239 research outputs found
Band structure and optical properties of opal photonic crystals
A theoretical approach for the interpretation of reflectance spectra of opal
photonic crystals with fcc structure and (111) surface orientation is
presented. It is based on the calculation of photonic bands and density of
states corresponding to a specified angle of incidence in air. The results
yield a clear distinction between diffraction in the direction of light
propagation by (111) family planes (leading to the formation of a stop band)
and diffraction in other directions by higher-order planes (corresponding to
the excitation of photonic modes in the crystal). Reflectance measurements on
artificial opals made of self-assembled polystyrene spheres are analyzed
according to the theoretical scheme and give evidence of diffraction by
higher-order crystalline planes in the photonic structure.Comment: to appear in PR
Role of covalency in the ground state properties of perovskite ruthenates: A first principle study using local spin density approximations
We investigate the electronic structure of SrRuO3 and CaRuO3 using full
potential linearized augmented plane wave method within the local spin density
approximations. The ferromagnetic ground state in SrRuO3 could exactly be
described in these calculations and the calculated spin magnetic moment is
found to be close to the experimentally observed values. Interestingly, the
spin polarized calculations for CaRuO3 exhibit large spin moment as observed in
the experiments but the magnetic ground state has higher energy than that in
the non-magnetic solution. Various calculations for different structural
configurations indicate that Ca-O covalency plays the key role in determining
the electronic structure and thereby the magnetic ground state in this system.Comment: 8 figure
Nod2 Deficiency in mice is Associated with Microbiota Variation Favouring the Expansion of mucosal CD4+ LAP+ Regulatory Cells
Nucleotide-binding Oligomerization Domain-2 (NOD2) mutations are associated with an increased risk to develop Crohn's Disease. In previous studies, we have shown that Nod2-/- mice manifest increased proportion of Lamina Propria (LP) CD4+ LAP+ Foxp3- regulatory cells, when compared with Nod2+/+ mice, while CD4+ Foxp3 + regulatory cells were not affected. Here, we investigated the Nod2 gut microbiota, by 16S rRNA pyrosequencing, at steady state and after TNBS-colitis induction in mice reared separately or in cohousing, correlating the microbial profiles with LP regulatory T cells proportion and tissue cytokines content. We found that enrichment of Rikenella and Alistipes (Rikenellaceae) in Nod2-/- mice at 8 weeks of age reared separately was associated with increased proportion of CD4+ LAP+ Foxp3- cells and less severe TNBS-colitis. In co-housed mice the acquisition of Rickenellaceae by Nod2+/+ mice was associated with increased CD4+ LAP+ Foxp3- proportion and less severe colitis. Severe colitis was associated with enrichment of gram-negative pathobionts (Escherichia and Enterococcus), while less severe colitis with protective bacteria (Barnesiella, Odoribacter and Clostridium IV). Environmental factors acting on genetic background with different outcomes according to their impact on microbiota, predispose in different ways to inflammation. These results open a new scenario for therapeutic attempt to re-establish eubiosis in Inflammatory Bowel Disease patients with NOD2 polymorphisms
TiOCl, an orbital-ordered system?
We present first principles density functional calculations and downfolding
studies of the electronic and magnetic properties of the layered quantum spin
system
TiOCl. We discuss explicitely the nature of the exchange pathes and attempt
to clarify the concept of orbital ordering in this material. An analysis of the
electronic structure of slightly distorted structures according to the phononic
modes allowed in this material suggests that this system is subject to large
orbital fluctuations driven by the electron-phonon coupling. Based on these
results, we propose a microscopic explanation of the behavior of TiOCl near the
phase transition to a spin-gapped system.Comment: Some figures are compressed, for higher quality please contact the
author
Impact of aging on the sintering behavior of bioactive-glass powder
Bioactive glasses (BGs) have been successfully used for several years as bone graft substitutes to fill defects and augment bone structures in orthopedic and dental procedures. Despite recent advances in the fabrication of reliable 3D scaffolds based on BG, the reproducibility of fabrication has only been marginally addressed and remains a challenge for their application. Recent studies have shown that BGs can react with moisture and atmospheric CO2 to form carbonates, affecting the properties and structure of the final product. In this study, factors that can affect the sintering behavior of BG powders were identified and investigated. A statistical analysis was then performed to optimize the BG sintering process, which revealed the possibility of obtaining BG scaffolds with reproducible density by acting on controllable factors such as aging and drying. In practice, this can be achieved by controlling the atmosphere during processing, handling, and storage of the material
Origin of Jahn-Teller distortion and orbital-order in LaMnO3
The origin of the cooperative Jahn-Teller distortion and orbital-order in
LaMnO3 is central to the physics of the manganites. The question is complicated
by the simultaneous presence of tetragonal and GdFeO3-type distortions and the
strong Hund's rule coupling between e_g and t_2g electrons. To clarify the
situation we calculate the transition temperature for the Kugel-Khomskii
superexchange mechanism by using the local density approximation+dynamical
mean-field method, and disentangle the effects of super-exchange from those of
lattice distortions. We find that super-exchange alone would yield T_KK=650 K.
The tetragonal and GdFeO3-type distortions, however, reduce T_KK to 550 K. Thus
electron-phonon coupling is essential to explain the persistence of local
Jahn-Teller distortions to at least 1150 K and to reproduce the occupied
orbital deduced from neutron scattering.Comment: 4 pages, 3 figures; published version (minor changes
Magnetic Field Effect on the Pseudogap Temperature within Precursor Superconductivity
We determine the magnetic field dependence of the pseudogap closing
temperature T* within a precursor superconductivity scenario. Detailed
calculations with an anisotropic attractive Hubbard model account for a
recently determined experimental relation in BSCCO between the pseudogap
closing field and the pseudogap temperature at zero field, as well as for the
weak initial dependence of T* at low fields. Our results indicate that the
available experimental data are fully compatible with a superconducting origin
of the pseudogap in cuprate superconductors.Comment: 4 pages, 3 figure
Charge Kondo effect toward a non-Fermi-liquid fixed point in the orbitally degenerate exchange model
We show that a Kondo-type model with an orbital degeneracy has a new
non-Fermi-liquid fixed point. Near the fixed point the spin degrees of freedom
are completely quenched, and the residual charge degrees of freedom lead to the
multi-channel Kondo effect. Anomalous behavior appears in electric and thermal
properties, but the magnetic susceptibility should show the local Fermi-liquid
behavior. The non-Fermi-liquid fixed point becomes unstable against
perturbations breaking the particle-hole symmetry. We derive these results
using the third-order scaling for a spherically symmetric model with a
fictitious spin. In contrast to the Coqblin-Schrieffer model, the present model
respects different time-reversal properties of multipole operators.Comment: 4 pages, 2 eps figures, to appear in J. Phys. Soc. Jpn. 68 No.
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