Helicon Propagation in Metals near the Cyclotron Edge

The theory of Doppler-shifted cyclotron resonance with helicon waves is elaborated by calculating the magnetic-field dependence of the surface impedance near the cyclotron absorption (Kjeldaas) edge. It is found that the location of the edge, defined as that field for which the field derivative of the surface reactance is a minimum, deviates significantly from the threshold field defined by the Doppler-shifted frequency criterion. The deviations are large when ${\omega }_c\tau <50\mathrm{}$. The theory is compared with the experimental work of Taylor on Na and K. The data agree well with the free-electron model for Na, but there is a large discrepancy (770 G for 10-Mc/sec helicons) in K. The surface impedance is also calculated for a metal with a spin-density-wave ground state, in order to test the hypothesis of such a ground state, previously made to explain other anomalies in K. The edge field derived for a spin-density-wave state appropriate to K is 730 G less than that required by the free-electron model, and agrees with the published data.