Metal-insulator transition in amorphous alloys

We focus on the central problem of discriminating between metallic and insulating behaviour in amorphous alloys formed between a semiconductor and a metal. For this, the logarithmic temperature derivative of the conductivity, w = d ln sigma / d ln T, has proved over recent years to be very helpful in determining the critical value x_c of the metal content x for the metal-insulator transition (MIT). We show that, for various amorphous alloys, recent experimental results on w(T,x) are qualitatively inconsistent with the usual assumptions of continuity of the MIT at T = 0 and of sigma(T,x_c) being proportional to a power of T. These results suggest that w(T,x_c) tends to 0 as T -> 0, in which case the MIT should be discontinuous at T = 0 (but only there), in agreement with Mott's hypothesis of a finite minimum metallic conductivity.