ELECTRON FOCUSING IN AG AND W SINGLE-CRYSTALS

The nearly ballistic propagation of electrons in Ag and of holes in W is investigated in real space. The carriers are excited by the illumin ation of a small area on one surface of the sample, and are detected o n the opposite side by a Cu point contact. So-called caustics, singula rities of the carrier flux in real space related to regions on the sur faces of constant energy in momentum space with zero Gaussian curvatur e, have been observed. These experiments will permit evaluation of Fer mi-surface models and band-structure calculations near the Fermi level .