Symmetry in the nuclear solid state physics

AbstractIn this paper some group-theoretical methods are discussed, which may be used to study both the hyperfine fields on nuclei of crystals, and the interaction of the nuclear resonant radiation with these crystals. Several types of hyperfine fields structures are considered. They are: the structure formed by the magnetic fields on nuclei, the electric field gradients structure and the combined one. It is shown, that the application of colour groups and tensor representations of space groups is not suitable for the study of the structures considered. Tensor representation also makes it possible to determine a set of possible reflections in Mössbauer diffraction patterns

Numerical simulation of the forbidden Bragg reflection spectra observed in ZnO.

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Thermal motion induced resonant forbidden reflections in wurtzite GaN

International audienceWe report on an experimental study of forbidden reflections in GaN (wurtzite structure) by resonant X-ray scattering at the Gallium K edge. Resonant reflections are explained by the coherent sum of a Thermal Motion Induced (TMI) scattering amplitude and a temperature independent term. We show that the shape of the TMI energy spectrum is the same for a number of reflections that are exactly forbidden by spacegroup symmetry, as well as one that relies on approximate cancellation due to special atomic sites. In addition to demonstrating new selection rules, several non-trivial aspects of the theory of TMI scattering in wurtzite crystals are quantitatively verified, including dependence on temperature, energy, azimuthal angle and polarization. The temperature-dependent and temperature independent spectra of GaN are very similar to those found in ZnO, suggesting strong similarities in the anisotropy of their electronic states. This is also supported by the strong linear dichroism observed in GaN. TMI spectra are determined by the evolution of the electronic anisotropy with nuclear position, and are likely to be extremely valuable for developing theories of electronic properties at elevated temperatures

Ab initio calculations of the forbidden Bragg reflections energy spectra in wurtzites versus temperature.

International audienceThermal-motion induced (TMI) scattering is caused by the influence of atomic displacements on electronic states in crystals and strongly depends on temperature. It corresponds to dipole-dipole resonant x-ray scattering, but is usually accompanied by dipole-quadrupole scattering. The phenomenological theory supposes the dipole-quadrupole term to be temperature independent (TI). As a result, the transformation of the energy spectra with temperature observed experimentally in ZnO and GaN corresponds to the interference between the TMI and TI terms. In the present paper the direct confirmation of this theoretical prediction is given. Ab initio molecular dynamics was used to simulate the sets of atomic sites at various temperatures followed by quantum mechanical calculation of resonant Bragg reflection energy spectra. The results of simulation are in excellent coincidence with experimental energy spectra of forbidden reflections and confirm the earlier phenomenological conjecture about the interference between the TI dipole-quadrupole and TMI dipole-dipole contributions to the resonant atomic factor

Atomic displacement effects in near-edge resonant “forbidden” reflections.

A survey of atomic displacement effects in the resonant scattering of synchrotron radiation is presented. It is shown that the dynamical displacements, associated with thermal vibrations, provide the thermal-motion-induced (TMI) “forbidden” reflections, while static displacements (e.g. induced by impurities) provide the point-defect-induced (PDI) “forbidden” reflections. Both kinds of reflections occur owing to perturbation of valent electrons wave functions by atomic displacements. The results of numerical calculations of TMI forbidden reflections in Ge and ZnO are compared with experimental data

“Shielding” of Cytokine Induction by the Periodontal Microbiome in Patients with Periodontitis Associated with Type 2 Diabetes Mellitus

Periodontal diseases, especially those with polymicrobial etiology, are often associated with type 2 diabetes mellitus, proceeding more severely and affecting the course of diabetes mellitus. Recently, this feature has been associated with the ability of periodontopathogen microflora to cause not only a local infectious process in the oral cavity, but also to interact with the human immune system and induce various systemic effects. We investigated changes in the salivary cytokine profile of patients with chronic periodontitis, associated and not associated with type 2 diabetes mellitus. We observed a statistically significant decrease of MCP-1/CCL2, GM-CSF, IL-5, IL-6, and IFN-γ in the saliva of patients with chronic periodontitis associated with type 2 diabetes mellitus in comparison with patients with chronic periodontitis only. All of these cytokines are associated with macrophage activation. These data are an important contribution to the elucidation of the mechanism of periodontopathogens involvement in the manifestation of the systemic effects of type 2 diabetes