PHASE-BOUNDARY IN THE DIMENSIONALITY OF ANGLE-DEPENDENT MAGNETORESISTANCE OSCILLATIONS IN THE CHARGE-TRANSFER SALT ALPHA-(BEDT-TTF)(2)KHG(SCN)(4)

Angle-dependent magnetoresistance oscillations (AMROs) have been studi ed in the isostructural charge-transfer salts alpha-(BEDT-TTF)(2)KHg(S CN)(4) and alpha-(BEDT-TTF)(2)NH4Hg(SCN)(4) (where BEDT-TTF is bis(eth ylenedithio)tetrathiafulvalene) in steady fields of up to 30 T. The sh apes of the approximately elliptical quasi-two-dimensional (Q2D) Fermi surfaces that these organic metals possess have been determined at 30 T and are found to be in broad agreement with recent band-structure c alculations. The Fermi surface of the salt alpha-(BEDT-TTF)(2)KHg(SCN) (4) undergoes a reconstruction at low fields and temperatures, resulti ng in a change in the dimensionality of the AMROs from Q2D character t o quasi-one-dimensional character. This change is associated with the kink transition that is observed in magnetic field sweeps and is attri buted to the formation of a spin-density wave ground state. The phase boundary of the change in the AMRO dimensionality has been followed to both the low-temperature high-field (about 23 T) and low-field high-t emperature (about 8 K) extremes. The data are compared with recently p roposed models of the AMROs and Fermi surfaces for these materials.