9,527 research outputs found
Optical observations of very low ionization HII regions in the large Magellanic cloud
Several very low ionization isolated HII regions were detected on a prism-objective plate of the Large Magellanic Cloud. Most of the objects show a very weak (OIII) lambda 5007 emission line and, on the other hand the (OII) lambda 6584 doublets are very intense. This kind of objects seem to be ideal in order to determine accurate N and O abundance, avoiding the use of large ionization correction factors in the N abundance determination. Spectrophotometric observations of these regions were carried out with the 4 m telescope and the 2-D Frutti spectrograph at Cerro Tololo, and with the 1.52 m and the Image Dissector Scanner (IDS) at La Silla, ESO. The wavelength range lambda lambda 3700 to 7000 A was covered. Calibrated fluxes of the emission lines detected were measured, and from these data preliminary results of physical conditions of the gas as well as some ionic abundances were derived. Comparisons of the observations with ionization structure models show that the effective temperatures of the ionizing stars are less than 35,000 K. Possible abundances gradients across the large megallanic cloud are discussed
Phonon Softening and Direct to Indirect Bandgap Crossover in Strained Single Layer MoSe2
Motivated by recent experimental observations of Tongay et al. [Tongay et
al., Nano Letters, 12(11), 5576 (2012)] we show how the electronic properties
and Raman characteristics of single layer MoSe2 are affected by elastic biaxial
strain. We found that with increasing strain: (1) the E' and E" Raman peaks
(E1g and E2g in bulk) exhibit significant red shifts (up to 30 cm-1), (2) the
position of the A1' peak remains at 180 cm-1 (A1g in bulk) and does not change
considerably with further strain, (3) the dispersion of low energy flexural
phonons crosses over from quadratic to linear and (4) the electronic band
structure undergoes a direct to indirect bandgap crossover under 3% biaxial
tensile strain. Thus the application of strain appears to be a promising
approach for a rapid and reversible tuning of the electronic, vibrational and
optical properties of single layer MoSe2 and similar MX2 dichalcogenides.Comment: http://link.aps.org/doi/10.1103/PhysRevB.87.12541
High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter CO 4-3 emission in the
Magellanic Clouds. The survey is comprised of 15 6'x6' maps obtained using the
AST/RO telescope toward the molecular peaks of the Large and Small Magellanic
Clouds. We have used these data to constrain the physical conditions in these
objects, in particular their molecular gas density and temperature. We find
that there are significant amounts of molecular gas associated with most of
these molecular peaks, and that high molecular gas temperatures are pervasive
throughout our sample. We discuss whether this may be due to the low
metallicities and the associated dearth of gas coolants in the Clouds, and
conclude that the present sample is insufficient to assert this effect.Comment: 18 pages, 3 figures, 5 tables. To appear in Ap
Photo-excitation of a light-harvesting supra-molecular triad: a Time-Dependent DFT study
We present the first time-dependent density-functional theory (TDDFT)
calculation on a light harvesting triad carotenoid-diaryl-porphyrin-C60.
Besides the numerical challenge that the ab initio study of the electronic
structure of such a large system presents, we show that TDDFT is able to
provide an accurate description of the excited state properties of the system.
In particular we calculate the photo-absorption spectrum of the supra-molecular
assembly, and we provide an interpretation of the photo-excitation mechanism in
terms of the properties of the component moieties. The spectrum is in good
agreement with experimental data, and provides useful insight on the
photo-induced charge transfer mechanism which characterizes the system.Comment: Accepted for publication on JPC, March 09th 200
Comparative Analysis of Molecular Clouds in M31, M33 and the Milky Way
We present BIMA observations of a 2\arcmin field in the northeastern spiral
arm of M31. In this region we find six giant molecular clouds that have a mean
diameter of 5713 pc, a mean velocity width of 6.51.2 \kms, and a mean
molecular mass of 3.0 1.6 10\Msun. The peak brightness
temperature of these clouds ranges from 1.6--4.2 K. We compare these clouds to
clouds in M33 observed by \citet{wilson90} using the OVRO millimeter array, and
some cloud complexes in the Milky Way observed by \cite{dame01} using the CfA
1.2m telescope. In order to properly compare the single dish data to the
spatially filtered interferometric data, we project several well-known Milky
Way complexes to the distance of Andromeda and simulate their observation with
the BIMA interferometer. We compare the simulated Milky Way clouds with the M31
and M33 data using the same cloud identification and analysis technique and
find no significant differences in the cloud properties in all three galaxies.
