Electromagnetic excitation of sound in metals

Excitation of sound oscillations in a metal by an electromagnetic wave incident on its surface is investigated. An analysis is presented of the different mechanisms of contactless conversion in the limit of local conductivity and under the conditions of the anomalous skin-effect. It is shown that in a sufficiently intense magnetic field when the cyclotron radius of the electron orbit is small compared to the wavelength of sound the principal role in the processes of conversion is played by inductive coupling. In the absence of a constant magnetic field the dynamic equilibrium between forces exerted on the lattice by the electrons and by the external field may be upset by the scattering of electrons at the metal boundary. The mechanism of excitation due to the diffuse scattering of electrons at the surface is very important in the UHF range. The special features of electromagnetic excitation of sound in semimetals, superconductors and ferromagnetic substances have been determined. The essential role of the forces exerted on the lattice by the electrons removed from the state of equilibrium is determined in semimetals by the large value, compared with normal metals, of the ratio of the deformation potential tensor to the Fermi energy. The technique of contactless excitation of sound is of interest as one of the methods of acoustic NMR and can be used to study different kinds of coupling of collective excitations in metals with sound waves. This refers both to the study of geometric and cyclotron resonances in the case of electromagnetic excitation of sound, and also to the problem of studying the heliconphonon and the doppleron-phonon resonances and the coupling with collective modes in ferromagnetic substances.