MAGNETOTRANSPORT AND FERMI-SURFACE TOPOLOGY OF BETA''-(BEDT-TTF)2AUBR2 - THE EFFECTS OF SPIN-DENSITY-WAVE FORMATION

Magnetoresistance measurements have been made on a number of single-cr ystal samples of the metallic charge-transfer salt beta''-(BEDT-TTF)2A uBr2, using magnetic fields up to 50 T. The experiments have been carr ied 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 Ha as oscillations, an anisotropic angle dependence, and, below 1 K, hyst eresis. Both the hysteresis in the magnetoresistance and frequency mix ing effects observed in the Shubnikov-de Haas spectrum can be explaine d 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 spectru m of beta''-(BEDT-TTF)2AuBr2 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 an gle dependence of the Shubnikov-de Haas oscillation amplitudes has bee n used to deduce the approximate shapes and orientations of these pock ets, which are found to be in good qualitative agreement with the prop osed model.