Large magnetic-field-induced Fermi surface changes in alpha -ET sub(2)KHg(NCS) sub(4)

We report angle-dependent magnetoresistance experiments on the charge-transfer salt alpha -ET sub(2)KHg(NCS) sub(4) using fields of up to 30 T. The data indicate that the widely reported high-field `kink' observed in the magnetoresistance of the alpha -ET sub(2)MHg(NCS) sub(4) (M identical with K, Tl, Rb) family of charge-transfer salts represents a transition from an imperfectly nested spin-density wave ground state, characterized by a quasi-one-dimensional Fermi surface sheet tilted with respect to the b*c plane, to a state with a cylindrical two-dimensional Fermi surface.