Model for tunneling experiments on the 90- and the 60-K YBa_2Cu_3O_{7-delta} phases

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 CuO2 planes with 2Δ1/kBTc1=4.1 and weak-coupling superconductivity with 2Δ2/kBTc2=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.