Competition between superconductivity and localization in two-dimensional ultrathin a-MoGe films

Abstract

The competition between superconductivity and localization in two dimensions has been systematically studied in ultrathin homogeneous superconducting films of amorphous MoGe. These films were model 2D systems, with essetially constant equivalent bulk properties down to film thicknesses of ∼10 Å. A strong and systematic reduction in the superconducting transition temperature T c with increasing 2D disorder (i.e., sheet resistance R □) has been observed. Experimental evidence argues aginst “proximity effects” as the primary cause of this behavior.Furthermore,results of superconductive tunneling indicate that this reduction does not arise from simple pairbreaking effects alone. Superconductivity appears particularly sensitive to disorder in 2D, far more so than the normal state properties. For example, by sheet resistances of R p[≅2000Ω , T c has been suppressed by 97% from the bulk value, whereas the normal state resistivity has only increased 40%. The results are found to be consistent with recent formulations of the theory, which not only take into account the effects of localization and the related enhancement in the Coulomb interaction, but also the dynamical nature of the screening.