Magnetoacoustic Effect and Ultrasonic Attenuation in Potassium

Abstract

The magnetic field variation of ultrasonic attenuation has been observed at 4.2°K and 1.2°K in highpurity single crystals of potassium. Longitudinal waves of frequencies 10 to 50 Mc/sec were propagated in four crystals oriented along [100] and along [110], the magnetic field being perpendicular to the direction of propagation. The attenuation was found to be an oscillatory function of the magnetic field strength and periodic in $\frac{1}{\lambda H}$. The Fermi momentum was found to be 7.6×${10}^{-20\mathrm{}}$ g cm/sec, in reasonable agreement with the free-electron value. No shifts in the positions of the extremal values of the attenuation were observed as the magnetic field was rotated in the (100) or (110) plane, indicating that the Fermi surface of potassium is spherical to better than 1%. The $\frac{1}{H\lambda }$ values for the extrema and the magnetic field dependence of the ultrasonic attenuation were found to be in agreement with the free-electron theories of ultrasonic attenuation.