UNIVERSAL CONDUCTANCE AT A PERCOLATIVE SUPERCONDUCTOR-INSULATOR TRANSITION IN 2 DIMENSIONS

Precisely at the percolation threshold, we show that a superconductor- insulator composite in 2d behaves like a normal metal, with a conducta nce g(e) which is a universal function of the material parameters. For a composite of BCS superconductor and insulator, we find that g(e) = C(DELTAg(n)epsilon(I))1/2, where epsilon(I) is the dielectric constant of the insulating component, DELTA = DELTAd/e2, DELTA is the BCS ener gy gap, d is the film thickness, and C is a dimensionless constant of order unity. g(e) and g(n) are the conductances of the film at thresho ld, and of the normal component, both measured in units of e2/hBAR. Ve ry near the threshold, g(e) falls to zero below a gap which vanishes a t p(c) according to the power law \p-p(c)\(s+t)/2, where s and t are t he standard percolation exponents. Above but not below p(c), the compo site exhibits a perfect-conductance delta-function whose strength fall s to zero as (p-p(c))t.