Doppler-shifted cyclotron resonance with helicons in aluminum

Abstract

Doppler-shifted cyclotron resonance (DSCR) has been observed with helicon waves propagating along the [001], [110], and [111] axes of pure aluminum crystals. Strong quantum oscillations are observed in all propagation directions, the periods of the quantum oscillations agreeing with those of the long-period de Haas-van Alphen oscillations. The dispersion relations of the helicons as well as the frequencies and field values of the DSCR surface impedance anomalies have been measured and compared to the values calculated from two current models for the aluminum Fermi surface. The nonlocal dispersion forB ∥ [001] follows closely the theoretical results, while the nonlocal effect forB ∥ [110] is weaker than expected. A helicon window and a negative nonlocal dispersion are observed forB ∥ [111], due to the resonance of the electron states on the second-zone cap region. Except for those appearing below the helicon window, all the field-frequency relations of the experimental DSCR anomalies agree with those calculated from the predicted resonance zones at the Fermi surface.