4,690 research outputs found
Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V2O3 and Ti2O3
The electronic structure of the corundum-type transition-metal oxides V2O3
and Ti2O3 is studied by means of the augmented spherical wave method, based on
density-functional theory and the local density approximation. Comparing the
results for the vanadate and the titanate allows us to understand the peculiar
shape of the metal 3d a_{1g} density of states, which is present in both
compounds. The a_{1g} states are subject to pronounced bonding-antibonding
splitting due to metal-metal overlap along the c-axis of the corundum
structure. However, the corresponding partial density of states is strongly
asymmetric with considerably more weight on the high energy branch. We argue
that this asymmetry is due to an unexpected broadening of the bonding a_{1g}
states, which is caused by hybridization with the e_g^{pi} bands. In contrast,
the antibonding a_{1g} states display no such hybridization and form a sharp
peak. Our results shed new light on the role of the a_{1g} orbitals for the
metal-insulator transitions of V2O3. In particular, due to a_{1g} - e_g^{pi}
hybridization, an interpretation in terms of molecular orbital singlet states
on the metal-metal pairs along the c-axis is not an adequate description.Comment: 7 pages, 3 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
Evidence of Songbird Intoxication From Rozol Application at a Black-Tailed Prairie Dog Colony
Concerns about avian poisonings from anticoagulant rodenticides have traditionally focused on secondary poisoning of raptors exposed by feeding on contaminated mammalian prey. However, ground foraging songbirds can be directly poisoned from operational applications of the anticoagulant rodenticide RozolH (0.005% chlorophacinone, active ingredient) applied as a grain bait, at black-tailed prairie dog Cynomys ludovicianus colonies. A dead western meadowlark Sturnella neglecta recovered from the study prairie dog colony displayed hemorrhaging in brain and pectoral muscle tissue, and it contained chlorophacinone residue concentrations of 0.59 and 0.49 mg/g (wet weight) in the liver and intestinal contents, respectively. Chlorophacinone residues from two Rozol-colored songbird droppings found at the study colony were 0.09 and 0.46 mg/g (wet weight). The timing of the meadowlark mortality and the occurrence of discolored droppings show that songbird exposure and poisoning can occur weeks after a Rozol application
Statistical mechanics of Floquet systems with regular and chaotic states
We investigate the asymptotic state of time-periodic quantum systems with
regular and chaotic Floquet states weakly coupled to a heat bath. The
asymptotic occupation probabilities of these two types of states follow
fundamentally different distributions. Among regular states the probability
decreases from the state in the center of a regular island to the outermost
state by orders of magnitude, while chaotic states have almost equal
probabilities. We derive an analytical expression for the occupations of
regular states of kicked systems, which depends on the winding numbers of the
regular tori and the parameters temperature and driving frequency. For a
constant winding number within a regular island it simplifies to Boltzmann-like
weights \exp(-\betaeff \Ereg_m), similar to time-independent systems. For
this we introduce the regular energies \Ereg_m of the quantizing tori and an
effective winding-number-dependent temperature 1/\betaeff, different from the
actual bath temperature. Furthermore, the occupations of other typical Floquet
states in a mixed phase space are studied, i.e. regular states on nonlinear
resonances, beach states, and hierarchical states, giving rise to distinct
features in the occupation distribution. Avoided crossings involving a regular
state lead to drastic consequences for the entire set of occupations. We
introduce a simplified rate model whose analytical solutions describe the
occupations quite accurately.Comment: 18 pages, 11 figure
Lensing signals in the Hubble Ultra-Deep Field using all 2nd order shape deformations
The long exposure times of the HST Ultra-Deep Field plus the use of an empirically derived position-dependent PSF, have enabled us to measure a cardioid/displacement distortion map coefficient as well as improving upon the sextupole map coefficient. We confirmed that curved background galaxies are clumped on the same angular scale as found in the HST Deep Field North. The new cardioid/displacement map coefficient is strongly correlated to a product of the sextupole and quadrupole coefficients. One would expect to see such a correlation from fits to background galaxies with quadrupole and sextupole moments. Events that depart from this correlation are expected to arise from map coefficient changes due to lensing, and several galaxy subsets selected using this criteria are indeed clumped
Time-dependent density functional theory: Past, present, and future