Thus we conclude that previous claims of differences in the molecular cloud
properties between these galaxies may have been due to differences in the
choice of cloud identification techniques. With the upcoming CARMA array,
individual molecular clouds may be studied in a variety of nearby galaxies.
With ALMA, comprehensive GMC studies will be feasible at least as far as the
Virgo cluster. With these data, comparative studies of molecular clouds across
galactic disks of all types and between different galaxy disks will be
possible. Our results emphasize that interferometric observations combined with
the use of a consistent cloud identification and analysis technique will be
essential for such forthcoming studies that will compare GMCs in the Local
Group galaxies to galaxies in the Virgo cluster.Comment: Accepted for Publication in the Astrophysical Journa
Carbon Nanotubes Band Assignation, Topology, Bloch States and Selection Rules
Various properties of the energy band structures (electronic, phonon, etc.),
including systematic band degeneracy, sticking and extremes, following from the
full line group symmetry of the single-wall carbon nanotubes are established.
The complete set of quantum numbers consists of quasi momenta (angular and
linear or helical) and parities with respect to the z-reversal symmetries and,
for achiral tubes, the vertical plane. The assignation of the electronic bands
is performed, and the generalized Bloch symmetry adapted eigen functions are
derived. The most important physical tensors are characterized by the same set
of quantum numbers. All this enables application of the presented exhaustive
selection rules. The results are discussed by some examples, e.g. allowed
interband transitions, conductivity, Raman tensor, etc.Comment: 11 pages, 2 figures, 2 tables; pdf available from:
http://www.ff.bg.ac.yu/qmf/qsg_e.ht
The barocaloric effect: A Spin-off of the Discovery of High-Temperature Superconductivity
Some key results obtained in joint research projects with Alex M\"uller are
summarized, concentrating on the invention of the barocaloric effect and its
application for cooling as well as on important findings in the field of
high-temperature superconductivity resulting from neutron scattering
experiments.Comment: 26 pages, 9 figure
Approximations based on density-matrix embedding theory for density-functional theories
Recently a novel approach to find approximate exchange–correlation functionals in density-functional theory was presented (Mordovina et al 2019 J. Chem. Theory Comput. 15 5209), which relies on approximations to the interacting wave function using density-matrix embedding theory (DMET). This approximate interacting wave function is constructed by using a projection determined by an iterative procedure that makes parts of the reduced density matrix of an auxiliary system the same as the approximate interacting density matrix. If only the diagonal of both systems are connected this leads to an approximation of the interacting-to-non-interacting mapping of the Kohn–Sham approach to density-functional theory. Yet other choices are possible and allow to connect DMET with other density-functional theories such as kinetic-energy density functional theory or reduced density-matrix functional theory. In this work we give a detailed review of the basics of the DMET procedure from a density-functional perspective and show how both approaches can be used to supplement each other. We do not present a specific realization of combining density-functional methods with DMET but rather provide common grounds to facilitate future developments that encompass both approaches. We do so explicitly for the case of a one-dimensional lattice system, as this is the simplest setting where we can apply DMET and the one that was originally presented. Among others we highlight how the mappings of density-functional theories can be used to identify uniquely defined auxiliary systems and projections in DMET and how to construct approximations for different density-functional theories using DMET inspired projections. Such alternative approximation strategies become especially important for density-functional theories that are based on non-linearly coupled observables such as kinetic-energy density-functional theory, where the Kohn–Sham fields are no longer obtainable by functional differentiation of an energy expression, or for reduced density-matrix functional theories, where a straightforward Kohn–Sham construction is not feasible
Self healing slip pulses along a gel/glass interface
We present an experimental evidence of self-healing shear cracks at a
gel/glass interface. This system exhibits two dynamical regimes depending on
the driving velocity : steady sliding at high velocity (> Vc = 100-125 \mu
m/s), caracterized by a shear-thinning rheology, and periodic stick-slip
dynamics at low velocity. In this last regime, slip occurs by propagation of
pulses that restick via a ``healing instability'' occuring when the local
sliding velocity reaches the macroscopic transition velocity Vc. At driving
velocities close below Vc, the system exhibits complex spatio-temporal
behavior.Comment: 4 pages, 6 figure
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