Determination of the Critical Conductivity Exponent for the Metal-Insulator Transition at Nonzero Temperatures: Universality of the Transition

A new approach has been developed for the determination of the critical conductivity exponent {μ} and the critical concentration Nc of the metal-insulator transition without extrapolation of the temperature dependence of the conductivity σ(T) to T0ex0ex=0ex0ex0. We propose to replace σ(0) by Δσ(T*)0ex0ex=0ex0exσN(T*)-σ_Nc(T*) at low T*, where σ(T)0ex0ex=0ex0exa+bTᵖ,p0ex0ex=0ex0ex1/2 or 1/3 is observed. Two series of samples of Ge:As and Ge:Sb were investigated. It is shown that μ0ex0ex=0ex0ex1 for both series. The normalized values of Δσ(T*)/ ̃ \~σ merge for Ge:As, Ge:Sb, Si:P, and Si:Sb into a universal line.