THE ANISOTROPIC ELECTRON-POSITRON MOMENTUM DISTRIBUTION IN DIAMOND

We have performed a high-resolution, high-sensitivity study of the ele ctron-positron momentum distribution in diamond, using the two-detecto r coincidence Doppler broadening technique of positron annihilation sp ectroscopy. The measurements were carried out with the (100), (110), a nd (111) axes each separately aligned collinearly with the detectors s o that the anisotropy of the momentum distribution could be mapped out . Small differences in the momentum profiles were enhanced with differ ence spectra, and compare well with results of a density-functional th eory calculation of the electron-positron momentum distribution in the diamond lattice. There has been much debate regarding previously unex plained anomalies in the annihilation characteristics of the bulk posi tron configuration in diamond. These new results strongly support a mo del describing the bulk: positron as delocalized and inhomogeneous, bu t with sufficient amplitude at me intra-bond region such that the anni hilation characteristics are determined by the bond electron density a nd momentum distribution.