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

A Microscopic Model for the Second-Harmonic Generation from C60

We discuss the microscopic origin of the Second-Harmonic Generation (SHG) resonance at ħω=1.81 eV, based on spectroscopic and thickness dependent SHG measurements on C60 thin films. We assign the three-level diagram responsible for the observed SHG resonance, and show it to be of magnetic-dipole-induced origin. Furthermore we explain the absence of almost any surface contributions, and the narrow line width of the involved HOMO-LUMO (11Ag→11T1g) excitation at 1.81 eV

Evaluar el programa microempresarial desarrollo socio económico aplicado por carbones del caribe en el municipio de la Jagua de Ibiríco (cesar)

Con el presente trabajo buscamos evaluar el programa microempresarial de desarrollo socioeconómico aplicado por la empresa Carbones del Caribe en el municipio de la Jagua de Ibirico (Cesar) con el fin de identificar los problemas que limiten sus acciones. Dado lo anterior los gobiernos de muchos países desarrollados o no, han tomado conciencia de la gran importancia de la pequeña empresa y han dispuesto recursos para estimular y apoyar al nuevo empresario; creando organismos, apropiando fondos y emitiendo disposiciones especiales que faciliten el despegue de las nacientes empresas, en algunos casos con el apoyo del sector privado, como el programa socioeconómico promovido por Carbones del Caribe en el municipio de la Jagua de Ibirico. El perfil de esas nuevas legiones de empresarios no sigue un patrón determinado; se encuentran entre ellos tanto mujeres como hombres, jóvenes llenos de vitalidad y adultos cargados

Fourier-Space Crystallography as Group Cohomology

We reformulate Fourier-space crystallography in the language of cohomology of groups. Once the problem is understood as a classification of linear functions on the lattice, restricted by a particular group relation, and identified by gauge transformation, the cohomological description becomes natural. We review Fourier-space crystallography and group cohomology, quote the fact that cohomology is dual to homology, and exhibit several results, previously established for special cases or by intricate calculation, that fall immediately out of the formalism. In particular, we prove that {\it two phase functions are gauge equivalent if and only if they agree on all their gauge-invariant integral linear combinations} and show how to find all these linear combinations systematically.Comment: plain tex, 14 pages (replaced 5/8/01 to include archive preprint number for reference 22

Normative equations for central augmentation index:Assessment of inter-population applicability and how it could be improved

Common reference values of arterial stiffness indices could be effective screening tool in detecting vascular phenotypes at risk. However, populations of the same ethnicity may differ in vascular phenotype due to different environmental pressure. We examined applicability of normative equations for central augmentation index (cAIx) derived from Danish population with low cardiovascular risk on the corresponding Croatian population from the Mediterranean area. Disagreement between measured and predicted cAIx was assessed by Bland-Altman analysis. Both, cAIx-age distribution and normative equation fitted on Croatian data were highly comparable to Danish low-risk sample. Contrarily, Bland-Altman analysis of cAIx disagreement revealed a curvilinear deviation from the line of full agreement indicating that the equations were not equally applicable across age ranges. Stratification of individual data into age decades eliminated curvilinearity in all but the 30–39 (men) and 40–49 (women) decades. In other decades, linear disagreement independent of age persisted indicating that cAIx determinants other than age were not envisaged/compensated for by proposed equations. Therefore, established normative equations are equally applicable to both Nordic and Mediterranean populations but are of limited use. If designed for narrower age ranges, the equations’ sensitivity in detecting vascular phenotypes at risk and applicability to different populations could be improved

Photonic realization of the relativistic Kronig-Penney model and relativistic Tamm surface states

Photonic analogues of the relativistic Kronig-Penney model and of relativistic surface Tamm states are proposed for light propagation in fibre Bragg gratings (FBGs) with phase defects. A periodic sequence of phase slips in the FBG realizes the relativistic Kronig-Penney model, the band structure of which being mapped into the spectral response of the FBG. For the semi-infinite FBG Tamm surface states can appear and can be visualized as narrow resonance peaks in the transmission spectrum of the grating

Effect of Partial Crystallization on the Structural and Luminescence Properties of Er3+-Doped Phosphate Glasses

