Doppler-Shifted Cyclotron Resonances with Helicons in Copper

The nonlocal propagation of helicons in a mixed electron and hole plasma is discussed. The conditions under which the Doppler-shifted cyclotron resonance (DSCR) of either type of carrier are effective are specified. The nonlocal magnetoconductivity, the dispersion relation for helicons, and the consequent surface impedance are evaluated for the case where the helicon wave vector is parallel to B and to the [001] axis of copper, assuming Roaf's model of the Fermi surface. The low-rf surface impedance of a copper single crystal of the above orientation is measured at 4.2^∘{}K. The standing-wave pattern is damped out before the nonlocal region is reached. The damping effect is interpreted to be the DSCR from the sections of the Fermi surface at k[001]=0.96 and 0.60 ^-1. Large surface-impedance anomalies are observed at low field. The field-frequency relation for the surface-impedance anomalies indicates that the electron orbits near the region k[001]=0.45 ^-1 are responsible. Detailed computation verifies this argument. The result is comparable to that of the magnetoacoustic attenuation measurements.