The sensitivity of positron annihilation characteristics to the structural changes occurring in supersaturated metallic systems has been demonstrated and explained since the early 1980s. This knowledge is currently applied for positron annihilation spectroscopy (PAS) investigations regarding the main phenomena that govern the structural evolution of a supersaturated alloy when various thermal treatments are applied: (a) kinetics of vacancy-type defects retained after quenching; (b) solute migration; (c) formation of solute clusters and/or GP zones; (d) precipitation. The analysis of PAS data gives information on the chemical composition, the defect structure and the density of decomposition products of nanometric size. The present article, that includes an exhaustive repertory of works published since 1987, discusses the value of the contribution that PAS studies can give to physical metallurgy in the field of age-hardenable systems. Most examples are taken from the direct experience of the authors.

http://iopscience.iop.org/article/10.1088/0953-8984/10/46/009/pdf

Current positron studies of structural modifications in age-hardenable metallic systems

Electron charge densities in PbSe and PbTe

Sulfur K- and L-edge XANES and electronic structure of zinc, cadmium and mercury monosulfides: a comparative study

Electron density distribution in GaAs using MEM

The positron distribution of charge along the (111) direction is calculated for elemental and compound semiconductors. The results show that the positron has a strong affinity for one sort of atom in binary semiconductors. This relative positron affinity may lead to the positron preferentially annihilating with the electrons of that sort of atom in compound semiconductors. The trends in going from Ge to CdTe through GaAs show an increase in positron affinity for the anion over the cation.



La densite de charge des positrons a ete calculee suivant la direction (111) pour des semiconductoeurs simples et composes. Les resultants montrent que le positron a une grande affimite pour un type d'atome dans les semiconducteurs binaires. Cette affinite relative peut amener le positron a s'annihiler de preferrence avec les electrons de ce type d'atome du compose. La densite de charge calculee pour plusieurs semiconducteurs, de Ge a CdTe en passant par GaAs, montre une affinite du positron croissante pour l'anion plutǒt que pour le cation.

Positron Distribution in Semiconductors

A computational technique is developed to evaluate electron and electron—positron momentum densities in silicon and germanium and the use of these to provide means of interpreting the k-space densities obtained by experimentalis's using the two-dimensional angular correlation of positron annihilation radiation technique (2D ACAR). As a consequence of the good agreement observed in the momentum densities and the LCW folded data obtained for silicon and germanium the independent particle approximation (IPM) coupled with the use of electron pseudo-wavefunctions represent an excellent starting point for a systematic many-particle description of the process, since significant deviations from the experimental angular correlation distribution can be ascribed to the electron—positron and electron—electron interactions.



Nous presentons dans ce papier le developpement des techniques de calcul computationnelles pour evaluer les densites de moment de l'electron et de l'electron—positron dans le cas du silicium et du germanium. L'utilisation de ces techniques donne les moyens d'interpreter les densitěs dans l'espace des k obtenues par les experimentateurs utilisant la correlation angulaire a deux dimensions de la technique d'annihilation des positrons (2D ACAR). Le bon accord observe entre les densites de moment et les donnees obtenues par le theoreme LCW pour le silicium et le germanium montre que l'approximation de la particule independante (IPM) associee aux pseudofonctions d'onde de l'electron constitue un excellent point de depart pour la description systematique du processus a plusieurs particules. les ecarts entre les resultats calcules et experimentaux de la distribution de la correlation angulaire peuvent ětre attribues aux interactions electron—electron et electron—positron.

Positron Annihilation in Si and Ge

The electron and positron charge densities are calculated as a function of position in the unit cell for two diamond and zincblende semiconductors, using wave functions derived from nonlocal pseudopotential band structure calculations for the electronic part, and the positron wave functions are calculated for single-particle approximation in manner which is free of any theoretical assumptions.



Les densites de charge electronique et positronique sont calculees en fonction de la position dans la maille cellulaire pour deux semiconducteurs de structures diamant et zinc-blende. Pour la partie electronique, les fonctions d'onde sont obtenues a partir des calculs de structure de bande utilisant le pseudopotentiel non local. Les fonctions d'onde du positron sont calculees a partir de l'approximation qui consiste a ne considerer qu'une seule particule.

Electron and Positron Distribution for the Plane (1110) in Si and GaAs

Electron and positron charge densities are calculated as a function of position in the unit cell for grey tin,α-Sn. Wavefunctions are used derived from pseudopotential band structure calculations and the independent particle approximation (IPM), respectively, for the electrons and positrons. Detailed plots of the distributions are along the 〈111〉 (the bond axis). Electron and electron-positron momentum densities are calculated for (001, 110) plane. The results are used to analyze the positron effect in α-Sn.



Les densites de charge electronique et positronique de l'etain α sont calculees en fonction de la position dans la maille cellulaire. Nous avons utilise les fonctions d'onde des electrons et des positrons obtenues a partir des calculs des structures de bandes par le pseudopotentiel et de l'approximation de la particule independante. Les courbes detaillees des distributions sont tracees suivant l'axe 〈111〉 qui est aussi celui de la liaison. Les densites de moment de l'electron et de l'electron-positron sont calculees suivant le plan (001, 110). Les resultats permettent d'analyser l'effet des positrons sur l'etain α.

A. Belaidi

Papers

Positron Annihilation in Si and Ge

Positron Distribution in Semiconductors

Electron and Positron Distribution for the Plane (1110) in Si and GaAs

Electron and positron distributions in grey tin

Conduction band-edge charge densities in II–VI compound semiconductors