Electromagnetic generation of ultrasound in metals

The electromagnetic generation of transverse acoustic waves in metals in the presence of a static magnetic field normal to the surface is discussed with reference to an isotropic effective mass $m^{*}$ of the conduction electrons. From a semiclassical argument, it is shown that in addition to the direct Lorentz force and the collision-drag force, each lattice ion experiences a Bragg-reflection force proportional to $\frac{m}{m^{*}}-1\mathrm{}$. In the nonlocal limit, when the ratio $\frac{m}{m^{*}}$ is greater than unity, this force causes the generated acoustic amplitude as a function of magnetic field to deviate significantly from the monotonic dependence that is expected from the free-electron theory of metals. However, this force does not provide significant modification to the free-electron theory for predicting the rotation of the plane of polarization, the attenuation coefficient of shear acoustic waves, and the properties of the helicon-phonon interaction.