Electronic structure calculations of vacancies and their influence on materials properties

We provide two examples to illustrate how electronic structure calculations contribute to our understanding of vacancies and their role in determining material properties. Diffusion and elctromigration in aluminium are known to depend strongly on vacancies. Electronic structure calculations show that the vacancy-impurity interaction oscillates with distance, and this leads to an explanation for both the increased elctromigration resistance and the slow impurity diffusion for copper in aluminium. Calculations of vacancies in plutonium have been used in conjunction with positron annihilation lifetime measurements to identify the presence of helium-filled vacanies. Helium stabilization of vacancies can provide the precursors for subsequent vacancy-related changes in materials properties

Concerning Order and Disorder in the Ensemble of Cu-O Chain Fragments in Oxygen Deficient Planes of Y-Ba-Cu-O

In connection with numerous X-ray and neutron investigations of some high temperature superconductors (YBa$_2$Cu$_3$O$_{6+x}$ and related compounds) a non-trivial part of the structure factor, coming from partly disordered Cu-O-$\dots$-O-Cu chain fragments, situated within basal planes, CuO$_x$, can be a subject of theoretical interest. Closely connected to such a diffusive part of the structure factor are the correlation lengths, which are also available in neutron and X-ray diffraction studies and depend on a degree of oxygen disorder in a basal plane. The quantitative measure of such a disorder can be associated with temperature of a sample anneal, $T_q$, at which oxygen in a basal plane remains frozen-in high temperature equilibrium after a fast quench of a sample to room or lower temperature. The structure factor evolution with $x$ is vizualized in figures after the numerical calculations. The theoretical approach employed in the paper has been developed for the orthorhombic state of YBCO.Comment: Revtex, 27 pages, 14 PostScript figures upon request, ITP/GU/94/0

Many-body effects in semiconductors

SIGLEAvailable from British Library Lending Division - LD:D58360/85 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

X-RAY ABSORPTION IN IONIC MATERIALS

Nous présentons des calculs des spectres XANES de l'oxyde de calcium, qui constitue un exemple typique de solide ionique. Nous discutons les prescriptions pour une modélisation du transfert de charge sans procédure d'itération par comparaison avec le calcul complet self-consistant de la structure de bande. Outre son application, directe aux systèmes ioniques, cette étude est importante pour la compréhension des limites théoriques des calculs réalisés dans le cas de systèmes complexes pour lesquels des potentiels self-consistants ne sont pas disponibles (tels les silicates et les biomolécules).Calculations of the XANES spectra for calcium oxide, as a typical ionic solid, are presented. Non-self-consistent prescriptions for modelling charge transfer are judged against a fully self-consistent band structure calculation. Apart from its immediate relevance to ionic systems, this study is important for an understanding of the theoretical limitations in calculations for complex systems (such as silicates or biomolecules) where no self-consistent potentials are available

Linear and circular dichroism in angle resolved Fe 3p photomission. Revision 1

Using a recently developed spin-polarized, fully relativistic, multiple scattering approach based on the layer KKR Green function method, we have reproduced the Fe 3p angle-resolved soft x-ray photoemission spectra and analyzed the associated large magnetic dichroism effects for excitation with both linearly and circularly polarized light. Comparison between theory and experiment yields a spin-orbit splitting of 1.0--1.2 eV and an exchange splitting of 0.9-- 1.0 eV for Fe 3p. These values are 50--100% larger than those hitherto obtained experimentally

The band model for the groundstate properties of gadolinium metal

SIGLEAvailable from British Library Document Supply Centre- DSC:6609.025(DL/SCI/P--654T) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Critical parameters of superconducting materials and structures

We report here the completion of a one year project to investigate the synthesis, electronic structure, defect structure, and physical transport properties of high temperature superconducting oxide materials. During the course of this project we produced some of the finest samples of single crystal detwinned YBa{sub 2}Cu{sub 3}O{sub 7}, and stoichiometrically perfect (Ba,K)BiO{sub 3}. We deduced the Fermi surface of YBa{sub 2}Cu{sub 3}O{sub 7}, (La,Sr){sub 2}CuO{sub 4}, and (Ba,K)BiO{sub 3} through the recording of the electron momentum density in these materials as measured by positron annihilation spectroscopy and angle resolved photoemission. We also performed extensive studies on Pr substituted (Y,Pr)Ba{sub 2}Cu{sub 3}O{sub 7} so as to further understand the origin of the electron pairing leading to superconductivity