Electromagnetic Generation of Ultrasonic Shear Waves in Potassium

Measurements have been made of the field dependence of both the electromagnetic generation and the attenuation of ultrasonic shear waves along the [110] direction in potassium in the non-local limit. In this limit in the configuration where the Lorentz force is responsible for the generation, the sound amplitude is nonlinear in the magnetic field strength through the region of the Kjeldaas absorption edge. We also observe an electromagnetically generated shear sound wave which is present at zero field and polarized at right angles to the direction of the rf magnetic field axis, and whose efficiency of generation drops nearly to zero at the Kjeldaas edge. The existing free-electron theory has been extended to the anisotropic case, and the experimental results are compared with theoretical predictions for both free-electron and charge-density-wave models of the Fermi surface using an experimental determination of the electron mean free path.