Time-dependent density functional theory (TDDFT) is presently enjoying
enormous popularity in quantum chemistry, as a useful tool for extracting
electronic excited state energies. This article discusses how TDDFT is much
broader in scope, and yields predictions for many more properties. We discuss
some of the challenges involved in making accurate predictions for these
properties.Comment: 12 pages, 4 figure
Tunnel junctions of unconventional superconductors
The phenomenology of Josephson tunnel junctions between unconventional
superconductors is developed further. In contrast to s-wave superconductors,
for d-wave superconductors the direction dependence of the tunnel matrix
elements that describe the barrier is relevant. We find the full I-V
characteristics and comment on the thermodynamical properties of these
junctions. They depend sensitively on the relative orientation of the
superconductors. The I-V characteristics differ from the normal s-wave RSJ-like
behavior.Comment: 4 pages, revtex, 4 (encapsulated postscript) figures (figures
replaced
Magnon Heat Transport in (Sr,La)_14Cu_24O_41
We have measured the thermal heat conductivity kappa of the compounds
Sr_14Cu_24O_41 and Ca_9La_5Cu_24O_41 containing doped and undoped spin ladders,
respectively. We find a huge anisotropy of both, the size and the temperature
dependence of kappa which we interpret in terms of a very large heat
conductivity due to the magnetic excitations of the one-dimensional spin
ladders. This magnon heat conductivity decreases with increasing hole doping of
the ladders. The magnon heat transport is analyzed theoretically using a simple
kinetic model. From this analysis we determine the spin gap and the temperature
dependent mean free path of the magnons which ranges by several thousand
angstroms at low temperature. The relevance of several scattering channels for
the magnon transport is discussed.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
Energetics of hydrogen impurities in aluminum and their effect on mechanical properties
The effects of hydrogen impurities in the bulk and on the surface of aluminum
are theoretically investigated. Within the framework of density functional
theory, we have obtained the dependence on H concentration of the stacking
fault energy, the cleavage energy, the Al/H surface energy and the Al/H/Al
interface formation energy. The results indicate a strong dependence of the
slip energy barrier in the direction the cleavage energy in the
[111] direction and the Al/H/Al interface formation energy, on H concentration
and on tension. The dependence of the Al/H surface energy on H coverage is less
pronounced, while the optimal H coverage is monolayer. The
calculated activation energy for diffusion between high symmetry sites in the
bulk and on the surface is practically the same, 0.167 eV. From these results,
we draw conclusions about the possible effect of H impurities on mechanical
properties, and in particular on their role in embrittlement of Al.Comment: 9 pages, 5 figure
Automatic annotation of bioinformatics workflows with biomedical ontologies
Legacy scientific workflows, and the services within them, often present
scarce and unstructured (i.e. textual) descriptions. This makes it difficult to
find, share and reuse them, thus dramatically reducing their value to the
community. This paper presents an approach to annotating workflows and their
subcomponents with ontology terms, in an attempt to describe these artifacts in
a structured way. Despite a dearth of even textual descriptions, we
automatically annotated 530 myExperiment bioinformatics-related workflows,
including more than 2600 workflow-associated services, with relevant
ontological terms. Quantitative evaluation of the Information Content of these
terms suggests that, in cases where annotation was possible at all, the
annotation quality was comparable to manually curated bioinformatics resources.Comment: 6th International Symposium on Leveraging Applications (ISoLA 2014
conference), 15 pages, 4 figure
Temperature dependent photoluminescence of organic semiconductors with varying backbone conformation
We present photoluminescence studies as a function of temperature from a
series of conjugated polymers and a conjugated molecule with distinctly
different backbone conformations. The organic materials investigated here are:
planar methylated ladder type poly para-phenylene, semi-planar polyfluorene,
and non-planar para hexaphenyl. In the longer-chain polymers the
photoluminescence transition energies blue shift with increasing temperatures.
The conjugated molecules, on the other hand, red shift their transition
energies with increasing temperatures. Empirical models that explain the
temperature dependence of the band gap energies in inorganic semiconductors can
be extended to explain the temperature dependence of the transition energies in
conjugated molecules.Comment: 8 pages, 9 figure
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