96 research outputs found

    Predicting ground-state configurations and electronic properties of the thermoelectric clathrates Ba8_{8}Alx_{x}Si46x_{46-x} and Sr8_{8}Alx_{x}Si46x_{46-x}

    Full text link
    The structural and electronic properties of the clathrate compounds Ba8_{8}Alx_{x}Si46x_{46-x} and Sr8_{8}Alx_{x}Si46x_{46-x} are studied from first principles, considering an Al content xx between 6 and 16. Due to the large number of possible substitutional configurations we make use of a special iterative cluster-expansion approach, to predict ground states and quasi-degenerate structures in a highly efficient way. These are found from a simulated annealing technique where millions of configurations are sampled. For both compounds, we find a linear increase of the lattice constant with the number of Al substituents, confirming experimental observations for Ba8_{8}Alx_{x}Si46x_{46-x}. Also the calculated bond distances between high-symmetry sites agree well with experiment for the full compositional range. For xx being below 16, all configurations are metallic for both materials. At the charge-balanced composition (x=16x=16), the substitutional ordering leads to a metal-semiconductor transition, and the ground states of Ba8_{8}Al16_{16}Si30_{30} and Sr8_{8}Al16_{16}Si30_{30} exhibit indirect Kohn-Sham band gaps of 0.36 and 0.30 eV, respectively, while configurations higher in energy are metals. The finding of semiconducting behavior is a promising result in view of exploiting these materials in thermoelectric applications.Comment: 9 figure

    Internal Josephson Oscillations for Distinct Momenta Bose-Einstein Condensates

    Full text link
    The internal Josephson oscillations between an atomic Bose-Einstein condensate (BEC) and a molecular one are studied for atoms in a square optical lattice subjected to a staggered gauge field. The system is described by a Bose-Hubbard model with complex and anisotropic hopping parameters that are different for each species, i.e., atoms and molecules. When the flux per plaquette for each species is small, the system oscillates between two conventional zero-momentum condensates. However, there is a regime of parameters in which Josephson oscillations between a vortex-carrying atomic condensate (finite momentum BEC) and a conventional zero-momentum molecular condensate may be realized. The experimental observation of the oscillations between these qualitatively distinct BEC's is possible with state-of-the-art Ramsey interference techniques.Comment: 8 pages, 6 figure

    Интеграционные процессы в условиях международной глобализации экономики

    Get PDF
    We present a double-charge model for the interaction between parallel polarizable hard spherocylinders subject to an external electric field. Using Monte Carlo simulations and free-energy calculations, we predict the phase behaviour for this model as a function of the density and electric field strength, at a fixed length-to-diameter ratio L/D = 5. The resulting phase diagram contains, in addition to the well-known nematic, smectic A, ABC crystal, and columnar phases, a smectic C phase, and a low temperature crystal X phase. We also find a string fluid at low densities and field strengths, resembling results found for dipolar spheres

    CELL: a Python package for cluster expansion with a focus on complex alloys

    Full text link
    We present the Python package CELL, which provides a modular approach to the cluster expansion (CE) method. CELL can treat a wide variety of substitutional systems, including one-, two-, and three-dimensional alloys, in a general multi-component and multi-sublattice framework. It is capable of dealing with complex materials comprising several atoms in their parent lattice. CELL uses state-of-the-art techniques for the construction of training data sets, model selection, and finite-temperature simulations. The user interface consists of well-documented Python classes and modules (http://sol.physik.hu-berlin.de/cell/). CELL also provides visualization utilities and can be interfaced with virtually any ab initio package, total-energy codes based on interatomic potentials, and more. The usage and capabilities of CELL are illustrated by a number of examples, comprising a Cu-Pt surface alloy with oxygen adsorption, featuring two coupled binary sublattices, and the thermodynamic analysis of its order-disorder transition; the demixing transition and lattice-constant bowing of the Si-Ge alloy; and an iterative CE approach for a complex clathrate compound with a parent lattice consisting of 54 atoms

