Superconducting transitions in amorphous molybdenum-germanium 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, the author has examined the role of dimensionality in the reduction of Tc in disordered superconductors. The author has 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 Tc 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 Tc due to the proximity effect. This has been accounted for by comparing the T, of the multilayers with a corresponding NISIN single layer sandwich structure, designed to have an identical proximity effect reduction of Tc, in addition to compensating for any effect of the SIN interface itself. X-ray diffraction measurements and cross-sectional TEM micrographs confirm that the layers are structurally well defined, uniform, and continuous