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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 Pb{sub {ital x}}Bi{sub 2{minus}{ital x}}Sr{sub 2}CaCu{sub 2}O{sub 8} superconductors. High-resolution spectroscopic measurements show that within {plus minus}0.5 eV of the Fermi level the Bi(Pb)-O electronic structure is independent of the Pb concentration ({ital x}{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 {minus}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.