1,222 research outputs found
The crucial importance of the -- hybridization in transition metal oxides
We studied the influence of the trigonal distortion of the regular octahedron
along the (111) direction, found in the layers. Under such a
distortion the orbitals split into one and two degenerated
orbitals. We focused on the relative order of these orbitals.
Using quantum chemical calculations of embedded clusters at different levels of
theory, we analyzed the influence of the different effects not taken into
account in the crystalline field theory; that is metal-ligand hybridization,
long-range crystalline field, screening effects and orbital relaxation. We
found that none of them are responsible for the relative order of the
orbitals. In fact, the trigonal distortion allows a mixing of the and
orbitals of the metallic atom. This hybridization is at the origin of the
-- relative order and of the incorrect prediction of the
crystalline field theory
Antiferro-quadrupolar correlations in the quantum spin ice candidate Pr2Zr2O7
We present an experimental study of the quantum spin ice candidate pyrochlore
coumpound \przr\ by means of magnetization measurements, specific heat and
neutron scattering up to 12 T and down to 60 mK. When the field is applied
along the and directions, field induced
structures settle in. We find that the ordered moment rises slowly, even at
very low temperature, in agreement with macroscopic magnetization.
Interestingly, for , the ordered moment appears on the
so called chains only. The spin excitation spectrum is essentially
{\it inelastic} and consists in a broad flat mode centered at about 0.4 meV
with a magnetic structure factor which resembles the spin ice pattern. For (at least up to 2.5 T), we find that a well defined mode
forms from this broad response, whose energy increases with , in the same
way as the temperature of the specific heat anomaly. We finally discuss these
results in the light of mean field calculations and propose a new
interpretation where quadrupolar interactions play a major role, overcoming the
magnetic exchange. In this picture, the spin ice pattern appears shifted up to
finite energy because of those new interactions. We then propose a range of
acceptable parameters for \przr\, that allow to reproduce several experimental
features observed under field. With these parameters, the actual ground state
of this material would be an antiferroquadrupolar liquid with spin-ice like
excitations
Fluctuations and skewness of the current in the partially asymmetric exclusion process
We use functional Bethe Ansatz equations to calculate the cumulants of the
total current in the partially asymmetric exclusion process. We recover known
formulas for the first two cumulants (mean value of the current and diffusion
constant) and obtain an explicit finite size formula for the third cumulant.
The expression for the third cumulant takes a simple integral form in the limit
where the asymmetry scales as the inverse of the square root of the size of the
system, which corresponds to a natural separation between weak and strong
asymmetry.Comment: 21 pages, 3 figure
A Comparison of Outflow Properties in AGN Dwarfs vs. Star Forming Dwarfs
Feedback likely plays a crucial role in resolving discrepancies between
observed and theoretical predictions of dwarf galaxy properties. Stellar
feedback was once believed to be sufficient to explain these discrepancies, but
it has thus far failed to fully reconcile theory and observations. The recent
discovery of energetic galaxy-wide outflows in dwarf galaxies hosting Active
Galactic Nuclei (AGN) suggests that AGN feedback may have a larger role in the
evolution of dwarf galaxies than previously suspected. In order to assess the
relative importance of stellar versus AGN feedback in these galaxies, we
perform a detailed Keck/KCWI optical integral field spectroscopic study of a
sample of low-redshift star-forming (SF) dwarf galaxies that show outflows in
ionized gas in their SDSS spectra. We characterize the outflows and compare
them to observations of AGN-driven outflows in dwarfs. We find that SF dwarfs
have outflow components that have comparable widths (W) to those of
outflows in AGN dwarfs, but are much less blue-shifted, indicating that SF
dwarfs have significantly slower outflows than their AGN counterparts. The
outflows in SF dwarfs are spatially resolved and significantly more extended
than those in AGN dwarfs. The mass loss rates, momentum and energy rates of
SF-driven outflows are much lower than those of AGN-driven outflows. Our
results indicate that AGN feedback in the form of gas outflows may play an
important role in dwarf galaxies and should be considered along with SF
feedback in models of dwarf galaxy evolution.Comment: 27 pages, 25 figures, Accepted for publication in Ap
Acceptability and feasibility of human papillomavirus vaccination for adolescents in school environments in Libreville
Background: High-risk oncogenic human papillomaviruses (HPV) are the cause of sexually transmitted viral infection. Its persistence is a risk factor for precancerous lesions of the cervix, which will constitute the base of cervical cancer. In the world, the prevalence of high-risk oncogenic HPV is 66.7%, which is higher among women starting their sexual activity.Methods: An analytical cross-sectional study was conducted in high schools in Gabon regarding parents. The variables selected were the socio-cultural and demographic characteristics of the parents, their knowledge of human papillomavirus vaccination and their acceptability of HPV vaccination and finally the feasibility of HPV vaccination. The statistical test used was Pearson's Chi-square, and a difference was considered significant for p<0.05.Results: The majority of parents, 89%, were informed of the existence of cervical cancer. However, 73.4% of them were unaware of the existence of vaccination against cervical cancer. Only 2.4% of parents had vaccinated their daughters against cervical cancer at the time of the study. These parents only 53.4% expressed an interest in vaccinating their daughters in 53.4% of cases. The ability to vaccinate children is associated with the socio-professional status of parents (p˂0.000).Conclusions: The majority of parents approved school-based vaccination against human papillomavirus infections despite its reported cost and lack of information. The integration of anti-HPV vaccination into the expanded programme on immunization in Gabon will improve immunization coverage
Charge density correlations in t-J ladders investigated by the CORE method
Using 4-site plaquette or rung basis decomposition, the CORE method is
applied to 2-leg and 4-leg t-J ladders and cylinders. Resulting range-2
effective hamiltonians are studied numerically on periodic rings taking full
advantage of the translation symmetry as well as the drastic reduction of the
Hilbert space. We investigate the role of magnetic and fermionic degrees of
freedom to obtain the most reliable representation of the underlying model.
