ELECTRON MOMENTUM DISTRIBUTIONS IN ELEMENTAL SEMICONDUCTORS PROBED BYPOSITRONS

Momentum distributions of positron-electron pairs in diamond, Si, and Ge are systematically studied using state-of-the-art techniques in exp eriment and theory, i.e., the positron two-dimensional angular correla tion of annihilation radiation (2D-ACAR) technique and two-component d ensity-functional (TCDF) theory. It is experimentally examined that al l samples are free from positron trapping. An interesting difference a mong the elemental semiconductors is then clarified, namely, a flat 2D -ACAR distribution of the [001] projection in the low momentum region is found in diamond, while a deep dip is observed at the origin in Si and Ge. These experimental results are compared with those of first-pr inciples TCDF calculations within the local-density approximation base d on the scheme by Puska, Seitsonen, and Nieminen [Phys. Rev. B 52, 10 947 (1995)] and the generalized-gradient approximation by Barbiellini et al. [Phys. Rev. B 53, 16 201 (1996)]. Good agreement between theor y and experiment confirms the validity of the TCDF. Analysis of calcul ational results clarifies that the unique electron momentum distributi on in diamond is due to the carbon p orbital sharply localized in real space. [S0163-1829(98)06519-9].