A temperature-dependent angular correlation study of positron annihilation in single crystal zinc

Abstract

The angular correlation peak counts from positrons annihilating in single crystals of zinc have been measured from 95 to 660K in the (1010) and (0001) directions. A significant anisotropy has been observed between the counts in the two orientations which increases with temperature but in the opposite sense to a/c. The anisotropy increases near 290K, and then decreases to the nearly isotropic distribution of positrons trapped at vacancies. The vacancy formation energy, hf, determined by the threshold temperature of 403K, is 0.52+or-0.01 eV for both orientations. A trapping-model determination of hf is found to depend on the temperature dependence assumed in the saturation region; bet fits are obtained for hf=0.59+or-0.03 eV from the (0001) data, and 0.60+or-0.03 eV from the (1010) data. Full angular correlation curves have been measured at 95, 340 and 600K for both orientations. The effects of the higher momentum components in the positron wavefunction are calculated and could account for the anisotropy observed beyond the Fermi momentum at 95 and 340K. It is suggested that the temperature dependence of the peak counts anisotropy may arise from a relatively faster decline of the Fermi cut-off in the (1010) direction relative to (0001), as expected from the band structure of zinc.