Tuning vortex fluctuations and the resistive transition in superconducting films with a thin overlayer

It is shown that the temperature of the resistive transition T_r of a superconducting film can be increased by a thin superconducting or normal overlayer due to an "anti-proximity effect" which manifests itself in an initial increase of T_r(d₂) with the overlayer thickness d₂ followed by a decrease of T_r(d₂) at larger d₂. The nonmonotonic dependence of T_r(d₂) resulting from the interplay of vortex fluctuations and the conventional proximity effect, was obtained by solving the Usadel equations to calculate the BKT transition temperature and the temperature of the resistive transition due to thermally-activated hopping of vortices. The model may explain the nonmonotonic dependence of T_r(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. The transition temperature can be optimized by tuning the overlayer parameters, which can significantly weaken vortex fluctuations and nearly restore the mean-field T_c.These results suggest that bilayers can be used as model systems for systematic investigations of optimization of fluctuations in superconductors.