Two-Dimensional Electrical Conductivity in Quench-Condensed Metal Films

The resistance of ultrathin films condensed on low-temperature substrates has been measured. All of the materials studied (Au, Cu, Al, Sn, and Pb) show a transition from activated to metallic behavior as the resistance is reduced below 30 000 \ensuremath{\Omega}/\ensuremath{\square}. This result gives strong evidence in support of the concept of a minimum two-dimensional conductivity. In Pb films with resistance >30 000 \ensuremath{\Omega}/\ensuremath{\square}, the onset of superconductivity is marked by an increase in activation energy.

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