ANGLE-DEPENDENT MAGNETORESISTANCE OSCILLATIONS AND FERMI-SURFACE REORDERING AT HIGH MAGNETIC-FIELDS IN ALPHA-(ET)(2)KHG(SCN)(4)

Angle dependent magnetoresistance oscillations (AMRO) have been studie d in the charge transfer salt alpha-(ET)2(K)Hg(SCN)(4) for magnetic fi elds in the range 0 - 30 T. This salt exhibits the onset of antiferrom agnetic order at temperatures T-N similar to 8-10 K and the presence b elow 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, ampl itude, and nature of the oscillations can be used to directly infer th e 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 osc illations from a closed cylindrical FS which is elongated in the c dir ection; the low field phase appears to be a spin density wave groundst ate, with a FS consisting of a sheet (which is quasi-1D in character a nd tilted at an angle of similar to 21 degrees to the bc plane) and s mall closed 2D pockets. It is suggested that the breakdown orbits betw een the pockets and the 1D sheets are able to account for the various Shubnikov-de Haas frequencies observed below the kink.