Observations of electronic inhomogeneity in heavily Pb-dopedBi2Sr2CaCu2Oysingle crystals by scanning tunneling microscopy

Abstract

Cleaved $\mathrm{ab}$ surfaces of heavily Pb-doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{y}$ single crystals, including phase separation into Pb-poor $\ensuremath{\alpha}$ and Pb-rich $\ensuremath{\beta}$ phases, were probed by cryogenic scanning tunneling microscopy/spectroscopy at 4.3 K. We could resolve individual Pb atoms substituted into Bi sites, which tend to be concentrated on the crest of modulation in the $\ensuremath{\alpha}$ phase, but are almost randomly dispersed throughout the $\ensuremath{\beta}$ phase. The scanning tunneling spectroscopy results revealed a considerably wide range of gap values $\ensuremath{\Delta},$ originating from the presence of both superconducting and pseudogap regions on a nanometer scale. Furthermore, we found that oxygen annealing substantially increases gap inhomogeneity. This strongly suggests that disorder of excess oxygen, possibly coupled with structural deformation of ${\mathrm{CuO}}_{2}$ sheets, destroys superconducting coherence, resulting in the appearance of a pseudogap.