Superconducting proximity effect through high-quality high-conductance tunnel barriers

The temperature dependence of the critical current of very high-conductance $\mathrm{Nb}-\mathrm{Al}{\mathrm{O}}_x-\mathrm{Al}-\mathrm{Al}{\mathrm{O}}_x-\mathrm{Nb}$ tunnel junctions with thin middle Al layers (thickness $d_{\mathrm{Al}}$ 2-8 nm) shows a tail above the Al critical temperature, whose amplitude strongly depends on $d_{\mathrm{Al}}$. This is interpreted as evidence of a proximity effect through tunnel barriers whose high quality and transparency can be independently assessed. These data have been modeled within the framework of the McMillan model using independently measurable barrier transmission factors, and obtaining an excellent quantitative agreement.