0 Fluctuation Effects in High Sheet Resistance Superconducting Films ∗

As the normal state sheet resistance, Rn, of a thin film superconductor increases, its superconducting properties degrade. For Rn ≃ h/4e superconductivity disappears and a transition to a nonsuperconducting state occurs. We present electron tunneling and transport measurements on ultrathin, homogeneously disordered superconducting films in the vicinity of this transition. The data provide strong evidence that fluctuations in the amplitude of the superconducting order parameter dominate the tunneling density of states and the resistive transitions in this regime. We briefly discuss possible sources of these amplitude fluctuation effects. We also describe how the data suggest a novel picture of the superconductor to nonsuperconductor transition in homogeneous 2D systems. Introduction It has been known for some time that increasing the normal state sheet resistance, Rn, of a homogeneously disordered (as opposed to islanded) superconducting film reduces its mean field transition temperature, Tco and superconducting energy gap, ∆[1, 2]. For Rn comparable to h/4e , Tco [3] and ∆[4] approach zero and a superconductor to non-superconductor transition To appear in the Russian review journal Uspekhi Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan