Electron-positron momentum density distribution in diamond

We have measured the momentum distribution of the electron-positron annihilating pair in diamond as a function of crystallographic orientation, using the Doppler broadening technique of positron annihilation spectroscopy. A 1% variation in the longitudinal projection of the momentum distribution was observed as the sample was rotated about the 〈110〉 axis. This anisotropic distribution shows an excellent agreement with a theoretical calculation performed based on two component density functional theory in the local density approximation. The results suggest strongly that the bulk positron density, though delocalized and inhomogeneous, has sufficient amplitude at the infra-bond regions such that the annihilation characteristics are dominated by the bond electron density and momentum distribution.