P.E.A. Turchi

A positron study of the electronic structure of YBa2Cu3O7- delta

The authors have measured the two-dimensional electron-positron momentum density (resolved momentum components along (100) and (010)) for single crystals of YBa2Cu3O7- delta at a significantly higher level of statistical precision for this orientation than that embraced in earlier published works. They have found in their data structures suggesting that the positron annihilates predominantly on oxygen sites in the copper-oxygen chains. These structures are also present in the results of a companion ligand field calculation and arise from spatial variations in the electron and positron wavefunction overlaps. Even at the present level of statistical precision, the authors are unable to argue for or against the existence of a Fermi surface.

Electron momentum density studies in high-Tc materials by positron annihilation spectroscopy: theory and experiment

The authors discuss positron behaviour and in particular the ground state positron probability in high-Tc materials. Special attention is paid to La2CuO4, for which they report detailed high-resolution two-dimensional angular correlation of positron annihilation radiation spectroscopy measurements in three sample orientations and as a function of temperature. Besides a large isotropic core-like electron contribution ( approximately=85%), the remaining anisotropic contribution is attributed to valence electrons which are modelled by a linear combination of atomic orbitals-molecular orbital method in conjunction with a localised ion scheme, within the independent particle model approximation. The covalency structure so derived is used to discuss the case of the parent doped material and YBa2Cu3O7. The authors also comment on the application of the Lock-Crisp-West theorem to La2CuO4, before adding some concluding remarks.

Positron annihilation in perovskite superconductors; Theory and experiment

Abstract Positron Annihilation Spectroscopy is shown to be of potential value for probing the electronic structure and the changes accompanying the superconducting transition of the new high Tc materials. The experimental results of electron-positron momentum distribution for La2CuO4 agree with a ligand field approach, suggesting a strong electron localization and the importance of the covalency.

The electronic properties of high Tc superconductors probed by positron annihilation

The discovery of superconductivity at 30K in Ba{sub .6}K{sub .4} BiO{sub 3} has generated considerable excitement in view of the contrasting properties of the Ba-K-Bi-O system when compared to the well known Cu-O based high temperature superconductors. Positron annihilation spectroscopy, which is a sensitive local probe of the electronic and defect properties of a solid, has been extensively applied in the study of Cu-O based superconductors. In the present paper, the results of positron lifetime as a function of temperature in Ba-K-Bi-O are presented and compared with the known results in the cuprate superconductors. Plausible reasons for the observed temperature dependence of positron lifetime are presented. 7 refs., 2 figs.

Electron-position momentum distribution measurements of high-Tc superconductors and related systems

Abstract We discuss our measurements of the 2D-angular correlation of positron annihilation radiation (ACAR) in La 2 CuO 4 , YBa 2 Cu 3 O 7 (YBCO), and NiO. The measurements for NiO are the first such 2D-ACAR measurements; the YBCO results are of a higher statistical quality than previously reported in the literature. The data are compared with complementary theoretical calculations and with each other. We discuss the implication of our analysis for ACAR studies of similar and related systems.

Systematics in position annihilation lifetime analysis of high Tc superconducting transitions

Abstract Values of the positron lifetime have previously been observed to change with temperature below Tc in high Tc superconducting oxides. We report new measurements on Ba.6K.4BiO3 and Nd1.85Ce.15CuO4.

Electronic Structure and Stability of Al-Ge Alloys Under Pressure

The synthesis of new phases under pressure and the study of their structure-properties relationships yield valuable information on the general laws of phase formation in multi-component alloys. They also provide an important insight in the conditions of occurrence of particular phases in systems which exhibit typical phase diagrams at ambient pressure [1]. Although pressure-temperature diagrams are available for most elements and some compounds [2], very little work has been done so far on the study of the possible competition between structural and chemical order effects in alloys as a function of pressure.

PHASE STABILITY FROM ELECTRONIC STRUCTURE OF ORDERED AND DISORDERED ALLOYS

A parameter-free approach to phase stability in substitutional alloys is applied to the study of ordering tendencies for a series of transition metal aluminides TM Al, where TM belongs to the 3d or 4d series in the periodic table, with special emphasis on the possible competition which may take place between the B2 order on the bcc lattice, and the L10 order on the fcc lattice, around equiatomic composition. The study is based upon an application of the linear muffin-tin orbital method in the atomic sphere approximation for the treatment of the ordered states, and of the generalized perturbation method to the Korringa Kohn Rostoker multiple scattering formulation of the coherent potential approximation for the treatment of the disordered state. For these alloys, the electronic origin of the ordering tendencies is rationalized as a function of the average number of valence electrons.