Energetics of positron states trapped at vacancies in solids

We report a computational first-principles study of positron trapping at vacancy defects in metals and semiconductors. The main emphasis is on the energetics of the trapping process including the interplay between the positron state and the defect's ionic structure and on the ensuing annihilation characteristics of the trapped state. For vacancies in covalent semiconductors the ion relaxation is a crucial part of the positron trapping process enabling the localization of the positron state. However, positron trapping does not strongly affect the characteristic features of the electronic structure, e.g., the ionization levels change only moderately. Also in the case of metal vacancies the positron-induced ion relaxation has a noticeable effect on the calculated positron lifetime and momentum distribution of annihilating electron-positron pairs.Comment: Submitted to Physical Review B on 17 April 2007. Revised version submitted on 6 July 200

Quantum Monte Carlo Study of Positron Lifetimes in Solids

Publisher Copyright: © 2022 American Physical Society.We present an analysis of positron lifetimes in solids with unprecedented depth. Instead of modeling correlation effects with density functionals, we study positron-electron wave functions with long-range correlations included. This gives new insight in understanding positron annihilation in metals, insulators, and semiconductors. By using a new quantum Monte Carlo approach for computation of positron lifetimes, an improved accuracy compared to previous computations is obtained for a representative set of materials when compared with experiment. Thus, we present a method without free parameters as a useful alternative to the already existing methods for modeling positrons in solids.Peer reviewe

Modeling the momentum distributions of annihilating electron-positron pairs in solids

Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material.Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and when using it in defect identification comparisons with theoretically predicted spectra is indispensable. In this article we present a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids. Valence electron states and their interaction with ion cores are described using the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states. We apply our numerical scheme to selected systems and compare three different enhancement (electron-positron correlation) schemes previously used in the calculation of momentum distributions of annihilating electron-positron pairs within the density-functional theory. We show that the use of a state-dependent enhancement scheme leads to better results than a position-dependent enhancement factor in the case of ratios of Doppler spectra between different systems. Further, we demonstrate the applicability of our scheme for studying vacancy-type defects in metals and semiconductors. Especially we study the effect of forces due to a positron localized at a vacancy-type defect on the ionic relaxations.Comment: Submitted to Physical Review B on September 1 2005. Revised manuscript submitted on November 14 200

Quantum Monte Carlo Study of Positron Lifetimes in Solids

Publisher Copyright: © 2022 American Physical Society.We present an analysis of positron lifetimes in solids with unprecedented depth. Instead of modeling correlation effects with density functionals, we study positron-electron wave functions with long-range correlations included. This gives new insight in understanding positron annihilation in metals, insulators, and semiconductors. By using a new quantum Monte Carlo approach for computation of positron lifetimes, an improved accuracy compared to previous computations is obtained for a representative set of materials when compared with experiment. Thus, we present a method without free parameters as a useful alternative to the already existing methods for modeling positrons in solids.Peer reviewe

On the sensitivity of positron annihilation signals to alloy homogeneity in InxGa1-xN

We present results of theoretical calculations of positron annihilation signals in InGaN alloys with and without vacancies. We demonstrate the sensitivity of the signals to the different configurations of the In/Ga atoms in In1-xGaxN supercells.Peer reviewe

Point defect balance in epitaxial GaSb

Positron annihilation spectroscopy in both conventional and coincidence Doppler broadening mode is used for studying the effect of growth conditions on the point defect balance in GaSb:Bi epitaxial layers grown by molecular beam epitaxy. Positron annihilation characteristics in GaSb are also calculated using density functional theory and compared to experimental results. We conclude that while the main positron trapping defect in bulk samples is the Ga antisite, the Ga vacancy is the most prominent trap in the samples grown by molecular beam epitaxy. The results suggest that the p–type conductivity is caused by different defects in GaSb grown with different methods.Peer reviewe

Vacancy-Impurity Complexes in Highly Sb-Doped Si Grown by Molecular Beam Epitaxy

Positron annihilation measurements, supported by first-principles electron-structure calculations, identify vacancies and vacancy clusters decorated by 1–2 dopant impurities in highly Sb-doped Si. The concentration of vacancy defects increases with Sb doping and contributes significantly to the electrical compensation. Annealings at low temperatures of 400–500 K convert the defects to larger complexes where the open volume is neighbored by 2–3 Sb atoms. This behavior is attributed to the migration of vacancy-Sb pairs and demonstrates at atomic level the metastability of the material grown by epitaxy at low temperature.Peer reviewe

Prognosis of recurrent myocardial infarction based on shortliffe fuzzy models using the electrical characteristics of biologically active points

This report discusses the employment of the electrical characteristics of biologically active points (BAP) in the meridian of the heart to solve the task of prognosticating recurrence of myocardial infarctions during the rehabilitation period using fuzzy decision rule

Positron localization effects on the Doppler broadening of the annihilation line: Aluminum as a case study

The coincidence Doppler broadening (CDB) technique is widely used to measure one-dimensional momentum distributions of annihilation photons, with the aim of obtaining information on the chemical environment of open-volume defects. However, the quantitative analysis of CDB spectra needs to include also purely geometrical effects. A demonstration is given here, on the basis of CDB spectra measured in quenched and in deformed pure aluminum. The comparison of the experimental results with ab initio computations shows that the observed differences come from the difference in free volume seen by positrons trapped in quenched-in vacancies or in vacancylike defects associated to dislocations. The computation reproduces accurately all details of CDB spectra, including the peak near the Fermi break, which is due to the zero-point motion of the confined positron.Peer reviewe