ELECTRONIC-PROPERTIES OF NIS2-XSEX SINGLE-CRYSTALS - FROM MAGNETIC MOTT-HUBBARD INSULATORS TO NORMAL METALS

The NiS2-xSex system represents one of the best examples of a Mott-Hub bard system, i.e. a system in which, under appropriate conditions of c oncentration, temperature, or pressure, a metal-insulator transition d riven by electron-electron interaction takes place. Here, the metallic phase is either antiferromagnet;ic (for 0.44 less than or equal to x less than or equal to 1) or paramagnetic (for x greater than or equal to 1), whereas the insulating phase is as a rule antiferromagnetic (in cluding the spin-canted phases). In this paper we review both the phys ical properties and outline the basic features of the theoretical appr oach to those correlated electron systems. Emphasis is placed on a qua litative understanding of the observed transformation of the system fr om semiconductor (or magnetic insulator) to metal.