Real Space Imaging of Ballistic Carrier Propagation in Bi Single Crystals

Carriers and phonons are excited in a small volume at the surface of a Bi single crystal by laser illumination. Their nearly ballistic propagation through the sample is studied at low temperature by real space imaging of the potential at the sample surfaces using a Cu point contact as probe electrode. A focusing of electrons due to the geometry of the Fermi surface and phonon focusing is observed. The application of a magnetic field allows the identification of the signal components. A theoretical model for the mechanism of carrier excitation and detection is proposed.