The anisotropic electron-positron momentum distribution in diamond

We have performed a high-resolution, high-sensitivity study of the electron-positron moruentum distribution in dinrnood using the two-detector coincidence Doppler broadening technrique of positron annihilation spectroscopy. The measurements were carried out with the (100), (110), and (111) axes each separately aligned collinearly with the detectors so that the anisotropy of the momentum distribution could be mapped out. Small differences in the momentum profiles were enhanced with difference spectra, and compare well with results of a density-functional theory calculation of the electron-positron momentum distribution in the diamond lastice. There has been much debate regarding previously unexpelanined anomalies in the annihilation characteristics of the bulk positron confriguration in diamond. These new results strongly support a model describing the bulk positron as delocalized and inhomogenous, but with sufficient amplititude at the intra-bond region such that the annihilation characteristics are determined by the bond electron, density and momentum distribution.