Tunneling density of states in the lead-bismuth-oxide superconductors

We have performed an extensive set of tunneling measurements on the high-${\mathit{T}}_{\mathit{c}}$ perovskites ${\mathrm{BaPbO}}_3$,${\mathrm{BaPb}}_{0.75}$ ${\mathrm{Sb}}_{0.25}$ ${\mathrm{O}}_3$, ${\mathrm{BaPb}}_{0.75}$ ${\mathrm{Bi}}_{0.25}$ ${\mathrm{O}}_3$, and ${\mathrm{Ba}}_{0.7}$ ${\mathrm{K}}_{0.3}$ ${\mathrm{BiO}}_3$. In these materials, the superconducting transition temperature varies from less than 1 to 30 K. Tunneling data show a linear background conductance similar to that observed in the cuprates and we have strong experimental evidence that this linear conductance results from intrinsic properties of the density of states in these materials. We also observe a remarkable correlation between ${\mathit{T}}_{\mathit{c}}$ and the slope of this linear background conductance.