Metal-insulator transition in amorphous alloys

We focus on the central problem of discriminating between metallic and insu lating behaviour in amorphous alloys formed between a semiconductor and a m etal. For this, the logarithmic temperature derivative of the conductivity, w = d In sigma/d In 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-in sulator transition (MIT). We show that, for various amorphous alloys, recen t experimental results on w(T,x) are qualitatively inconsistent with the us ual assumptions of continuity of the MIT at T = 0 and of sigma(T,x(c)) bein g 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 metall ic conductivity. (C) 1999 Elsevier Science Ltd. All rights reserved.