Semiclassical description of cyclotron resonance in quasi-two-dimensional organic conductors: Theory and experiment

We discuss cyclotron-resonance-like behavior in quasi-two-dimensional organic conductors, both from a theoretical perspective and from an experimental point of view. We demonstrate how the conductivity in the least dispersive direction can dominate the magnetoelectrodynamic response of highly anisotropic metals. Consequently, we develop a detailed semiclassical model for the conductivity in this direction, taking into consideration the unique Fermi surface topologies common to these materials. This can result in multiple cyclotron-resonance-like features in the conductivity along the least conducting direction, which arise from periodic motion in a plane perpendicular to the applied magnetic field; we refer to these features as ''periodic orbit resonances.'' It is shown that the details of these periodic orbit resonances are highly sensitive to the precise shape of the Fermi surface; indeed, both quasi-two-dimensional, and quasi-one-dimensional, Fermi surface sections will contribute to this effect. We also discuss compelling experimental evidence supporting our model, as well as several other consequences of such a semiclassical treatment; e.g., magnetoresistance and periodic orbit resonance linewidths. The outcome of this work is a clearer understanding of cyclotron-resonance-like features observed recently in several bis(ethylenedithio)-tetrathiafulvalene charge-transfer salts. In light of our findings, we urge caution when analyzing experimental data. In particular, care should be exercised in experiments on materials possessing both quasi-one- and two-dimensional Fermi surfaces, bearing in mind that either type of carrier can contribute to the periodic orbit resonances.