Doping Dependence of the Electronic Structure of Ba_{1-x}K_{x}BiO_{3} Studied by X-Ray Absorption Spectroscopy

We have performed x-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy (XPS) studies of single crystal Ba_{1-x}K_{x}BiO_{3} (BKBO) covering the whole composition range $0 \leq x \leq 0.60$. Several features in the oxygen 1\textit{s} core XAS spectra show systematic changes with $x$. Spectral weight around the absorption threshold increases with hole doping and shows a finite jump between $x=0.30$ and 0.40, which signals the metal-insulator transition. We have compared the obtained results with band-structure calculations. Comparison with the XAS results of BaPb_{1-x}Bi_{x}O_{3} has revealed quite different doping dependences between BKBO and BPBO. We have also observed systematic core-level shifts in the XPS spectra as well as in the XAS threshold as functions of $x$, which can be attributed to a chemical potential shift accompanying the hole doping. The observed chemical potential shift is found to be slower than that predicted by the rigid band model based on the band-structure calculations.Comment: 8 pages, 8 figures include

The application of interpolating MLS approximations to the analysis of MHD flows

The element-free Galerkin method (EFGM) is a very attractive technique for solutions of partial differential equations, since it makes use of nodal point configurations which do not require a mesh. Therefore, it differs from FEM-like approaches by avoiding the need of meshing, a very demanding task for complicated geometry problems. However, the imposition of boundary conditions is not straightforward, since the EFGM is based on moving-least-squares (MLS) approximations which are not necessarily interpolants. This feature requires, for instance, the introduction of modified functionals with additional unknown parameters such as Lagrange multipliers, a serious drawback which leads to poor conditionings of the matrix equations. In this paper, an interpolatory formulation for MLS approximants is presented: it allows the direct introduction of boundary conditions, reducing the processing time and improving the condition numbers. The formulation is applied to the study of two-dimensional magnetohydrodynamic flow problems, and the computed results confirm the accuracy and correctness of the proposed formulation. (C) 2002 Elsevier B.V. All rights reserved

Structural changes of superconducting YBa₂Cu₄O₈ under high pressure

Structural changes in YBa2Cu4O8 have been studied as a function of pressure using TOF neutron powder diffraction with a sample in a helium-gas pressure cell. Its structure contains double chains of edge-sharing [CuO4] squares and two-dimensional CuO2 planes. The distance between an apical oxygen atom and an in-plane copper atom decreases with increasing pressure. The distance between barium and oxygen atoms in the chain and between in-plane copper atoms located on both sides of a Y layer also change significantly with pressure. All of these movements of atoms are closely related to charge transfer from the CuO chain to the CuO2 plane. Calculation of Madelung energies based on our structural parameters shows that the concentration of holes in the conducting CuO2 plane increases appreciably with increasing pressure, which is in good agreement with the observed rise in Tc under high pressure.</p

Structural changes of superconducting YBa₂Cu₄O₈ under high pressure

Structural changes in YBa2Cu4O8 have been studied as a function of pressure using TOF neutron powder diffraction with a sample in a helium-gas pressure cell. Its structure contains double chains of edge-sharing [CuO4] squares and two-dimensional CuO2 planes. The distance between an apical oxygen atom and an in-plane copper atom decreases with increasing pressure. The distance between barium and oxygen atoms in the chain and between in-plane copper atoms located on both sides of a Y layer also change significantly with pressure. All of these movements of atoms are closely related to charge transfer from the CuO chain to the CuO2 plane. Calculation of Madelung energies based on our structural parameters shows that the concentration of holes in the conducting CuO2 plane increases appreciably with increasing pressure, which is in good agreement with the observed rise in Tc under high pressure.</p

A FEM approach to the equations of magneto-aerodynamics

In this work, a Finite Element Method treatment is outlined for the equations of Magnetoaerodynamics. In order to provide a good basis for numerical treatment of Magneto-aerodynamics, a full version of the complete equations is presented and FEM contribution matrices are deduced, as well as further terms of stabilization for the compressible flow case

Pressure-induced structural changes in the superconductor (Nd_1-xSr_x (Nd_1-yCe_y) CuO_4-z. A neutron diffraction study

The crystal structure of (Nd0.59Sr0.41) (Nd0.73Ce0.27)CuO4-z (z≈0.4) was investigated by means of TOF neutron powder diffraction at six hydrostatic pressure up to 0.637 GPa with a He gas pressure cell. The superconducting transition temperature of this oxide has a positive pressure coefficient, dTc/dP, of 4.0 K GPa-1. As the pressure is increased, the a and c lattice parameters of its tetragonal cell both decrease linearly, which leads to a similar decrease in the length of a Cu-O (1) bond within a CuO2 sheet. Increasing pressure also decrease the distance Cu and axial O(2) atoms at a similar rate. Unfortunately, larger uncertainties for the Cu-O(2) bond length make a quantitative comparison difficult. It is clear, however, that no pronounced changes are observed in individual Cu-O bond lengths when compared with the overall cell compression. We conclude that an understanding of the pressure effect on Tc must be gained by considering pressure-induced changes in the overall structure rather than focussing attention on a single Cu-O bond.</p

Pressure-induced structural changes in the superconductor (Nd_1-xSr_x (Nd_1-yCe_y) CuO_4-z. A neutron diffraction study

The crystal structure of (Nd0.59Sr0.41) (Nd0.73Ce0.27)CuO4-z (z≈0.4) was investigated by means of TOF neutron powder diffraction at six hydrostatic pressure up to 0.637 GPa with a He gas pressure cell. The superconducting transition temperature of this oxide has a positive pressure coefficient, dTc/dP, of 4.0 K GPa-1. As the pressure is increased, the a and c lattice parameters of its tetragonal cell both decrease linearly, which leads to a similar decrease in the length of a Cu-O (1) bond within a CuO2 sheet. Increasing pressure also decrease the distance Cu and axial O(2) atoms at a similar rate. Unfortunately, larger uncertainties for the Cu-O(2) bond length make a quantitative comparison difficult. It is clear, however, that no pronounced changes are observed in individual Cu-O bond lengths when compared with the overall cell compression. We conclude that an understanding of the pressure effect on Tc must be gained by considering pressure-induced changes in the overall structure rather than focussing attention on a single Cu-O bond.</p