Nonlocal electromagnetic generation and detection of ultrasound in potassium

The amplitude of electromagnetic generation of ultrasound in single-crystal potassium has been measured at $T=4.2$ K as a function of magnetic field from 0 to 15 kG at a frequency of 8.97 MHz, using inductive means for both generation and detection of the acoustic wave. At zero magnetic field the measured amplitude is several times larger than the predicted free-electron value, and between 0 and 3 kG the experimental curve is well outside the predicted behavior. From 9 to 14 MHz the frequency dependence of the measured amplitude exhibits the expected behavior. An attempt has been made to rework the theory in the usual approximations to explain the discrepancy observed. It is shown that no simple modification of free-electron theory can account for the measured enhancement of the zero-field electromagnetic-generation amplitude. The experimental results are discussed in the light of previous measurements in potassium.