Metal-insulator transition in amorphous alloys

The logarithmic temperature derivative of the conductivity, w = d ln sigma/ d ln T, has proved to be very helpful for determining the critical value x( c) of the metal content x. We show that, for various amorphous alloys, rece nt experimental results on w(T, x) are qualitatively inconsistent with the usual assumptions of continuity of the metal-insulator transition at T = 0 and of sigma(T,x(c)) being proportional to T-p With p = 1/2 or 1/3: There a re samples with simultaneously w << p and dw/dT < 0 at the lowest experimen tally accessible temperatures. The results suggest that w(T, x(c)) tends to 0 as T --> 0, in which case the MIT would be discontinuous at T = 0 (but o nly there). (C) 2000 Elsevier Science B.V. All rights reserved.