Magnetotransport and Fermi-surface topology of [bis(ethylenedithio)tetrathiafulvalene]2KHg(SCN)4

Abstract

We have performed measurements of the angle-dependent magnetoresistance in the organic metal (BEDT-TTF${)}_2$KHg(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 ${\mathit{B}}_{\mathit{F}}$=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 ∼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 ${\mathit{B}}_{\mathit{F}}$=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 ∼22 T slowly moves to lower field. (BEDT-TTF${)}_2$KHg(SCN${)}_4$ is known to order antiferromagnetically at ∼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 ∼22 T, resulting in the fall in resistance and the simpler high-field behavior.