Electron tunneling in the high-Tcbismuthate superconductors

We have performed extensive tunneling measurements on some high-${\mathit{T}}_{\mathit{c}}$ superconducting oxides of bismuth: ${\mathrm{BaPb}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Bi}}_{\mathit{x}}$ ${\mathrm{O}}_3$, ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Rb}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$, and ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$. The results on high-quality single crystals and epitaxial films indicate that these materials are weak-coupled superconductors with 2Δ/${\mathit{kT}}_{\mathit{c}}$=3.5±0.3. The observed quasiparticle excitation spectrum agrees with the BCS model. We also do not observe structure indicating electron-phonon coupling sufficiently strong to account for the ${\mathit{T}}_{\mathit{c}}$ of these materials.