Microscopic electronic inhomogeneity in the high-T-c superconductor Bi2Sr2CaCu2O8+x

The parent compounds of the copper oxide high-transition-temperature (high- T-c) superconductors are unusual insulators (so-called Mott insulators). Su perconductivity arises when they are 'doped' away from stoichiometry(1). Fo r the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen a toms, which introduce positive charge carriers ('holes') into the CuO2 plan es where the superconductivity is believed to originate. Aside from providi ng the charge carriers, the role of the oxygen dopants is not well understo od, nor is it clear how the charge carriers are distributed on the planes. Many models of high-T-c superconductivity accordingly assume that the intro duced carriers are distributed uniformly, leading to an electronically homo geneous system as in ordinary metals. Here we report the presence of an ele ctronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations usin g scanning tunnelling microscopy and spectroscopy. The inhomogeneity is man ifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatial ly and vary on the surprisingly short length scale of similar to 14 Angstro m. Our analysis suggests that this inhomogeneity is a consequence of proxim ity to a Mott insulator resulting in poor screening of the charge potential s associated with the oxygen ions left in the BiO plane after doping, and i s indicative of the local nature of the superconducting state.