Generation of combination-frequency sound harmonics in conductors

The generation of combination-frequency harmonics of longitudinal sound waves in conductors is investigated in the case where all the wavelengths are much smaller than the electron mean free path. It is shown that when the primary waves propagate in the same direction, resonant (moving in phase with the sound waves) electrons can provide the main contribution to the nonlinear response characterizing difference-frequency generation, whereas the sum-frequency nonlinear response is associated with nonresonant electrons and coincides with the static response. Difference-frequency generation is considerably more efficient than sum-frequency generation in the given situation.

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