Vertical Nb/TiOx/Nb Josephson Junctions Controlled by In-Plane Hot-Electron Injection

Abstract

We fabricate vertical $\mathrm{Nb}/{\mathrm{Ti}\mathrm{O}}_{x}/\mathrm{Nb}$ Josephson junctions where the ${\mathrm{Ti}\mathrm{O}}_{x}$ layer is mostly metallic but has a high-resistance interface with the top $\mathrm{Nb}$ electrode. Thus, the junctions are in essence of the superconductor--insulator--normal-metal--superconductor type. The ${\mathrm{Ti}\mathrm{O}}_{x}$ layer extends beyond the junction area and allows one to send an in-plane current through the normal-metal layer of the junction. We investigate and analyze dc properties (critical current, current-voltage characteristics) as well as the behavior of the junctions in external microwave fields. We find a strong dependence of all properties on the voltage ${V}_{i}$ applied across the ${\mathrm{Ti}\mathrm{O}}_{x}$ lead. The dependence on ${V}_{i}$ can be mapped to the dependence of the junction parameters on the bath temperature $T$, when ${V}_{i}$ is converted to an effective electronic temperature via ${T}_{e}=[{T}^{2}+(a{V}_{i}{)}^{2}{]}^{1/2}$, with $a=3.14\phantom{\rule{0.2em}{0ex}}\mathrm{K}/\mathrm{mV}$. This relation was used before in the context of a metallic nanowire contacted by superconducting electrodes. While the geometry of our ${\mathrm{Ti}\mathrm{O}}_{x}$ layer is far from that of a nanowire, the scaling works perfectly well for all experimental data. Our data also indicate that the electronic temperature is homogeneous throughout the junction area, a result that is unexpected for the large junction sizes we use. The device allows easy and relatively fast in situ manipulation of the junction parameters.