MAGNETOTRANSPORT AND FERMI-SURFACE TOPOLOGY OF [BIS(ETHYLENEDITHIO)TETRATHIAFULVALENE]2KHG(SCN)4

We have performed measurements of the angle-dependent magnetoresistance in the organic metal (BEDT-TTF)2KHg(SCN)4 [Where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene] in fields of up to 30 T and at temperatures down to 0.4 K. Shubnikov-de Haas (SdH) oscillations were observed over a wide range of fields and field orientations; at fields below 22 T, the dominant oscillation for B perpendicular to the highly conducting layers has a fundamental field B(F) = 670 +/- 5 T. However, we have observed that another series with a fundamental field which is sample dependent and some 100-200 T higher than the dominant series is also present, resulting in a beat structure in the envelope of the oscillations. In addition, significant hysteresis is seen in the magnetoresistance. At approximately 22 T, the resistance falls sharply, the hysteresis almost vanishes, the measured effective mass increases, and the SdH oscillations simplify to a single series, with a frequency B(F) = 656 +/- 10 T similar to that predicted by band-structure calculations using the room-temperature crystal structure. On rotating about the a axis the transition at approximately 22 T slowly moves to lower field. (BEDT-TTF)2KHg(SCN)4 is known to order antiferromagnetically at approximately 8 K, and we suggest that the effects observed below the transition may result from a magnetic superlattice in the direction perpendicular to the highly conducting planes; using this model, we are able to account qualitatively for most of the features observed. The interplane interactions responsible for the ordering are overcome by the external magnetic field at approximately 22 T, resulting in the fall in resistance and the simpler high-field behavior.