We have determined the localization length $\xi$ and the impurity dielectric susceptibility ${\chi }_{\mathrm{imp}}$ as a function of Ga acceptor concentrations (N) in nominally uncompensated ${}^{70}\mathrm{G}\mathrm{e}:\mathrm{G}\mathrm{a}$ just below the critical concentration ${(N}_c)$ for the metal-insulator transition. Both $\xi$ and ${\chi }_{\mathrm{imp}}$ diverge at $N_c$ according to the functions $\xi \propto (1\mathrm{}-{N/N}_c{)}^{-\nu }$ and ${\chi }_{\mathrm{imp}}\propto {(N}_c/N-{1)}^{-\zeta },$ respectively, with $\nu =1.2\mathrm{}\pm 0.3$ and $\zeta =2.3\mathrm{}\pm 0.6$ for ${0.99N}_c<N<\mathrm{}{N}_c.$ Outside of this region $(N<\mathrm{}{0.99N}_c),$ the values of the exponents drop to $\nu =0.33\mathrm{}\pm 0.03$ and $\zeta =0.62\mathrm{}\pm \mathrm{0.05.}$ The effect of the small amount of compensating dopants that are present in our nominally uncompensated samples, may be responsible for the change of the critical exponents at $N\approx {0.99N}_c.$

• Received 10 April 2000

DOI:https://doi.org/10.1103/PhysRevB.62.R2255