Spin gaps, pair binding energies and charge correlations are computed and
compared to available ED and DMRG data for the full Hamiltonian. Strong
evidences for short-range diagonal stripe correlations are found in periodic
4-leg t-J ladders.Comment: Computation of Luttinger liquid parameters (charge velocity and
charge correlation exponent) adde
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Refractive index matched, nearly hard polymer colloids
Refractive index matched particles serve as essential model systems for colloid scientists, providing nearly hard spheres to explore structure and dynamics. The poly(methyl methacrylate) latexes typically used are often refractive index matched by dispersing them in binary solvent mixtures, but this can lead to undesirable changes, such as particle charging or swelling. To avoid these shortcomings, we have synthesized refractive index matched colloids using polymerization-induced self-assembly (PISA) rather than as polymer latexes. The crucial difference is that these diblock copolymer nanoparticles consist of a single core-forming polymer in a single non-ionizable solvent. The diblock copolymer chosen was poly(stearyl methacrylate)-poly(2,2,2trifluoroethyl methacrylate) (PSMA-PTFEMA), which self-assembles to form PTFEMA core spheres in n-alkanes. By monitoring scattered light intensity, n-tetradecane was found to be the optimal solvent for matching the refractive index of such nanoparticles. As expected for PISA syntheses, the diameter of the colloids can be controlled by varying the PTFEMA degree of polymerization. Concentrated dispersions were prepared, and the diffusion of the PSMA-PTFEMA nanoparticles as a function of volume fraction was measured. These diblock copolymer nanoparticles are a promising new system of transparent spheres for future colloidal studies
A historical vertebrate collection from the Middle Miocene of the Peruvian Amazon
The Miocene aquatic and terrestrial fossil record from western Amazonia constitute a clear evidence of the palaeoenvironmental diversity that prevailed in the area, prior to the establishment of the Amazon River drainage. During the Miocene, the region was characterized by a freshwater megawetland basin, influenced by episodic shallow-marine incursions. A fossil vertebrate collection from the middle Miocene strata of the Pebas Formation is here studied and described. This historical collection was recovered in 1912 along the banks of the Itaya River (Iquitos, Peru), during a scientific expedition led by two scientists of the University of Zurich, Hans Bluntschli and Bernhard Peyer. Our findings include a total of 34 taxa, including stingrays, bony fishes, turtles, snakes, crocodylians, and lizards. Fishes are the most abundant group in the assemblage (~ 23 taxa), including the first fossil record of the freshwater serrasalmids Serrasalmus, and Mylossoma, and the hemiodontid Hemiodus for the Pebas system, with the latter representing the first fossil be discovered for the entire Hemiodontidae. The presence of a representative of Colubroidea in the middle Miocene of Iquitos supports the hypothesis of arrival and dispersal of these snakes into South America earlier than previously expected. This fossil assemblage sheds light on the palaeoenvironments, and the geographical/temporal range of several aquatic/terrestrial lineages inhabiting the Amazonian region
Locked and Unlocked Polygonal Chains in 3D
In this paper, we study movements of simple polygonal chains in 3D. We say that an open, simple polygonal chain can be straightened if it can be continuously reconfigured to a straight sequence of segments in such a manner that both the length of each link and the simplicity of the chain are maintained throughout the movement. The analogous concept for closed chains is convexification: reconfiguration to a planar convex polygon. Chains that cannot be straightened or convexified are called locked. While there are open chains in 3D that are locked, we show that if an open chain has a simple orthogonal projection onto some plane, it can be straightened. For closed chains, we show that there are unknotted but locked closed chains, and we provide an algorithm for convexifying a planar simple polygon in 3D with a polynomial number of moves
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