    Tongue ties do not widen the upper airways in racehorses

    Get PDF
    Background There is contradictory evidence on the potential benefits of tongue ties on upper airway function and their efficacy in inhibiting intermittent dorsal displacement of the soft palate (DDSP) in racehorses. Objectives To test the hypothesis that tongue ties increase the pharyngeal diameter and decrease the occurrence of dynamic airway obstruction in racehorses. Study design Prospective, crossover blinded clinical study. Methods Data of 22 Thoroughbred and 8 Standardbred racehorses examined using overground endoscopy under full-intensity exercise on training racetracks with and without fixation of the tongue by use of tongue ties were analysed. Equivalent exercise intensity was ensured by measuring heart rate (bpm), speed (GPS) and venous lactate. Pharyngeal diameter was expressed as pharyngeal-epiglottis (PE) ratios and laryngeal abduction accordingly as laryngeal-median-ratios. Data were analysed using multivariable repeated-measurements ANOVA. Results The PE ratio increased significantly from 1.11 ± 0.19 to 1.28 ± 0.30 in all horses between rest and full-intensity exercise (p < 0.01). Multi-variable analysis revealed that this effect decreased significantly by the application of tongue ties (1.15 ± 0.27, p < 0.01). Tongue ties did not influence maximum laryngeal width (p = 0.09) and area (p = 0.2) significantly. DDSP was found in 4 of 30 examinations with tongue tie and in 1 of 30 examinations without tongue tie. Main limitations The study population was not randomly chosen and was heterogeneous. Few horses had a respiratory noise and the prevalence of upper respiratory tract disorders was too low for statistical comparison of the rate of DDSP with and without tongue ties. Conclusions A positive effect of tongue ties on pharyngeal or laryngeal diameters was not found in this study. Therefore, the results of this study do not support the use of tongue ties to enhance upper airway function.Hintergrund Zu potenziellen Vorteilen von Zungenbändern auf die Funktion der oberen Atemwege und deren Wirksamkeit zur Unterbindung von „DDSP“ (Dorsal displacement of the soft palate) bei Rennpferden gibt es im Widerspruch stehend publizierte Evidenz. Ziel der Studie Überprüfung der Hypothese, dass Zungenbänder den Pharyngealdurchmesser vergrößern und das Auftreten von dynamischen Atemwegsobstruktionen bei Rennpferden verringert. Studiendesign Klinische, prospektive Crossover-Blindstudie. Methodik Analysiert wurden Daten von 22 Vollblutpferden und acht Standardbred Trabrennpferden, untersucht mithilfe Dynamischer Atemwegs-Endoskopie unter Vollbelastung auf Trainingsrennbahnen mit und ohne Fixation der Zunge durch ein Zungenband. Eine gleichwertige Belastungsintensität wurde mithilfe von Messungen der Herzfrequenz (bpm), Geschwindigkeit (GPS) und venöser Laktatwerte sichergestellt. Pharyngealdurchmesser wurden als Pharyngeal-Epiglottis (PE) Verhältnis und laryngeale Abduktion, folglich Laryngeal-Median-Verhältnis, dargestellt. Daten wurden mithilfe multivariabler ANOVA mit wiederholten Messungen analysiert. Ergebnisse Das PE Verhältnis erhöhte sich signifikant von 1.11 ± 0.19 auf 1.28 ± 0.3 bei allen Pferden zwischen Ruhe und Vollbelastung (p <0.01). Mehrvariablen-Analysen ergaben, dass sich dieser Effekt durch Einsatz von Zungenbändern signifikant verringert (1.15 ± 0.27, p<0.01). Zungenbänder hatten keinen signifikanten Einfluss auf maximale laryngeale Weite (p = 0.09) und Fläche (p = 0.2). DDSP wurde bei 4/30 Untersuchungen mit Zungenband und 1/30 Untersuchungen ohne Zungenband beobachtet. Haupteinschränkungen Die Studienpopulation wurde nicht zufällig ausgewählt und war heterogen. Wenige Pferde zeigten Stridor und die Prävalenz von oberen Atemwegserkrankungen war zu niedrig für den statistischen Vergleich der DDSP-Rate mit und ohne Zungenbänder. Schlussfolgerung Ein positiver Effekt von Zungenbändern auf pharyngeale oder laryngeale Durchmesser wurde in dieser Studie nicht beobachtet. Folglich unterstützen die Ergebnisse dieser Studie den Einsatz von Zungenbändern zur Verbesserung der Funktion der oberen Atemwege nicht

    Graphene Oxide alpha Bi2O3 Composites for Visible Light Photocatalysis, Chemical Catalysis and Solar Energy Conversion

    Get PDF
    The growing challenges of environmental purification by solar photocatalysis, precious metal free catalysis and photocurrent generation in photovoltaic cells are receiving the utmost global attention. Here we demonstrate the one pot green chemical synthesis of a new stable heterostructured, eco friendly, multifunctional micro composite consisting of amp; 945; Bi2O3 micro needles intercalated with anchored graphene oxide GO micro sheets 1.0 wt for the above mentioned applications in a large economical scale. The bare amp; 945; Bi2O3 micro needles display twice as better photocatalytic activities than commercial TiO2 Degussa P25 while the GO hybridized composite exhibit 4 6 times enhanced photocatalytic activities than neat TiO2 photocatalyst in the degradation of colored aromatic organic dyes crystal violet and rhodamine 6G under visible light irradiation 300 W tungsten lamp . The highly efficient activity is associated with the strong surface adsorption ability of GO for aromatic dye molecules, the high carrier acceptability and efficient electron hole pair separation in Bi2O3 by individual adjoining GO sheets. Introduction of Ag nanoparticles 2.0 wt further enhances the photocatalytic performance of the composite over 8 folds due to a plasmon induced electron transfer process from Ag nanoparticles via GO sheets into the conduction band of Bi2O3. The new composites are also catalytically active. They catalyze the reduction of 4 nitrophenol to 4 aminophenol in presence of borohydride ions. Photoanodes assembled from GO amp; 945; Bi2O3 and Ag GO amp; 945; Bi2O3 composites display an improved photocurrent response power conversion efficiency 20 higher over those prepared without GO in dye sensitized solar cells DSSCs
    corecore