Experimental Study of Retardation Effects in Cyclotron Resonance

When the electron transit time through the skin layer becomes comparable to the rf period, Azbel'-Kaner cyclotron resonance is substantially altered. As Drew has shown explicitly, the retardation effect leads to an exponentially decreasing amplitude of successive subharmonics and a progressive change of the resonance line shape. We present and analyze data on a particular non-free-electron-like orbit on the Fermi surface of Ga, which at frequencies in the range 8-54 GHz shows strong retardation. The data confirm the expected exponential decay and related line-shape changes, and give evidence for their frequency and mean-free-path dependence.