Observation of a subgap density of states in superconductor-normal metal bilayers in the Cooper limit

Abstract

We present transport and tunneling measurements of Pb-Ag bilayers with thicknesses, $d_{Pb}$ and $d_{Ag}$, that are much less than the superconducting coherence length. The transition temperature, $T_c$, and energy gap, $\Delta$, in the tunneling Density of States (DOS) decrease exponentially with $d_{Ag}$ at fixed $d_{Pb}$. Simultaneously, a DOS that increases linearly from the Fermi energy grows and fills nearly 40% of the gap as $T_c$ is 1/10 of $T_c$ of bulk Pb. This behavior suggests that a growing fraction of quasiparticles decouple from the superconductor as $T_c$ goes to 0. The linear dependence is consistent with the quasiparticles becoming trapped on integrable trajectories in the metal layer.

5 pages and 4 figures. This version is just the same as the old version except that we try to cut the unnecessary white space in the figures and make the whole paper look more compact

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• Tuning vortex fluctuations and the resistive transition in superconducting films with a thin overlayer
• "Super weakly" coupled superconductivity in ultrathin superconductor-normal-metal bilayers
• Early stage morphology of quench condensed Ag, Pb and Pb/Ag hybrid films