Defect spectroscopy with positrons: calculations

A versatile program for calculating positron states and their annihilation characteristics has been developed. The positron wave function is solved for using finite-element techniques in three dimensions for arbitrary geometry. The positron potential is constructed using the full electrostatic potential and the local-density approximation for correlation. Lattice atom relaxations (including those induced by the positron) are incorporated in terms of the lattice Green's function. Applications to positron defect spectroscopy considered so far include (i) vacancy clusters in metals, (ii) vacancy-carbon complexes in Fe, and (iii) positron surface states on clean and oxidized Al. Experiences with the scheme have been encouraging, and the predictive power of the method is deemed good. (Auth.)