COMPETITION BETWEEN SUPERCONDUCTIVITY AND LOCALIZATION IN ULTRATHIN AMORPHOUS MOLYBDENUM-GERMANIUM FILMS (TWO-DIMENSIONAL, FLUX-FLOW RESISTANCE).

摘要

The primary result of this work was a systematic examination of the suppression of superconductivity with increasing disorder in two-dimensional (2D) model systems. For this study, single ultrathin films of homogeneous amorphous molybdenum-germanium (Mo-Ge) were synthesized using special vertical multilayer deposition techniques. These films exhibit essentially constant equivalent bulk properties down to film thicknesses of about 10 (ANGSTROM)ngstroms, and therefore represent model 2D systems.; With increasing 2D disorder there is a rapid suppression of the superconducting transition temperature. More mundane origins such as simple pairbreaking effects or "proximity effects" from neighboring layers do not appear to explain this result. The results are qualitatively and quantitatively consistent with the most recent theories of localization effects upon superconductivity, where the disorder leads to anomalous electron diffusion. Among other predictions, this anomalous diffusion leads to an increase in the Coulomb interaction, which inherently competes with superconductivity. Additional differences are found in the measured critical fields, fluctuation conductivity and superconductive tunneling in these films.; It is of added importance to establish the validity of these theories, for initial predictions indicate that disorder has profound implications upon bulk superconductivity. Such effects would be of considerable technological importance.; Additionally, several normal state properties were measured in the same films. As the localization theory for normal state properties is more developed than for the superconducting state, it provides additional checks upon the validity of the localization treatment for our films. The measured temperature dependence of the resistance, magnetoresistance, density of states anomaly, and the rise in film resistivity with increasing sheet resistance appear to be self-consistent and in agreement with theory.; Additional topics include the observation of a new universal behavior for the superconducting fluxflow in thin films. These measurements were made upon the same homogeneous ultrathin Mo-Ge films used for the localization studies above. Also, initial studies were made on the effect of disorder upon superconductivity in the aluminum-silicon system. These films differ from our Mo-Ge films in that they are inhomogeneous and have a much reduced spin-orbit scattering rate.