On the role of classical and quantum notions in channeling and the development of fast positrons as a solid state probe of valence electron and spin densities

Abstract

The subtleties of the interplay between classical and quantum pictures are revealed particularly clearly in connection with the effect of channeling. The role of this interplay is described and related to a new probe for valence electron and spin magnetic densities in thin films, which is based upon two-photon annihilation in flight of channeled MeV positrons. The channeling effect is used to steer positrons into the interstices of a crystal, where the valence electron density is high. Transmission measurements have been performed to test our quantitative understanding of the spatial and momentum distribution of channeled positrons. We calculate annihilation rates in a gold crystal and show that the probe is highly sensitive to valence electrons. Channeling states in iron are shown to be sensitive to different regions of the spin magnetic density. With a polarized beam of MeV positrons and the possibility for selective state population, we show that the spatial distribution of the magnetization density can be probed.