Pair weakening and tunnel channels at cuprate interfaces

The very high superconducting transition temperatures ${\mathit{T}}_{\mathit{c}}$ of cuprates are not yet substantiated in applications which seem intimately related to the layered nature of the cuprous oxides. Based on a highly directional p-d hybridization and correlation, the layering causes a correlated quasi-two-dimensional conduction in the ${\mathrm{CuO}}_2$ planes. The basis for the superconductivity seems to be the low carrier density (${\mathit{n}}_{\mathit{s}}$≥${10}^{21}$/${\mathrm{cm}}^3$ eV) close to the metal-insulator transition (MIT) in two dimensions. Thus, any perturbation degrading hybridization and correlation renders the cuprate insulating, which then by disorder contains localized states (${\mathit{n}}_{\mathit{L}}$≤${10}^{21}$/${\mathrm{cm}}^3$). In this layered material the correlated conduction is only weakly hindered by point defects. More prominent perturbations for the conduction are external or internal surfaces with reduced hybridization by a reduced dimensionality and by disorder in energy, distance, or bond angle which occurs intrinsically by relaxation of dangling bonds. This intrinsic, insulating seam with its localized states weakens superconductivity and supports tunneling in various channels. The weakening by on-site Coulomb repulsion close to the MIT smears out and roughens the metal-insulator interface and, e.g., causes reduced and locally varying energy gaps and leakage current ${\mathit{j}}_{\mathit{b}\mathit{l}}$.