Radio-Frequency Size Effect in Potassium: Relation to Cyclotron Resonance, Helicon Waves, and Sound Waves

Studies are reported of the transmission of 1- to 100-MHz rf energy through a thin potassium slab when a magnetic field is applied either perpendicular or nearly parallel to the surface of the slab. In the perpendicular-field geometry for fields below the helicon Doppler-shifted cyclotron resonance edge, the phase of the transmitted signal increases linearly with applied field as predicted by Gantmakher and Kaner. Near zero applied field, the signal shows the increase in amplitude and extra 1/2π phase shift predicted by Baraff. When the field is nearly parallel to the slab surface, an attenuation of the transmitted signal is observed at the value of field where the effective phase velocity of the rf current distribution matches the phase velocity of an acoustic wave.