Microwave Properties of Ba(0.6)K(0.4)BiO(3) Crystals

We report on field-induced variations of the microwave surface resistance at 9.6 GHz of Ba(0.6)K(0.4)BiO(3) crystals. Energy losses have been investigated as a function of the static magnetic field in the range of temperatures 4.2 K - Tc. By analyzing the experimental results in the framework of the Coffey and Clem model we determine the temperature dependence of the first-penetration field, upper critical field and depinning frequency. The results show that the pinning energy of this bismuthate superconductor is weaker than those of cuprates.Comment: 6 pages, 8 embedded figure

Non-cubic layered structure of Ba(1-x)K(x)BiO3 superconductor

Bismuthate superconductor Ba(1-x)K(x)BiO3 (x=0.27-0.49, Tc=25-32K) grown by an electrolysis technique was studied by electron diffraction and high-resolution electron microscopy. The crystalline structure thereof has been found to be non-cubic, of the layered nature, and non-centrosymmetric, with the lattice parameters a ~ ap, c ~ 2ap (ap is a simple cubic perovskite cell parameter) containing an ordered arrangement of barium and potassium. The evidence for the layered nature of the bismuthate superconductor removes the principal crystallographic contradiction between bismuthate and cuprate high-Tc superconductors.Comment: 4 pages, 3 figures, to be published in Physical Review B as a Rapid Communicatio

Observation of a Transition from BCS to HTSC-like Superconductivity in Ba_{1-x}K_xBiO_3 Single Crystals

A study of temperature dependences of the upper critical field B_{c2}(T) and surface impedance Z(T)=R(T)+iX(T) in Ba_{1-x}K_xBiO_3 single crystals that have transition temperatures in the range 6 x>0.4) reveals a transition from BCS to unusual type of superconductivity. B_{c2}(T) curves corresponding to the crystals that have T_c>20 K have positive curvature (like in some HTSC), and those of the crystals with T_c<15 K fall on the usual Werthamer-Helfand-Hohenberg curve. R(T) and X(T) dependences of the crystals with T_c~30 K and T_c~11 K are respectively linear (like in HTSC) and exponential (BCS) in the temperature range T << T_c. The experimental results are discussed in connection with the extended saddle point model by Abrikosov.Comment: 5 pages, 5 figure

SIMULATION NUMERIQUE DE LA CUISSON ET DE L'USINAGE DE MATERIAUX COMPOSITES

The thesis aims to simulate composite molds manufacturing, by curing of prepregs used for LCM processes. We also present solutions for predicting the effects of machining. Numerical simulation of curing involves solving thermo-mechanical problems, together with a representation of the thermo-physico chemical cross-linking reaction of the resin. A macroscopic weakly coupled thermo-physico mechanical approach is employed in the threedimensional numerical model, which is validated by numerical/experimental comparison on the angular variation of L-shaped part, from two kinds of composite materials: T300/914 and Hextool. The second part of the work deals with the simulation of composite materials machining. Numerical simulation of the machining is based on modeling of material removal process and internal stresses redistribution. The machining is modelled as a thermomechanical problem using a hybrid model. The mechanisms involved in machining are converted into equivalent thermo-mechanical loadings which are determined experimentally, and then applied to the finished surface of the workpiece. The simulation led to thecalculation of the corresponding distortions. The machining simulation requires precise knowledge of the residual stresses and the deformed shape induced by curing process. The simulated residual distortions are faced to experimental measurements after machining. The simulations are performed using the Sysply® finite element software, issued by ESI-GROUP.La thèse a pour but de simuler la fabrication des moules en matériaux composites, par cuisson de plis pré-imprégnés, utilisés pour les procédés LCM. Nous présentons également des solutions pour prédire les effets de leur usinage. La simulation numérique de la cuisson fait appel à la résolution de problèmes thermomécaniques, et également à une représentation thermo-physico chimique de la réaction de réticulation de la résine. Une approche macro thermo-physico mécanique couplée faiblement est employée dans le modèle numérique tridimensionnel, validé en le confrontant à une étude expérimentale, au niveau de la variation angulaire d'une pièce en forme de L, pour deux matériaux composites de type différent : T300/914 et Hextool. La seconde partie du travail concerne la simulation de l’usinage des matériaux composites. La simulation numérique de l’usinage s’appuie sur la modélisation de l’enlèvement de matière et de la redistribution des contraintes internes. L’usinage estmodélisé comme un problème thermomécanique en utilisant un modèle hybride. Les mécanismes associés à l’usinage sont convertis en chargements thermomécaniques équivalents qui sont déterminés expérimentalement et ensuite appliqués à la surface de la pièce usinée. La simulation conduit au calcul des distorsions induites correspondantes. La simulation de l’usinage nécessite de connaître précisément les contraintes et déformées résiduelles induites par la phase de cuisson. Les distorsions résiduelles simulées par ce modèle sont confrontées à l’expérience. Les simulations sont réalisées avec le logiciel de calcul par éléments finis Sysply®, commercialisé par ESI-GROUP

Conservation laws derived from systemic approach and symmetry

We prove mathematically for the first time that a conserved value must exist in economics. We derive it by using symmetry arguments in the systemic approach. Recently, it has been shown that a conserved value and its rigorous application can avoid any form of financial crises. So it is proven that a financial crisis is for sure avoidable without any other economic sacrifices.peer-reviewe

Suppression Of The Plasmon Resonance In Au/cds Colloidal Nanocomposites

The nature of exciton-plasmon interactions in Au-tipped CdS nanorods has been investigated using femtosecond transient absorption spectroscopy. The study demonstrates that the key optoelectronic properties of composite heterostructures comprising electrically coupled metal and semiconductor domains are substantially different from those observed in systems with weak interdomain coupling. In particular, strongly coupled nanocomposites promote mixing of electronic states at semiconductor-metal domain interfaces, which causes a significant suppression of both plasmon and exciton excitations of carriers

Heteroepitaxial Growth Of Colloidal Nanocrystals Onto Substrate Films Via Hot-injection Routes

Hot-injection synthesis of colloidal nanocrystals (NCs) in a substrate-bound form is demonstrated. We show that polycrystalline films submerged into hot organic solvents can nucleate the heteroepitaxial growth of semiconductor NCs, for which the ensuing lattice quality and size distribution are on the par with those of isolated colloidal nanoparticles. This strategy is demonstrated by growing lead chalcogenide NCs directly onto solvent-submerged TiO(2) substrates. The resulting PbX/VTiO(2) (X = S, Se, Te) nanocomposites exhibit heteroepitaxial interfaces between lead chalcogenide and oxide domains and show an efficient separation of photoinduced charges, deployable for light-harvesting applications. The extendibility of the present method to other material systems was demonstrated through the synthesis of CdS/TiO(2) and Cu(2)S/TiO(2) heterostructures, fabricated from PbS/TiO(2) composites via cation exchange. The photovoltaic performance of nanocrystal/substrate composites comprising PbS NCs was evaluated by incorporating PbS/TiO2 films Into prototype solar cells