Lifetimes of positrons trapped at Si vacancies

We present first-principles calculations on positron lifetimes at various vacancies in Si. Lattice relaxation and electron redistribution caused by the vacancy and by the positron are included using the two-component density-functional theory. The calculated lifetimes are in agreement with the experimental data for the monovacancies and the divacancies and provide a new assignment of the longer lifetimes measured previously to stable multivacancies. {} 1996 The American Physical Society.