The Onset of Superconductivity in Ultrathin Films of Metals Grown on a Layer of Amorphous Germanium.
Electrical transport measurements have been performed on ultrathin films of metals which have been quench-evaporated onto both bare glazed alumina substrates and glazed alumina substrates overcoated with a thin layer of amorphous germanium. Both systems exhibit the onset of superconductivity with increasing thickness of the metal. The films grown on bare substrates exhibit electrical transport characteristics in early stages of film growth which strongly suggest that they consist of separated clusters and, accordingly, have been termed "granular". The films grown on germanium, on the other hand, become electrically connected at much lower material coverages and do not exhibit the features associated with granularity in their electrical transport characteristics. These observations have led to the inference that these films grow in a uniform rather than granular fashion, with disorder limited to microscopic length scales. Such films have been termed "homogeneous". The inferred morphologies of these two types of films have played a significant role in the interpretation of data and theoretical modeling of the insulator-superconductor transition in each type of system. As films of both types must be kept at liquid helium temperatures and in an ultrahigh vacuum environment, direct studies of film morphology would be extremely difficult and none has yet been reported. In a recent experiment, the evaporation of a germanium overlayer onto a granular metal film near the threshold for superconductivity produced a dramatic and unexpected change in electrical transport properties of the film. This suggests that, contrary to previous assumptions, the germanium underlayer in the "homogeneous" films described above may have actually played an active role in electrical transport. Based on these findings, new morphological models for the films grown on germanium are proposed in this work. The nature of the onset of superconductivity in such models is then deduced from basic principles, resulting in possible new insights into the nature of the insulator-superconductor transition in two-dimensional granular and homogenous systems. A design for the next-generation of apparatus for fabrication and study of ultrathin films and suggestions for further investigations are also presented.
- Pub Date:
- Physics: Condensed Matter; Engineering: Materials Science