Model for tunneling experiments on the 90- and 60-KYBa2Cu3O7−δphases

The tunneling conductances of 90- and 60-K YBCO-based junctions in both the superconducting and the normal state have been quantitatively modeled by a superposition of two BCS-like contributions and a linear one. Strong-coupling superconductivity in the ${\mathrm{CuO}}_2$ planes with 2${\mathrm{\Delta }}_1$/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}1}$=4.1 and weak-coupling superconductivity with 2${\mathrm{\Delta }}_2$/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}2}$=2.1 along the CuO chains are inferred. A consistent explanation of the experiments is found by assuming an internal proximity effect between strongly and weakly superconducting layers.