First-principles calculation of coincidence Doppler broadening of positron annihilation radiation

We report a first-principles method for calculating the momentum density of positron-electron pairs in materials, which can be accurately measured, in a wide momentum range, by means of coincidence Doppler broadening (CDB) of positron annihilation radiation. The calculation is based on the two-component density-functional theory within the local-density approximation. The electron and positron wave functions are calculated by means of the full-potential linearized-augmented-plane-wave method with use of semicore orbitals and of the pure plane-wave method, respectively. This hybrid basis set accurately determines the wave functions of core and valence electrons and is free from any shape or symmetry assumption for the positron wave function. The method is applied to two typical systems. i.e., Al and graphite having isotropic and anisotropic positron densities, respectively. The calculations agree well with experiments over the entire measurable momentum region. Especially, the calculations well reproduce the anisotropic high-momentum CDB tails of graphite, which originate from the quasi-two-dimensionally distributed positron. This reproduction suggests that the present method is applicable for a variety of materials.