We report the first measurement of cyclotron resonance in a metallic charge-transfer salt. Carrier pockets are observed with eA'ective masses of 0.40m, and 0.94m,. In contrast, masses of 2.0m, and 2.4m, are seen in magnetotransport. We propose that the transport masses are enhanced by electronelectron interactions, whereas, following Kohn's theorem, the mass measured by cyclotron resonance is independent of these interactions. These measurements therefore represent the first direct gauge of the electron-electron interaction in a metallic charge-transfer salt.

We report magnetoresistance measurements on ET, KHg(SCN), and /3"ET2 AuBr,. Both show Shubnikov-de Haas oscillation frequencies additional to those expected from band structure calculations, and other magnetoresistance anomalies occur (e.g. hysteresis, "kink" structures). These effects are explained qualitatively by band structure modifications produced by magnetic ordering.

Magnetoresistance measurements have been made on a number of single-crystal samples of the metallic charge-transfer salt P"-(BEDT-TTF}, AuBr"using magnetic fields up to 50 T. The experiments have been carried out for a wide range of orientations of the sample with respect to the magnetic field and for temperatures ranging between 80 mK and 4.2 K. The magnetoresistance exhibits a complex series of Shubnikov -de Haas oscillations, an anisotropic angle dependence, and, below 1 K, hysteresis. Both the hysteresis in the magnetoresistance and frequency mixing effects observed in the Shubnikov -de Haas spectrum can be explained by the effects of Shoenberg magnetic interaction, and this mechanism has been successfully used to model the observed Fourier spectrum of the magnetoresistance. The complex Shubnikov-de Haas frequency spectrum of P"-(BEDT-TTF}zAuBrz is proposed to result from the effects of a spin-density wave on the band structure, which alters the original Fermi surface to produce three two-dimensional carrier pockets. The angle dependence of the Shubnikov-de Haas oscillation amplitudes has been used to deduce the approximate shapes and orientations of these pockets, which are found to be in good qualitative agreement with the proposed model.