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Some materials are metals and conduct electricity well; others are insulators and do not. In some cases, metals can be converted to insulators and vice versa by changing some fundamental parameter, such as pressure. In their Perspective, Abrahams and Kotliar discuss results reported in the same issue by Husmann et al . ( p. 1874 ) in which a continuous metal-insulator transition has been observed in a nickel selenide-sulfide compound. Understanding such transitions, which have posed puzzles for years, may help unravel the physics of highly correlated electrons, as well as lead to new materials.
Abrahams et al. (1996) studied this question.