1. Department of Physics, Boston University, Boston, Massachusetts 02215, USA
2. Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA
3. Department of Superconductivity, University of Tokyo, Yayoi, 2-11-16 Bunkyo-ku, Tokyo 113-8656, Japan
4. Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506-0055, USA
5. Department of Physics, University of California, Berkeley, California 94720, USA
6. Present addresses: Department of Applied Physics, Stanford University, Stanford, Califronia 94305, USA (H.E.); and National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA (E.W.H.).

Correspondence to: S. H. Pan¹ Correspondence and requests for materials should be addressed to S.H.P. (e-mail: Email: shpan@bu.edu).

Abstract

The parent compounds of the copper oxide high-transition-temperature (high-T_c) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are 'doped' away from stoichiometry¹. For the compound Bi₂Sr₂CaCu₂O_8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers ('holes') into the CuO₂ planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-T_c superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi₂Sr₂CaCu₂O_8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of 14 Å. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.