Electron-positron momentum density in diamond, Si, and Ge

The electron-positron momentum densities in diamond, Si, and Ge are calculated using a first-principles method. Comparison of the theoretical momentum densities with the experiment shows that the electron-positron correlation effects are very important in Si and Ge, while this effect is negligible in diamond because the electrons are tightly bound. Our analysis shows that only the upper two bands, which consist of the sp³ hybridized orbitals, contribute to the structures at the low-momentum region of the momentum density. Diamond does not show these structures at the low-momentum region is due to its smaller lattice constant and weak electron-positron correlation effects. {} 1996 The American Physical Society.