Origin of the Tc depression and the role of charge transfer and dimensionality in ultrathin YBa2Cu3O7-δ

We describe measurements on trilayers and multilayers of YBa2Cu3O7-δ (YBCO) with intervening layers of Y1-xPrxBa2Cu3O7-δ, with thicknesses d of YBCO down to that of a single unit cell. The zero-resistance transition temperature Tco decreases as d decreases, and increases as x decreases, and we show that a single unit-cell layer is still superconducting. We describe a transition from a ''bulk regime'' in the interior of the YBCO to a ''surface regime'' near the interfaces, and show that the Tco depression is correlated with a depressed conductance in the region adjacent to the interfaces. The results indicate that the changes are related to the presence of the interfaces, primarily to charge transfer between the layers, with only minor indications of a change in the intrinsic properties of the YBCO from the bulk down to the thickness of a single unit cell. The values of Tco in multilayers are consistent with those on trilayers, and so rule out any contributions from very long-range interactions, but they indicate the presence of interactions with length scales of several hundred angstroms across the barrier layers