Abstract
Recent studies of the phonon spectra of metals using “point-contact spectroscopy” (PCS) are reviewed, with particular emphasis on the possibility of studying the electron-phonon interaction in single crystals. Various regimes of current flow in contacts are discussed, including ballistic, diffusion, and thermal regimes. The former (the Knudsen limit) proves to be the most appropriate for spectroscopic purposes; the PC spectrum (the derivative d 2 V/dI 2)in this case reveals a complicated structure containing information about singular points of the phonon density of states as well as the anisotropy (and energy dependence) of matrix elements of the electron-phonon interaction. Experimental results are presented for Cu, the most suitable metal for obtaining contacts of sufficiently good quality. The origins of the PC spectrum background at high energies and the singularities of PC spectra at low energy are analyzed. They are attributed to the nonequilibrium phonons reabsorbed in the junction, and to the influence of electron-impurity scattering upon the effectiveness of electron-phonon collisions.
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