Quantum localization versus classical percolation and the metal-nonmetal transition in Hg-Xe films

The electrical conductivities of films which are mixtures of Hg and Xe have been examined in the normal (nonsuperconducting) state as a function of composition. These films, which were quench condensed at T=4.2 K, were typically 5000 thick, and are believed to be microscopically homogeneous and fully three dimensional. Aspects of quantum theories of localization appear to more accurately describe the metal-nonmetal transition than classical percolation. Three distinct regimes of conductivity are evident: conventional transport for low Xe concentration, incipient localization, and strong localization with a negative temperature coefficient of resistance. Data in the second regime are compared with the scaling theory of localization, and a correlation length is inferred. The transition to the insulating regime appears to be continuous, but a minimum metallic conductivity at T=0 K cannot be ruled out. The behavior of the superconducting transition with composition has also been examined within the context of localization theory.