Linear normal conductance in copper oxide tunnel junctions

We have measured the tunneling spectra of a large number of YBa2Cu3O7-based junctions that show linear background conductances. When conservation of states between the superconducting and the normal characteristics is observed, we have found a linear dependence of the slope of the G(V) curves on the zero-bias conductance values G(0). Tunneling data on the Bi-Sr-Ca-Cu-O and La-Sr-Cu-O systems seem to confirm this behavior. We infer that linear background conductances reflect the normal density of states of the cuprate superconductors and we speculate that this behavior is due to the interlayer coupling mechanism between the CuO2 planes.