Investigation and calculation of positron lifetimes of monovacancies in crystals

The first-principles calculations of positron lifetimes of mono-vacancies in crystals were investigated. We use the two-component density functional theory to respectively compute positron lifetimes of neutral charge state of VAl defect in aluminium, VSi defect in silicon, VC, VSi and VC+CSi defects in 3C silicon carbide, VGa and VAs defects in gallium arsenide, taking into account atomic relaxation due to vacancy and electronic structural relaxation due to the presence of the positron. Three different calculation schemes are used. We find that the electron density inside the vacancy more or less increases due to the presence of the positron if the ionic positions are kept fixed, and the positron becomes more localized after the electronic structural relaxation for the case of VAl defect in aluminium and VSi defect in 3C silicon carbide, but it is opposite for the case of VGa defect in gallium arsenide and VC defect in 3C silicon carbide. The results with no consideration of the relaxation are even much closer to the experimental ones, therefore the atomic relaxation due to the position play an important role in calculating the positron lifetime of mono-vacancies in crystals.