"Super weakly" coupled superconductivity in ultrathin superconductor-normal-metal bilayers

TL;DR

This study investigates ultrathin Pb/Ag bilayers, revealing that their superconducting gap follows BCS behavior despite anomalous DOS, with a systematic decrease in gap ratio indicating super weak coupling due to DOS depletion.

Contribution

It demonstrates that ultrathin superconductor-normal-metal bilayers exhibit BCS-like superconductivity despite anomalous DOS, introducing a semi-quantitative model for the reduced gap ratios.

Findings

Superconducting gap follows BCS predictions in ultrathin bilayers.

Gap ratio decreases by over 20% below BCS value in low-$T_c$ samples.

Anomalous DOS at Fermi energy correlates with reduced pairing available.

Abstract

Tunneling measurements of the temperature dependence of the electronic density of states (DOS) of ultrathin bilayers of Pb and Ag reveal that their superconducting energy gap, $\Delta (T)$, evolves similarly to BCS predictions despite the presence of a large anomalous DOS at the Fermi energy that persists as $T\to 0$. The gap ratio, ${2\Delta (0)}\over {k_BT_c}$, systematically decreases more than 20% below the BCS universal value of 3.52 in bilayers with the lowest superconducting transition temperatures, $T_c$; a behavior we deem super weakly coupled. A semi-quantitative model suggests that the reduced gap ratios result from a systematic depletion of the DOS available for pairing that occurs with the growth of the anomalous DOS at the Fermi energy.