Sound propagation in two-dimensional metals

Electronic damping and renormalization of the sound velocity for metals with a cylindrical Fermi surface (layered metals), in which the electron-phonon relaxation has a specific two-dimensional character,1 are studied. The usually used τ approximation is inapplicable even for high (ωτ ≫l) frequencies of the sound propagating in the plane of the two-dimensional layers: relaxation occurs in several stages, whose times differ substantially. It is shown that the effects of the spatial and temporal dispersion in sound propagation become significant at much lower frequencies than in a three-dimensional metal, and they are of a different character. In particular, resonance absorption is qualitatively different from Landau absorption: it is sensitive to the relaxation processes, and it depends not only on the temperature but also on the frequency, and in an unusual manner also. The renormalization of the sound velocity is anomalously large and also sensitive to the scattering processes. Thus the study of sound propagation is an additional (to the electrical conductivity) method with appreciable advantages for studying two-dimensional effects in electron-phonon relaxation. It is shown that absorption and renormalization of the velocity increase substantially as the angle between the wave vectors of the sound and the normal vector to the plane of the two-dimensional layers decreases.

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