"Super weakly" coupled superconductivity in ultrathin   superconductor-normal-metal bilayers
  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. 
Zhenyi Long, M. D. Stewart Jr and James M. Valles Jr