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 mu and the critical concentration N-c of the metal-insulator transition without extrapolation of the temperature dependence of the conductivity sigma(T) to T = 0. We propose to replace sigma(0) by Delta sigma(T*) = sigma(N)(T*) - sigma(Nc)(T*) at low T*, where sigma(T) = a + bT(p),p = 1/2 or 1/3 is observed. Two series of samples of Ge:A and Ge:Sb were investigated. It is shown that mu = 1 for both series. The normalized values of Delta sigma(T*)/sigma merge for Ge:As, Ge:Sb, Si:P, and Si:Sb into a universal line.