Ilja Makkonen

Three real-space discretization techniques in electronic structure calculations

A characteristic feature of the state-of-the-art of real-space methods in electronic structure calculations is the diversity of the techniques used in the discretization of the relevant partial differential equations. In this context, the main approaches include finite-difference methods, various types of finite-elements and wavelets. This paper reports on the results of several code development projects that approach problems related to the electronic structure using these three different discretization methods. We review the ideas behind these methods, give examples of their applications, and discuss their similarities and differences.

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 vacancytype defect on the ionic relaxations. DOI: 10.1103/PhysRevB.73.035103

Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculations

Electron-positron momentum distributions measured by the coincidence Doppler broadening method can be used in the chemical analysis of the annihilation environment, typically a vacancy-impurity complex in a solid. In the present work, we study possibilities for a quantitative analysis, i.e., for distinguishing the average numbers of different atomic species around the defect. First-principles electronic structure calculations self-consistently determining electron and positron densities and ion positions are performed for vacancy-solute complexes in

Identifying vacancy complexes in compound semiconductors with positron annihilation spectroscopy: a case study of InN

\mathrm{Al}\text{\ensuremath{-}}\mathrm{Cu}

Identification of vacancy defect complexes in transparent semiconducting oxides ZnO, In2O3 and SnO2

,

Energetics of positron states trapped at vacancies in solids

\mathrm{Al}\text{\ensuremath{-}}\mathrm{Mg}\text{\ensuremath{-}}\mathrm{Cu}

Point defect balance in epitaxial GaSb

, and

Characteristics of S adsorption on Pd vicinal surfaces

\mathrm{Al}\text{\ensuremath{-}}\mathrm{Mg}\text{\ensuremath{-}}\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Ag}

Electrical compensation via vacancy–donor complexes in arsenic-implanted and laser-annealed germanium

alloys. The ensuing simulated coincidence Doppler broadening spectra are compared with measured ones for defect identification. A linear fitting procedure, which uses the spectra for positrons trapped at vacancies in pure constituent metals as components, has previously been employed to find the relative percentages of different atomic species around the vacancy [A. Somoza et al. Phys. Rev. B 65, 094107 (2002)]. We test the reliability of the procedure by the help of first-principles results for vacancy-solute complexes and vacancies in constituent metals.

Enhancement models of momentum densities of annihilating electron-positron pairs: The many-body picture of natural geminals

We present a comprehensive study of vacancy and vacancy-impurity complexes in InN combining positron annihilation spectroscopy and ab-initio calculations. Positron densities and annihilation characteristics of common vacancy-type defects are calculated using density functional theory and the feasibility of their experimental detection and distinction with positron annihilation methods is discussed. The computational results are compared to positron lifetime and conventional as well as coincidence Doppler broadening measurements of several representative InN samples. The particular dominant vacancy-type positron traps are identified and their characteristic positron lifetimes, Doppler ratio curves and lineshape parameters determined. We find that VIn and their complexes with VN or impurities act as efficient positron traps, inducing distinct changes in the annihilation parameters compared to the InN lattice. Neutral or positively charged VN and pure VN complexes on the other hand do not trap positrons. The predominantly introduced positron trap in irradiated InN is identified as the isolated VIn, while in as-grown InN layers VIn do not occur isolated but complexed with one or more VN. The number of VN per VIn in these complexes is found to increase from the near surface region towards the layer-substrate interface.