Superconducting Transitions in Amorphous Molybdenum - Ultrathin Films and Multilayers.

The primary goal of this work was to clarify the role of enhanced Coulomb interactions in the destruction of superconductivity in disordered systems of reduced dimensions. Through a systematic study of the critical temperatures in single film sandwich and multilayer structures, we have examined the role of dimensionality in the reduction of T _{rm c} in disordered superconductors. We have observed a continuous crossover from two to three dimensional behavior as electron diffusion between individual superconducting layers in the multilayer becomes possible. This demonstrates unambiguously that the reduction in T _{rm c} is an artistic 2D effect and is not simply due to interface or proximity effects, as has often been assumed in the past. Multilayers were fabricated by sequential cosputtering of alternate layers of superconducting and nonsuperconducting amorphous Mo-Ge alloys. The effects of screening at short length scales in these films are probed via a systematic variation of both the distance between superconducting layers and the conductivity of the nonsuperconducting layers in a multilayer structure. As the conductivity of the nonsuperconducting layer increases, electron diffusion becomes more three dimensional. However this increased conductivity also introduces a reduction in T_ {rm c} due to the proximity effect. This has been accounted for by comparing the T_ {rm c} of the multilayers with a corresponding N/S/N single layer sandwich structure, designed to have an identical proximity effect reduction of T _{rm c}, in addition to compensating for any effect of the S/N interface itself. X-ray diffraction measurements and cross-sectional TEM micrographs confirm that the layers are structurally well defined, uniform, and continuous. Low temperature resistivity measurements agree with values expected for thick films and show that the electrical properties of single film sandwich and multilayer structures can be compared on an equal basis. The Kosterlitz-Thouless transition due to phase fluctuations was also studied in these ultrathin films and 2D-like multilayers through measurement of the temperature dependence of the ac impedence. These results are compared with existing theory of the dependence of T_ {rm KT} on sheet resistance and the temperature dependence is compared to that expected from BCS theory in the dirty limit.