Observations of electronic inhomogeneity in heavily Pb-doped Bi2Sr2CaCu2Oy single 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 $\alpha$ and Pb-rich $\beta$ 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 $\alpha$ phase, but are almost randomly dispersed throughout the $\beta$ phase. The scanning tunneling spectroscopy results revealed a considerably wide range of gap values $\Delta ,$ 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.