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 σ/d ln T, has proved over recent years to be very helpful in determining the critical value xc 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 σ(T,xc) being proportional to a power of T. These results suggest that w(T,xc) 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.