Search results for "Local symmetry"
showing 10 items of 15 documents
Optically detected magnetic resonance studied via the blue luminescence of Ti-doped
1998
The UV-excited blue and green luminescence bands of Ti:sapphire are characterized by ODMR. These emission bands are attributed to two centres, which show very similar properties and are created as a result of a charge-transfer transition of an electron from to ions. In both centres, the d electron of and the hole of are strongly coupled and form triplet states. Doublet systems can be ruled out as sources of the blue-green luminescence. The angular dependence of the ODMR can be explained with an appropriate spin Hamiltonian assuming orthorhombic local symmetry. The orientation of the principal axes of the g-tensor and the crystal-field tensor, found for both centres, suggest that the hole is…
A simple algorithm to evaluate the local symmetry at each point of a closed contour
1995
In this work, contour symmetry is evaluated as a numeric feature for each point of the shape outline, using only the positions of a local vicinity of points. A measure is defined, named Local Symmetric Deficiency (LSD), so that the lower this quantity is, the higher the symmetry will be in the local region considered. This approach is simpler than related previous ones both from a conceptual point of view and for its implementation, since it is reduced just to a suitable manipulation of the Freeman chain code of the curve studied. Its computational cost is very low and it has the advantages of a parallel algorithm, since values for LSD can be computed for each point independently.
Charge-density analysis of 1-nitroindoline: refinement quality using free R factors and restraints
2011
Nitramines and related N-nitro compounds have attracted significant attention owing to their use in rocket fuel and as explosives. The charge density of 1-nitroindoline was determined experimentally and from theoretical calculations. Electron-density refinements were performed using the multipolar atom formalism. In order to design the ideal restraint strategy for the charge-density parameters, R-free analyses were performed involving a series of comprehensive refinements. Different weights were applied to the charge-density restraints, namely the similarity between chemically equivalent atoms and local symmetry. Additionally, isotropic thermal motion and an anisotropic model calculated by …
Theoretical investigation of the self-trapped hole in alkali halides. I. Long-range effects within the model hamiltonian approach
1994
A small-radius polaron model of the self-trapped hole (Vk-center) in alkali halide crystals is presented. Along with the usual contributions, the electronic polarization is also included in accordance with the electronic polaron theory of Toyozawa. It is shown that the exact solution of the problem within the Landau-Pekar approximation leads to multi-hole quantum states accompanied by the relevant electronic and lattice polarizations. As an example the KCl crystal is considered, for which the Vk-center structure as well as the self-trapping energy are computed. While solving our equations, the local symmetry of the defect is taken into account allowing us to consider a comparatively spread …
(K)over-bar* mesons in dense matter
2010
We study the properties of (K) over bar* mesons in nuclear matter using a unitary approach in coupled channels within the framework of the local hidden gauge formalism and incorporating the (K) over bar pi decay channel in matter. The in-medium (K) over bar *N interaction accounts for Pauli blocking effects and incorporates the (K) over bar* self-energy in a self-consistent manner. We also obtain the (K) over bar* (off-shell) spectral function and analyze its behavior at finite density and momentum. At a normal nuclear matter density, the (K) over bar* meson feels a moderately attractive potential, while the (K) over bar* width becomes five times larger than in free space. We estimate the t…
Photoionization of adsorbates on sites with non-axial local symmetry. An example of the symmetry group
1996
We present an analysis of the influence of the local adsorption site symmetry on the angular distribution of photoelectrons ejected from adsorbed atoms or molecules. This analysis applies to photoelectron spectra obtained with photons of any energy in the VUV (photoelectron spectroscopy) or x-ray (XPS) regions, provided that the associated wave of the electron `sees' the neighbouring surface atoms. The adsorbate - substrate system is modelled by a cluster of symmetry group consisting of an adsorbate and several atoms of the substrate. The analysis applies also to clean surfaces of symmetry and to molecules of the symmetry group fixed in space. This local symmetry analysis is an alternative …
Indium Doping in Barium Cerate: the Relation between Local Symmetry and the Formation and Mobility of Protonic Defects
2007
The solid solution series Ba(In,Ce)O3-ä has been investigated with respect to structure, formation, and mobility of protonic defects. Compared to the limited solubility of Y2O3 in BaCeO3 and BaZrO3, the complete solubility of In2O3 is suggested to reflect a relation between absolute hardness of the dopant and the ease of insertion into the hosting lattices. Extended X-ray absorption fine structure (EXAFS) was used to probe the local environment of In3+ in barium cerate: in the surroundings of the dopant, the orthorhombic structure is strongly modified, resulting in an increase of local symmetry. The InO6 octahedra are very regular, and there is no indication for any defect clustering. This …
The width of the omega meson in the nuclear medium
2013
We evaluate the width of the omega meson in nuclear matter. We consider the free decay mode of the omega into three pions, which is dominated by rho IEuro decay, and replace the rho and pi propagators by their medium-modified ones. We also take into account the quasielastic and inelastic processes induced by a vector-baryon interaction dominated by vector meson exchange, as well as the contributions coming from the mechanism with medium-modified K , propagators. We obtain a substantial increase of the omega width in the medium, reaching a value of 121 +/- 10 MeV at normal nuclear matter density for an omega at rest, which comes mainly from omega N -> pi pi N, omega NN -> pi NN processes ass…
Charm and hidden charm scalar mesons in the nuclear medium
2009
We study the renormalization of the properties of low-lying charm and hidden charm scalar mesons in a nuclear medium, concretely of the D-s0(2317) and the theoretical hidden charm state X(3700). We find that for the D-s0(2317), with negligible width at zero density, the width becomes about 100 MeV at normal nuclear-matter density, while in the case of the X(3700) the width becomes as large as 200 MeV. We discuss the origin of this new width and trace it to reactions occurring in the nucleus, while offering a guideline for future experiments testing these changes. We also show how those medium modifications will bring valuable information on the nature of the scalar resonances and the mechan…
Case in favor of the N*(1700)(3/2(-))
2013
Using an interaction extracted from the local hidden-gauge Lagrangians, which brings together vector and pseudoscalar mesons, and the coupled channels rho N (s wave), pi N (d wave), pi Delta (s wave), and pi Delta (d wave), we look in the region ofv root s = 1400-1850 MeV and find two resonances dynamically generated by the interaction of these channels, which are naturally associated to N*(1520)(3/2(-)) and N*(1700)(3/2(-)). N*(1700)(3/2(-)) appears neatly as a pole in the complex plane. The free parameters of the theory are chosen to fit the pi N (d-wave) data. Both the real and imaginary parts of the pi N amplitude vanish in our approach in the vicinity of this resonance, which is simila…