HALL-EFFECT NEAR THE METAL-INSULATOR-TRANSITION

Hall-coefficient and dc-conductivity measurements have been made, with use of the van der Pauw geometry, on uncompensated Si:As samples on both sides of the metal-insulator transition (7.77×1018<N<32.8×1018 cm-3, 8.55×1018<nc<8.60×1018 cm-3) in the temperature range 300 to 0.5 K. Much of the data was taken in temperature sweeps between 4.2 and 0.5 K at magnetic fields between 0.5 and 15 T. The insulating samples exhibit variable-range-hopping (VRH) behavior for RH(N,H,T) that is similar to the VRH behavior of (N,H,T) and is Mott VRH in the temperature range of these experiments. The ratio of the Hall VRH characteristic temperature T0H and the Mott characteristic temperature T0 as H0 and Nnc- is in good agreement with the theoretical prediction of Gruenewald et al. that (T0H/T0)1/4=5/8. The metallic results indicate RH(n,H,T)RH(n,H)[1+mH(n,H)T1/2] at sufficiently low temperature, analogous to earlier results for (n,H,T) and suggest a coefficient of the T1/2 term for xy of order mxy1.5mxx. The values of RH-1(n, H0, T0 K) do not show the apparent critical behavior observed for Ge:Sb, Kr:Bi, and a-Si:Pt and are essentially in agreement with the weak-localization theoretical predictions of Fukuyama and of Shapiro and Abrahams. It is speculated that the differing critical behavior of these metal-insulator systems results from a spin-orbit contribution (extraordinary contribution) to the Hall coefficient. © 1990 The American Physical Society.