Susceptibility of Si:P across the metal-insulator transition. II. Evidence for local moments in the metallic phase

Metal–Insulator Transition in Three-Dimensional Semiconductors

We use a random gap model to describe a metal–insulator transition in three-dimensional semiconductors due to doping, and find a conventional phase transition, where the effective scattering rate is the order parameter. Spontaneous symmetry breaking results in metallic behavior, whereas the insulating regime is characterized by the absence of spontaneous symmetry breaking. The transition is continuous for the average conductivity with critical exponent equal to 1. Away from the critical point, the exponent is roughly 0.6, which may explain experimental observations of a crossover of the exponent from 1 to 0.5 by going away from the critical point.

Spin-glass freezing and RKKY interactions near the metal-insulator transition in amorphous Gd-Si alloys

Magnetization measurements of giant negative magnetoresistive amorphous GdxSi1-x ( 0.04<x<0.19) show strong mixed ferromagnetic and antiferromagnetic interactions and spin-glass freezing. The interactions appear to be an RKKY-like indirect exchange mediated by the conduction electrons, but are strikingly independent of their localization at the metal-insulator (MI) transition, presumably because the localization length exceeds the inter-Gd distance. The susceptibility per Gd atom in the paramagnetic state shows a nontrivial dependence on composition, with a maximum at the MI transition.