Enhancement of superconductivity in thin aluminium films

Abstract

The results of superconductive tunneling measurements are presented, on very thin (35–80 Å) aluminium films which exhibit enhanced transition temperatures Tc up to 2.16 °K, and also exhibit a unique energy gap. Stability of the enhanced Tc films is attained by investigating the films in a vacuum; however, upon breaking vacuum, oxide growth occurs, and the tunneling barrier becomes impenetrable. The temperature dependence of the energy gap is BCS-like, and the BCS gap parameter 2Δ(O)/kBTc lies within experimental error of the usual BCS value of 3.528 for a weakly coupled superconductor. Existing theories of the enhancement of Tc are discussed in the light of present experimental evidence. However, it is found that none are capable of explaining reasonably the experimental data accumulated to date. It appears that all thin film systems reported to date as having shown an enhancement of Tc are basically the same. Electron microscope photomicrographs show that, structurally, the films are composed of crystallites having a wide range of sizes. Any new theory will have to contain a volume density of such crystallites as a key parameter in determining the enhanced Tc of the film.