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 critica l conductivity exponent mu and the critical concentration N-c of the m etal-insulator transition without extrapolation of the temperature dep endence of the conductivity sigma(T) to T = 0. We propose to replace s igma(0) by Delta sigma(T) = sigma(N)(T*) - sigma(Nc)(T*) at low T*, w here sigma(T) = a + bT(p),p = 1/2 or 1/3 is observed. Two series of sa mples 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.