmetry reasons. Armchair triangles can be used as
building blocks for other types of flakes that support the ghost states. Edge
roughness has only a small effect on the level structure of the triangular
flakes, but the effect is considerably enhanced in the other types of flakes.
In round flakes, the states near the Fermi level depend strongly on the flake
radius, and they are always localized on the zigzag parts of the edge
Leaving no-one behind? Social inequalities and contrasting development impacts in rural Rwanda
Two polar viewpoints have emerged regarding Rwandaâs post-genocide development: (1) that economic development has improved the wellbeing of Rwandans and (2) that repressive policies have negatively impacted many. Assessing the impacts and inclusiveness of policies through trends among different social groups is timely in the context of the Sustainable Development Goalsâ pledge to âleave no-one behindâ. This study examines rural Rwandansâ perspectives on the changes affecting them. A multidimensional wellbeing approach was applied through mixed-method research involving 115 rural households in two locations in western Rwanda, in 2011â12. Findings reveal that the household-level impact was heavily influenced by socio-economic power and socio-ethnic grouping. Negative impacts, including restricted freedom and loss of material and cultural resources are disproportionately felt by the poorest. The indigenous Batwa suffer particularly detrimental impacts. The findings suggest that strategies deemed successful in making progress towards the Millennium Development Goals in Rwanda need, as a minimal measure, to be supported by social protection programs that specifically target the landless, vulnerable and cultural minorities. However, to align Rwandaâs development policies with the ambitions of the Sustainable Development Goals (SDGs), a dramatic strategic shift is required to âleave no-one behindâ and avoid the reproduction of poverty and exacerbation of inequality
Finsler geometry modeling of reverse piezoelectric effect in PVDF
We apply the Finsler geometry (FG) modeling technique to study the electric field-induced strain in ferroelectric polymers. Polyvinylidene difluoride (PVDF) has a negative longitudinal piezoelectric coefficient, which is unusual in ferroelectrics, and therefore the shape changes in this material are hard to predict. We find that the results of Monte Carlo simulations for the FG model are in good agreement with experimental strain-electric field curves of PVDF-based polymers in both longitudinal and transverse directions. This implies that FG modeling is suitable for reproducing the reverse piezoelectric effect in PVDF
Thermal gradient of in-flight polymer particles during cold spraying
International audienceThe manufacture of polymer coatings via the cold-spray process remains challenging owing to the viscoelastic-viscoplastic behavior exhibited by polymers. One crucial step to improve cold-spray polymer coating is to determine the particles' thermal history during their flight from inside the nozzle to their impact on the substrate. In this study, we propose estimating the velocity and temperature of an isolated polymer particle traveling through a nozzle with a sharp change in its cross-section. The preliminary results show that the geometric discontinuity constricts the flow, thereby increasing the particle velocity. Moreover, a significant thermal gradient is expected inside the particle, which in turn leads to a gradient of mechanical properties of the polymeric particle during impact
The coronal line regions of planetary nebulae NGC6302 and NGC6537: 3-13um grating and echelle spectroscopy
We report on advances in the study of the cores of NGC6302 and NGC6537 using
infrared grating and echelle spectroscopy. In NGC6302, emission lines from
species spanning a large range of ionization potential, and in particular
[SiIX]3.934um, are interpreted using photoionization models (including CLOUDY),
which allow us to reestimate the central star's temperature to be about
250000K. All of the detected lines are consistent with this value, except for
[AlV] and [AlVI]. Aluminium is found to be depleted to one hundredth of the
solar abundance, which provides further evidence for some dust being mixed with
the highly ionized gas (with photons harder than 154eV). A similar depletion
pattern is observed in NGC6537. Echelle spectroscopy of IR coronal ions in
NGC6302 reveals a stratified structure in ionization potential, which confirms
photoionization to be the dominant ionization mechanism. The lines are narrow
(< 22km/s FWHM), with no evidence of the broad wings found in optical lines
from species with similar ionization potentials, such as [NeV]3426A. We note
the absence of a hot bubble, or a wind blown bipolar cavity filled with a hot
plasma, at least on 1'' and 10km/s scales. We also provide accurate new
wavelengths for several of the infrared coronal lines observed with the
echelle.Comment: Accepted for publication in MNRA
Spatially resolved spectroscopy of planetary nebulae and their halos I. Five galactic disk objects
Strong mass loss off stars at the tip of the asymptotic giant branch (AGB)
profoundly affects properties of these stars and their surroundings, including
the subsequent planetary nebula (PN) stage. With this study we wanted to
determine physical properties of mass loss by studying weakly emitting halos,
focusing on objects in the galactic disk. Halos surround the, up to several
thousand times, brighter central regions of PNe. Young halos, specifically,
still contain information of the preceeding final mass loss stage on the AGB.
In the observations we used the method of integral field spectroscopy with the
PMAS instrument. This is the first committed study of halos of PNe that uses
this technique. We improved our data analysis by a number of steps. In a study
of the influence of scattered light we found that a moderate fraction of
intensities in the inner halo originate in adjacent regions. As we combine line
intensities of distant wavelengths, and because radial intensity gradients are
steep, we corrected for effects of differential atmospheric refraction. In
order to increase the signal-to-noise of weak emission lines we introduced a
dedicated method to bin spectra of individual spatial elements. We also
developed a general technique to subtract telluric lines - without using
separate sky exposures. By these steps we avoided introducing errors of several
thousand Kelvin to our temperature measurements in the halo. For IC3568 we
detected a halo. For M2-2 we found a halo radius that is 2.5 times larger...
(abridged)Comment: 27 pages, 29 figures, A&A (in press), Abridged abstract, Corrected
and clarified various minor issues; the section on scattered light is
significantly clarifie
Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. III - Further observations of NGC 5548 at optical wavelengths
The results of the second year of an intensive ground-based spectroscopic and photometric study of variability in the bright Seyfert 1 galaxy NGC 5548 are reported in order to study the relationship between continuum and emission-line variability. Relative to the first year of the monitoring program, the nucleus of NGC 5548 was considerably fainter and the continuum variations slower during the second year, but the continuum H-beta cross-correlation results for the two years are nearly identical. The variations in the broad H-beta emission-line lag behind those in the continuum by somewhat less than 20 days, as concluded from the first year's dat
Mapping and Spectroscopy of the Planetary Nebula NGC 7009 in the Visual and Infrared
NGC 7009 is a fascinating example of a high excitation, elliptical planetary
nebula (PN) containing circum-nebular rings, and FLIERs and jets along the
major axis. We present visual spectroscopy along multiple position angles
through the nucleus, taken with the Observatorio Astronomico Nacional (Mexico);
mid-infrared (MIR) spectroscopy and imaging acquired using the Infrared Space
Observatory (ISO) and Spitzer Space Telescope (SST), and narrow band imaging
obtained using the Hubble Space Telescope (HST). The data show that the
mid-infrared (MIR) continuum is dominated by a broad ~ 100K continuum, and a
strong excess attributable to crystalline silicate emission. The primary peaks
in this excess are similar to those observed in Forsterite and clino- and
ortho-enstatite. We use the ground-based spectroscopy, and ratioing of HST
images to investigate the presence of shocks in the ansae and interior
envelope. It is concluded that line ratios in the ansae may be partially
consistent with shock excitation, although these features are primarily
dominated by photo-ionisation. We also note evidence for shock excitation at
the limits of the interior elliptical shell, and for multiple bow-shock
structures centered upon the ansae.Comment: 22 pages, 19 figures. Accepted for publication in MNRAS. 52 pages in
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