Reducing the positional modulation of NbO6-octahedra in SrxBa1-xNb2O6 by increasing the Barium content: A single crystal neutron diffraction study at ambient temperature for x=0.61 and x=0.34

We report on the influence of the Barium content on the modulation amplitude in SrxBa1-xNb2O6 compounds by comparing Sr0.61Ba0.39Nb2O6 (SBN61) and Sr0.34Ba0.66Nb2O6 (SBN34). Our single crystal neutron diffraction results demonstrate that the amplitude of the positional modulation of the NbO6 octahedra is reduced with increasing barium content, indicating that the origin of the modulation is the partial occupation of the pentagonal channels by Sr and Ba atoms. By increasing the Sr content the bigger Ba atoms are replaced by the smaller Sr atoms, which leads to a larger deformation of the surrounding lattice and hence to a larger modulation amplitude. The more homogeneous the filling of these channels with one atomic type (Ba) the lower the modulation amplitude. Our results also show that the structure can be described with a two-dimensional incommensurate harmonic modulation. No second order modulation has been observed, both by single crystal diffraction measurements and q-scans. The positional modulation of the Nb atoms is much smaller than that of the oxygen atoms, such that the modulation can be seen as a rotational modulation of almost rigid NbO6-octahedra

Inter-site Coulomb interaction and Heisenberg exchange

Based on exact diagonalization results for small clusters we discuss the effect of inter-site Coulomb repulsion in Mott-Hubbard or charge transfer insulators. Whereas the exchange constant J for direct exchange is substantially enhanced by inter-site Coulomb interaction, that for superexchange is suppressed. The enhancement of J in the single-band models holds up to the critical value for the charge density wave (CDW) instability, thus opening the way for large values of J. Single-band Hubbard models with sufficiently strong inter-site repulsion to be near a CDW instability thus may provide `physical' realizations of t-J like models with the `unphysical' parameter ratio J/t=1.Comment: Revtex file, 4 PRB pages, with 5 embedded ps-files. To appear in PRB, rapid communications. Hardcopies of figures or the entire manuscript may also be obtained by e-mail request to: [email protected]

Quasi Regular Polyhedra and Their Duals with Coxeter Symmetries Represented by Quaternions I

In two series of papers we construct quasi regular polyhedra and their duals which are similar to the Catalan solids. The group elements as well as the vertices of the polyhedra are represented in terms of quaternions. In the present paper we discuss the quasi regular polygons (isogonal and isotoxal polygons) using 2D Coxeter diagrams. In particular, we discuss the isogonal hexagons, octagons and decagons derived from 2D Coxeter diagrams and obtain aperiodic tilings of the plane with the isogonal polygons along with the regular polygons. We point out that one type of aperiodic tiling of the plane with regular and isogonal hexagons may represent a state of graphene where one carbon atom is bound to three neighboring carbons with two single bonds and one double bond. We also show how the plane can be tiled with two tiles; one of them is the isotoxal polygon, dual of the isogonal polygon. A general method is employed for the constructions of the quasi regular prisms and their duals in 3D dimensions with the use of 3D Coxeter diagrams.Comment: 22 pages, 16 figure

Personal reflections on the history of aperiodic crystals from early days to the state of the art

International audienceAperiodic crystals occur as modulated, intergrowth and quasicrystal structures. Their characterization in terms of symmetries possibly requires not only a higher-dimensional description, but also non-Euclidean transformations (like scaling). These crystallographic symmetries, observed in nature, have been investigated but only partially classified. Turning points in the structure determination of aperiodic crystals are presented in historical perspective up to the present time, together with methods and programs. The WEB allows to get more information on all that, on additional tools and on databases. An attempt to present challanges towards lines of future research concludes the review

Estudo da curva de crescimento de bubalinos da raça Murrah criados no estado do Rio Grande do Sul.

Objetivou-se identificar um modelo, dentre oito citados na literatura, que melhor descreva a curva de crescimento de bubalinos da raça Murrah criados no estado de Rio Grande do Sul. Foram utilizadas informações de 42 machos e 48 fêmeas da raça Murrah, nascidos entre 2009 e 2011, totalizando 624 pares de registros peso-idade do nascimento até 750 dias de idades. Os parâmetros dos modelos foram estimados por meio de regressão não-linear pelo procedimento NLIN (SAS system), utilizando-se o método de Gauss Newton. Os critérios para a escolha do melhor ajuste foram: o quadrado médio dos resíduos (QMR), o desvio médio absoluto (DMA), o índice e a avaliação visual do gráfico dos valores observados e estimados. Com base no do QMR, DMA e o índice, constatou-se que o modelo Richards (1959) apresentou melhor ajuste, seguido pelo modelo Von Bertalanffy (1957). Com isto, o modelo Richards (1959) dentre os testados foi capaz de descrever o crescimento de búfalos da raça Murrah criados no estado do Rio Grande do Sul

Characterization of sub-nanosecond pulsed laser amplification with Er:Yb co-doped phosphate glass fibers

We present an experimental characterization of the amplification of sub-nanosecond duration laser pulses at a wavelength of 1538 nm in short custom-made Er:Yb phosphate glass fibers with different core diameters. The fibers vary in their diameter from 100 µm (highly multi-mode) down to 12 µm (single-mode). The peak power, energy per pulse, and spectral shape of the amplified signal are presented. With our input pulses, the measurements show that the large core diameter fibers do not increase the amplification of the 1538 nm signal. We believe this is due to the high re-absorption of the Er3+ ions in the phosphate fiber. The optimal fiber geometry was found to have a core diameter of 20 µm with a length of 14 cm. The maximum peak power is 8.25 kW, corresponding to a net gain of 10.9 dB, with a pulse duration of 0.7 ns and a repetition rate of 40 kHz