Angle-dependent magnetoresistance oscillations and Fermi surface reordering at high magnetic fields in α-(ET)2KHg(SCN)4

Angle dependent magnetoresistance oscillations (AMRO) have been studied in the charge transfer salt α-(ET)2KHg(SCN)4 for magnetic fields in the range 0 – 30 T. This salt exhibits the onset of antiferromagnetic order at temperatures TN ∼8–10 K and the presence below this temperature of a region of sharp negative magnetoresistance at a field around 22 T known as the “kink”. AMRO have been measured in this salt for a wide range of applied fields since the period, amplitude, and nature of the oscillations can be used to directly infer the character of the Fermi surface (FS) as a function of field. The data indicate that a profound change in the band structure occurs at this kink transition; the high field phase is characterised by quasi-2D oscillations from a closed cylindrical FS which is elongated in the c direction; the low field phase appears to be a spin density wave groundstate, with a FS consisting of a sheet (which is quasi-1D in character and tilted at an angle of ∼21° to the b∗c plane) and small closed 2D pockets. It is suggested that the breakdown orbits between the pockets and the 1D sheets are able to account for the various Shubnikov-de Haas frequencies observed below the kink.