Electronic Properties of NiS_2-xSe_x Single Crystals: From Magnetic Mott−Hubbard Insulators to Normal Metals

The NiS 2- x Se x system represents one of the best examples of a Mott−Hubbard system, i.e. a system in which, under appropriate conditions of concentration, temperature, or pressure, a metal−insulator transition driven by electron−electron interaction takes place. Here, the metallic phase is either antiferromagnetic (for 0.44 ≤ x ≤ 1) or paramagnetic (for x ≥1), whereas the insulating phase is as a rule antiferromagnetic (including the spin-canted phases). In this paper we review both the physical properties and outline the basic features of the theoretical approach to those correlated electron systems . Emphasis is placed on a qualitative understanding of the observed transformation of the system from semiconductor (or magnetic insulator) to metal.