Polarization-dependent angle-resolved photoemission of the layered cuprate Ba2Cu3O4Cl2

We report polarization-dependent angle-resolved photoemission studies of single crystalline Ba₂Cu₃O₄Cl₂. This antiferromagnet adopts a layered body-centered tetragonal structure related to that of the cuprates, but with anextra copper site resulting in a plane with Cu₃O₄ stoichiometry. For the -M direction of the Brillouin zone we observe a dispersive feature with a bandwidth of 400 ±80 meV, which approaches to within 0.9 eV of the Fermi level at (/2,/2). The magnitude of the observed dispersion and its polarization dependence are consistent with its origin as a purely antibonding combination of O2p_x,y and atomic orbitals from the cuprate-like CuO₂ sub-system, i.e. Zhang-Rice singlets.     

Identification of RuO2 as the active phase in CO oxidation on oxygen-rich ruthenium surfaces

CO oxidation over ruthenium dioxide (RuO₂) dominates the CO/CO₂ conversion rate over the catalytically active oxygen-rich Ru(0001) surfaces. In sharp contrast, chemisorbed O overlayers on Ru(0001) (with and without dissolved oxygen) are virtually inactive with respect to CO oxidation.     

Magnetoacoustic measurements on pure and Zn doped YBCO crystals

Investigations on the temperature dependence in the range from 4 to 100 K of the ultrasonic attenuation (T, B) and the relative change Δv(T,B)/v of the sound velocity for various applied magnetic fields have been made. The ultrasonic attenuation was measured by the pulse-echo technique and the relative change of the sound velocity Δv/v by a phase-comparison method. The measurements serve for the examination of the elastic behaviour of the flux line lattice (FLL) of pure and Zn doped YBCO crystals and YBCO/Ag composites. These crystals are important for applications as permanent magnets. In the framework of the thermally assisted flux flow model (TAFF) the activation energies U(B) were deduced from the ultrasonic data.     

Modelling of Complete Robot Dynamics Based on a Multi-Dimensional, RBF-like Neural Architecture

A neural network based identification approach of manipulator dynamics is presented. For a structured modelling, RBF-like static neural networks are used in order to represent and adapt all model parameters with their non-linear dependences on the joint positions. The neural architecture is hierarchically organised to reach optimal adjustment to structural apriori-knowledge about the identification problem. The model structure is substantially simplified by general system analysis independent of robot type. But also a lot of specific features of the utilised experimental robot are taken into account.A fixed, grid based neuron placement together with application of B-spline polynomial basis functions is utilised favourably for a very effective recursive implementation of the neural architecture. Thus, an online identification of a dynamic model is submitted for a complete 6 joint industrial robot.     

Experimental Estimation of Model Error Bounds Based on Modified Stochastic Approximation

A modification of the estimation algorithm stochastic approximation is presented. With assumptions to the statistical distribution of the training data it becomes possible, to estimate not only the mean value but also well directed deviating values of the data distribution. Thus, detailed error models can be identified by means of parameter-linear formulation of the new algorithm. By definition of suitable probabilities, these parametric error models are estimating soft error bounds. That way, an experimental identification method is provided that is able to support a robust controller design. The method was applied at an industrial robot, which is controlled by feedback linearisation. Based on a dynamic model realised by a neural network, the presented approach is utilised for the robust design of the stabilising decentral controllers.     

EELS and XPS investigations of Ba2Cu3O4Cl2

Electron energy-loss spectroscopy in transmission and x-ray photoelectron spectroscopy have been applied to investigate single crystalline Ba₂Cu₃O₄Cl₂ to obtain the loss-function along the high symmetry directions of the cuprate plane, the Cu2p and O1s excitation edges, and to study the valence band and the core level electronic structure. The lossfunction shows a gap of 1.65eV, practically dispersionless structures between 2 and 8eV, and a volume plasmon located at 25eV. The dielectric function is derived. The excitation edges are a measure for the number of holes located in the Cu like or O like states parallel or perpendicular to the Cu₃O₄ planes. The Cu₂p_3/2 XPS shows three features in the main line and a satellite structure due to two independent copper sites and indicating a strong 3d electron correlation. By interpreting forward scattering features the valence band structure can be assigned to its atomic origin.     

Melt texturing and properties control of Nd1+yBa2-yCu3Ox bulk materials

Melt textured NdBaCuO samples with T_c = 94 K and j_c(0 T, 77 K) = 46 kA/cm² have been prepared on an isothermal route. The thermodynamic background of this procedure which aims at the production of a Ba rich Nd-123 is pointed out.     

X-Ray photoemission and electron-energy loss spectroscopic studies of Sr2CuO2Cl2

Sr₂CuO₂Cl₂. represents a nearly ideal case of a 2-D antiferromagnetic CuO₂-plane and thus is of particular interest in the context of the origins of high T_C superconductivity. We present high energy spectroscopic results on single crystalline Sr₂CuO₂Cl₂. Using electron-energy loss spectroscopy in transmission we have measured the loss function along the high symmetry directions in the CuO₂-plane. X-ray photoemission spectroscopy is applied to study the valence band and the Cu 2p spectrum. Comparison of the experiment to a cluster calculation is given.     

Effect of rotation of feed and seed rods on the quality of Na0.75CoO2 single crystal grown by traveling solvent floating zone method

High purity Na_0.75CoO₂ single crystals have been grown by floating zone method. We found the rotation of feed and seed rods play a crucial role in growing high quality single crystal. Systematic investigations suggest the occurrence of a phase separation at microscopic level, such as the separation into Na-rich and Na-poor domains during the growth, and formation of impurity phase(s) depending on growth conditions. Na_xCoO₂ (x = 0.30, 0.60) crystals have been prepared by sodium deintercalation from Na_0.75CoO₂. Powder X-ray and energy dispersive X-ray analyses have confirmed the phase purity and homogeneity of the samples. Magnetic susceptibility measurements of x = 0.60 and 0.75 crystals indicate a bulk phase transition at 22 K and an anomaly around 339 K and 334 K respectively which are attributed to structural transition. In addition some unique features related to Na_0.60CoO₂ were observed.     

Univariant reactions in the YBCO system and the influence of oxygen on soldification and microstructure

Several aspects of the Y−Ba−Cu−O phase diagram have been investigated by thermal analysis, isopiestic, and soaking methods in various gas environments in more detail. On the basis of the univariant reactions at p(O₂)≈0.21×10⁵ Pa (especially e₁ and m₁) and oxygen balance considerations, consequences concerning single crystal growth as well as texturing techniques have been derived. It is shown that oxygen diffusion through the melt can become an important technological factor. Phase diagram investigations at reduced oxygen partial pressure, especially the stability field of YBa₂Cu₃O_7−δ and the pressure dependence of the e₁ reaction, give rise to modified techniques of melt processing at lower temperatures.