Annealed disorder, rare regions, and local moments: A novel mechanism for metal-insulator transitions

It in known that for noninteracting electron systems, annealed magnetic dis order leads to a new mechanism, and a new universality class, for a metal-i nsulator transition. The transition is driven by a vanishing of the thermod ynamic density susceptibility rather than by localization effects. The crit ical behavior in d = 2 + epsilon dimensions is determined, and the underlyi ng physics is discussed. It is further argued that annealed magnetic disord er, in addition to underlying quenched disorder, describes local magnetic m oments in electronic systems.