Er-doped phosphate glass ceramics were fabricated by melt-quenching technique followed by a heat treatment. The effect of the crystallization on the structural and luminescence properties of phosphate glasses containing Al2O3, TiO2, and ZnO was investigated. The morphological and structural properties of the glass ceramics were characterized by Field Emission-Scanning Electron Microscopy (FE-SEM), X-ray Diffraction (XRD), and micro-Raman spectroscopy. Additionally, the luminescence spectra and the lifetime values were measured in order to study the influence of the crystallization on the spectroscopic properties of the glasses. The volume ratio between the crystal and the glassy phases increased along with the duration of the heat treatment. The crystallization of the glass ceramics was confirmed by the presence of sharp peaks in the XRD patterns and different crystal phases were identified depending on the glass composition. Sr(PO3)(2) crystals were found to precipitate in all the investigated glasses. As evidenced by the spectroscopic properties, the site of the Er3+ ions was not strongly affected by the heat treatment except for the fully crystallized glass ceramic which does not contain Al2O3, TiO2, and ZnO. An increase of the lifetime was also observed after the heat treatment of this glass. Therefore, we suspect that the Er3+ ions are incorporated in the precipitated crystals only in this glass ceramic

Theory of Superconducting TcT_{c} of doped fullerenes

We develop the nonadiabatic polaron theory of superconductivity of $M_{x}C_{60}$ taking into account the polaron band narrowing and realistic electron-phonon and Coulomb interactions. We argue that the crossover from the BCS weak-coupling superconductivity to the strong-coupling polaronic and bipolaronic superconductivity occurs at the BCS coupling constant $\lambda\sim 1$ independent of the adiabatic ratio, and there is nothing ``beyond'' Migdal's theorem except small polarons for any realistic electron-phonon interaction. By the use of the polaronic-type function and the ``exact'' diagonalization in the truncated Hilbert space of vibrons (``phonons'') we calculate the ground state energy and the electron spectral density of the $C_{60}^{-}$ molecule. This allows us to describe the photoemission spectrum of $C_{60}^{-}$ in a wide energy region and determine the electron-phonon interaction. The strongest coupling is found with the high-frequency pinch $A_{g2}$ mode and with the Frenkel exciton. We clarify the crucial role of high-frequency bosonic excitations in doped fullerenes which reduce the bare bandwidth and the Coulomb repulsion allowing the intermediate and low-frequency phonons to couple two small polarons in a Cooper pair. The Eliashberg-type equations are solved for low-frequency phonons. The value of the superconducting $T_{c}$, its pressure dependence and the isotope effect are found to be in a remarkable agreement with the available experimental data.Comment: 20 pages, Latex, 4 figures available upon reques

Contenido de clorofilas totales en doce clones de cacao (Theobroma cacao L.)

Total chlorophyll content in twelve cocoa clones (Theobroma cacao L.) Resumen El contenido de pigmentos fotosintéticos, en particular las clorofilas, está relacionado con la adaptación de los genotipos a las condiciones climáticas en que estos se desarrollan. En varias especies, la cantidad de clorofila está relacionada con el rendimiento de los cultivos. El objetivo de la presente investigación fue determinar el contenido de clorofilas totales en 12 clones de Theobroma cacao L. que están siendo empleados como progenitores en un programa de mejoramiento genético en la Facultad de Ingeniería Agronómica de la Universidad Técnica de Manabí. El contenido de clorofilas totales se midió en las hojas adultas de tres plantas de cada clon, en condiciones de secano, en los meses de enero, junio y julio de 2017, con el medidor de clorofila Minolta SPAD-502®. Se encontraron diferencias significativas entre los clones en cada uno de los tres muestreos, en particular en el último realizado en la estación seca, lo que se atribuye a la mayor adaptabilidad de algunos clones a las condiciones de baja disponibilidad de agua. No se observaron diferencias significativas en la evolución del contenido de clorofilas de los clones. Palabras clave: cacao; Theobroma cacao L.; clorofilas; clones. Abstract The content of photosynthetic pigments, particularly chlorophylls, is related to the adaptation of genotypes to the climatic conditions in which they develop. In several species, the amount of chlorophyll is related to the yield of crops. The objective of the present investigation was to determine the total chlorophyll content in 12 clones of Theobroma cacao L. that are being used as progenitors in a breeding program in the Facultad de Ingeniería Agronómica of the Universidad Técnica de Manabí. The total chlorophyll content was measured in the adult leaves of three plants of each clone, under rain-fed conditions, in the months of January, June and July of 2017, with the chlorophyll meter Minolta SPAD-502®. Significant differences were found between the clones in each of the three samplings, particularly in the last one carried out in the dry season, which is attributed to the greater adaptability of some clones to the conditions of low water availability. No significant differences were observed in the evolution of the chlorophyll content of the clones. Keywords: cocoa; Theobroma cacao L.; chlorophylls; clones