Electronic effect of lead substitution in single-crystal Bi(Pb)-Sr-Ca-Cu-O superconductors determined by scanning tunneling microscopy

Scanning tunneling microscopy has been used to characterize directly the surface electronic structure of a series of single-crystal ${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{Bi}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ superconductors. High-resolution spectroscopic measurements show that within \ifmmode\pm\else\textpm\fi{}0.5 eV of the Fermi level the Bi(Pb)-O electronic structure is independent of the Pb concentration (x\ensuremath{\le}0.7). Extended spectroscopic measurements demonstrate that the density of unfilled states at +1 eV systematically decrease as the concentration of Pb increases, although the filled states between 0 and -2 eV are essentially independent of the concentration of Pb. In addition, analysis of the differential conductivity indicates that the surface conductivity varies from weakly metallic to semiconducting as the tip-surface separation increases.