Metal-insulator phase transitions and related critical phenomena in NiS2-xSex

Recent research on various physical properties of the NiS2-xSex system is r eviewed. It is emphasized that with increasing x<0.71 the system evolves gr adually from the insulating to the metallic state; at intermediate x metal- insulator transitions are encountered. These phenomena and a variety of res ults involving x ray and electron spectroscopy are linked to the transition from itinerant to localized electronic states induced by strong electron-e lectron correlation effects. At alloy concentration close to x=0.44 pressur e may be used to tune the electrical properties, such that quantum critical fluctuations are encountered at cryogenic temperatures; these are rational ized in terms of current theoretical concepts. Major recent advances: Recen t advances in the field include (1) the publication of a revised phase diag ram of the Ni2-xSex system that establishes more realistic phase boundaries for the various magnetic and nonmagnetic transitions as a function of temp erature 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 inter pretation of these transitions in terms of itinerant-localized transitions of 3 d electrons. (C) 2001 Elsevier Science Ltd. All rights reserved.