Superfluid anisotropy in YBCO: Evidence for pair tunneling superconductivity

Proximity effect and pair tunneling models are applied as alternative scenarios to explain the recently measured $ab$-plane and $c$-axis components of the superfluid tensor in Copper-Oxide superconductors which contain chains, such as $YBa_2Cu_3O_{6.95}$. It is argued that conventional proximity effect models, which couple chains and planes via single electron tunneling only, are incompatible with the experimental observations. On the other hand several surprising features of the experimental data are readily explained by the presence of a microscopic pair tunneling process.