Metal–insulator phase transitions and related critical phenomena in NiS2−xSex

Recent research on various physical properties of the NiS2−xSex system is reviewed. It is emphasized that with increasing x<0.71 the system evolves gradually from the insulating to the metallic state; at intermediate x metal–insulator transitions are encountered. These phenomena and a variety of results involving x ray and electron spectroscopy are linked to the transition from itinerant to localized electronic states induced by strong electron–electron correlation effects. At alloy concentration close to x=0.44 pressure may be used to tune the electrical properties, such that quantum critical fluctuations are encountered at cryogenic temperatures; these are rationalized in terms of current theoretical concepts. Major recent advances: Recent advances in the field include (1) the publication of a revised phase diagram of the Ni2−xSex system that establishes more realistic phase boundaries for the various magnetic and nonmagnetic transitions as a function of temperature T and of x, (2) the elucidation of quantum critical phenomena that are noted at cryogenic temperatures for the alloy x=0.44, and (3) the interpretation of these transitions in terms of itinerant-localized transitions of 3 d electrons.