Tuning Vortex Fluctuations and the Resistive Transition in Superconducting Films With a Thin Overlayer


It is shown that the temperature of the resistive transition Tr of a superconducting film can be increased by a thin superconducting or normal overlayer. For instance, deposition of a highly conductive thin overlayer onto a dirty superconducting film can give rise to an “antiproximity effect,” which manifests itself in an initial increase of Tr (d2) with the overlayer thickness d2 followed by a decrease of Tr (d2)at larger d2. Such a nonmonotonic thickness dependence of Tr (d2) results from the interplay of the increase of a net superfluid density mitigating phase fluctuations and the suppression of the critical temperature Tc due to the conventional proximity effect. This behavior of Tr (d2) is obtained by solving the Usadel equations to calculate the temperature of the Berezinskii-Kosterlitz-Thouless transition, and the temperature of the resistive transition due to thermally activated hopping of single vortices in dirty bilayers. The theory incorporates relevant material parameters such as thicknesses and conductivities of the layers, the interface contact resistance between them, and the subgap quasiparticle states, which affect both phase fluctuations and the proximity effect suppression of Tc. The transition temperature Tr can be optimized by tuning the overlayer parameters, which can significantly weaken vortex fluctuations and nearly restore the mean-field critical temperature. The calculated behavior of Tr (d2) may explain the nonmonotonic dependence of Tr (d2) observed on (Ag,Au,Mg,Zn)-coated Bi films, Ag-coated Ga and Pb films, or NbN and NbTiN films on AlN buffer layers. These results suggest that bilayers can be used as model systems for systematic investigations of optimization of fluctuations in superconductors

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