Electron focusing in metals and semimetals

Partly ballistic propagation of carriers and phonons in single crystals of the metals Ag and W and the semimetal Bi is studied in real space. The carr iers are excited by heating a small surface area of the sample by illuminat ion or electron beam irradiation. Alternatively they are injected through p oint contacts. They spread out from this point-like source setting up a cha racteristic anisotropic current pattern representing some kind of fingerpri nt of the material's band structure. If the sample thickness is comparable to the mean free path of the carriers, they produce a potential pattern on the opposite sample surface. We present images of these patterns revealing the phenomenon of electron focusing, i.e. caustics produced by a diverging density of trajectories in specific directions. The influence of magnetic f ields is studied. We give some basic theoretical analysis of the data in te rms of fully ballistic transport. An outlook into possible future developme nts and applications of the technique and the completion of the theoretical model is given. (C) 2000 Elsevier Science B.V. All rights reserved.