Studying conduction-electron/interface interactions using transverse electron focusing

Understanding the microscopic processes by which the electrons within condu ctors are scattered at surfaces and interfaces is important for both fundam ental physics and technology. The authors review what has been learned sc, far about scattering of electrons at a variety of surfaces and interfaces u sing a technique, transverse electron focusing (TEF), that involves two poi nt contacts in a uniform magnetic field. Transverse electron focusing is a sort of beta-ray spectrometer in a metal, except that, whereas the beta-ray spectrometer requires a narrow beam because the energy and momentum of a f ree electron can be arbitrary, in TEF the electrons of interest all have th e Fermi energy and momentum, so focusing occurs even for electrons injected isotropically in angle. Transverse electron focusing is unique in its abil ity to probe localized and selectable portions of the interface from inside the conductor, using conduction electrons on only small parts of the Fermi surface. The authors first briefly review the essential features of TEF an d of ideal and rough surfaces and describe the three techniques now used fo r injecting and detecting electrons: needle contacts, lithographically fabr icated contacts. and light-induced injection. They then turn to measurement s in metals and semimetals of the probability of specular reflection q from a given interface for electrons at the Fermi energy impinging at perpendic ular incidence. They examine how q varies over different crystal faces for different electron orbits on the Fermi surfaces of a variety of conductors and how it is affected by changes in the de Broglie wavelength lambda(dB), by chemical etching, ion etching, or physical damage, by a molecular overla yer condensed from the surrounding atmosphere, and, for semimetals, by surf ace band bending (surface charge). The authors also explain how to measure the dependence of q upon the angle of incidence q(theta), which gives infor mation about surface structure. Transverse electron focusing studies of a v ariety of quasiparticle effects arising because the electrons are in a soli d are described. These include (a) scattering of excitations moving on "hol elike" orbits-q can depend upon the sign of the particle charge; (b) scatte ring involving a surface reciprocal-lattice vector G(r), including surface resonances induced by an artificial grating etched onto a Bi Surface; and ( c) scattering between different parts of the Fermi surface-intervalley scat tering (IVS)-including scattering in which the sign of the quasiparticle ch arge changes. The authors review studies of scattering of electrons from a normal-metal (or semimetal)/superconductor interface, which involves an unu sual phenomenon called Andreev reflection, in which the signs of both the c harge and mass change. Also described are TEF studies of scattering of ligh t-excited electrons from an intercrystalline boundary and recent TEF measur ements of q for scattering from the boundary of the two-dimensional electro n gas. The authors conclude with a list of future TEF studies of conduction -electron/interface interactions that they believe to be interesting and im portant. [S0034-6861(99)01105-8].