Scaling analysis of the low temperature conductivity in neutron-transmutation-doped ⁷⁰Ge:Ga

We report on the scaling analysis of low temperature electron transport properties of nominally uncompensated neutron-transmutation-doped ⁷⁰Ge:Ga samples in the critical regime for the metal-insulator transition. Ga concentration (N) and temperature (T) dependent conductivities σ(N,T) are shown to collapse onto a single universal curve using finite temperature scaling of a form σ(N,T) ∝ T^x f(|N/N_c - 1|/T^y) with x ≈ 0.38 and y ≈ 0.32 for the very small region of N = N_c ± 0.004N_c. The conductivity critical exponent μ = x/y = 1.2 ± 0.2 found from this analysis is significantly larger than μ ≈ 0.5 found from the analysis we performed previously on the same series of samples covering the much larger region of the concentration N_c < N < 1.4N_c. Determination of the true critical region, either N = N_c ± 0.4% or N = N_c ± 40%, is necessary in the future for the reliable determination of μ in Ge:Ga.