Marcel Dickmann (Applied Physics & Measurement Technology, UniBwM)
Positron Annihilation Lifetime Spectroscopy (PALS) is a powerful method to investigate open-volume defects like vacancies or vacancy clusters in crystalline materials. The occurrence of atomistic defects has a major influence on material properties, such as electrical conductivity or magnetic and optical characteristics. PALS allows the characterization of such defects by shooting positrons into the materials and measuring the time between implantation and annihilation with electrons. By determining the positron lifetimes in the material, defects can be characterized by type and concentration. The principle is based on the fact that positrons are captured in vacancy-like defects, which increases their lifetime due to the reduced local electron density (see Fig. 1).
However, the correlation of a measured lifetime to a defect type needs calculations based on Two-Component Density Functional Theory (TC-DFT). These calculations require a large amount of computing power and need high-performance clusters such as HSUper to produce reliable results. For our calculations we use the open software suite ABINIT, which allows to determine positron and electron densities and derived material properties. ABINIT was developed for the calculation of electron densities using pseudopotentials and plane wave basis sets or projector augmented waves. The software is particularly optimized for the use on large computing clusters.
In order to achieve optimum performance of our calculations in terms of computation time and accuracy, we would like to find the most suitable settings for running ABINIT on HSUper. The project for performance engineering offers us the opportunity to collaborate with IT experts and to carry out test calculations with different program settings in order to create an ideal computing environment for our calculations.
Figure 1 – Positron density calculations in diamond: In a perfect crystal (A) the positron can be described as a de-localized plane wave between the atoms. In an open-volume defect like a nitrogen vacancy (NV) center (B), the positron probability density is confined. The different electron density of both systems leads to a change in the positron lifetime, which can be used for defect characterization.