Nanoscale Heterogeneity in the Electronic Structure of Bi2Sr2CaCu2O8+δ

We study the spatially resolved differential tunneling conductance spectra from as-grown and underdoped samples of the cuprate high-T_c superconductor Bi₂Sr₂CaCu₂O_8+δ. The data are acquired at 4.2K using atomically-resolved scanning tunneling spectroscopy on the BiO cleave surface of these crystals. For optimally doped samples, the tunneling spectra are consistent with superconductivity over ~99% of the surface but display strong heterogeneity in spectral characteristics with a spatial scale of ~3nm. For underdoped samples, the spectra reveal ~3nm diameter domains with electronic characteristics usually associated with superconductivity that are embedded in an apparently distinct electronic background. If these observations represent the doping dependence of bulk nanoscale heterogeneity in the electronic structure, their trend implies that some form of nanoscale electronic phase separation will occur at very low doping in Bi₂Sr₂CaCu₂O_8+δ. Neither the microscopic origin of these phenomena nor the identity of the putative second electronic phase has yet been identified.