Localization length and impurity dielectric susceptibility in the   critical regime of the metal-insulator transition in homogeneously doped   p-type Ge
  We have determined the localization length \xi and the impurity dielectric susceptibility \chi_{\rm imp} as a function of Ga acceptor concentrations (N) in nominally uncompensated ^{70}Ge:Ga just below the critical concentration (N_c) for the metal-insulator transition. Both \xi and \chi_{\rm imp} diverge at N_c according to the functions \xi\propto(1-N/N_c)^{-\nu} and \chi_{\rm imp}\propto(N_c/N-1)^{-\zeta}, respectively, with \nu=1.2\pm0.3 and \zeta=2.3\pm0.6 for 0.99N_c< N< N_c. Outside of this region (N<0.99N_c), the values of the exponents drop to \nu=0.33\pm0.03 and \zeta=0.62\pm0.05. 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\approx0.99N_c. 
Michio Watanabe, Kohei M. Itoh, Youiti Ootuka, and Eugene E. Haller