Response of positrons to clustering in Al-Zn alloys

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Abstract
The positron annihilation characteristics, namely the positron lifetime and the angular correlation curve, has been calculated for small Zn clusters in an FCC Al matrix. The positron potential is obtained from the full lattice electrostatic potential and correlation potential, within the local density approximation. The wavefunction and eigenvalue of the lowest-lying positron state is determined by solving the Schrodinger equation in the finite different scheme. The results show that the positron forms a bound state for clusters containing as few as nine Zn atoms. The positron lifetime decreases with increasing Zn cluster size with a tendency to saturate for clusters larger than 13 atoms. The angular correlation curves are much more sensitive to the variation in cluster size, especially the S parameter which measures the ratio of core annihilations to valence annihilations. The results of the calculation suggest that the kinetics of the very early stages of precipitation in alloys can be analysed by careful positron annihilation experiments.