Universal Character of Tunnel Conductivity of Metalinsulator-Metal Heterostructures with Nanosized Oxide Barriers

We discuss an universal effect in tunnel characteristics of layered metal - insulator – metal structures, where the dielectric barrier is formed by a nanoscale disordered oxide film: a universal distribution of the insulator layer transparencies, which does not depend on specific microscopic characteristics of it. Experimental results for superconducting three- and four-layered structures with inhomogeneous tunnel barriers, confirming the existence of a universal distribution of transparencies, are given jointly with a simple theoretical interpretation, based on the equipartition hypothesis of a product of the barrier height on the way which an electron passes within it.