1. Department of Physics, University of California, Berkeley, California 94720, USA
2. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4301, USA
3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4. Department of Superconductivity, University of Tokyo, Yayoi, 2-11-16 Bunkyoku, Tokyo 113-8656, Japan
5. Department of Applied Physics, Stanford University, Stanford, California 94205-4060, USA

Correspondence to: J. C. Davis^1,3 Correspondence and requests for materials should be addressed to J.C.D. (e-mail: Email: jcdavis@socrates.berkeley.edu).

Abstract

Granular superconductivity occurs when microscopic superconducting grains are separated by non-superconducting regions; Josephson tunnelling between the grains establishes the macroscopic superconducting state¹. Although crystals of the copper oxide high-transition-temperature (high-T_c) superconductors are not granular in a structural sense, theory suggests that at low levels of hole doping the holes can become concentrated at certain locations resulting in hole-rich superconducting domains^{2, 3, 4, 5}. Granular superconductivity arising from tunnelling between such domains would represent a new view of the underdoped copper oxide superconductors. Here we report scanning tunnelling microscope studies of underdoped Bi₂Sr₂CaCu₂O₈₊ that reveal an apparent segregation of the electronic structure into superconducting domains that are 3 nm in size (and local energy gap <50 meV), located in an electronically distinct background. We used scattering resonances at Ni impurity atoms⁶ as 'markers' for local superconductivity^{7, 8, 9}; no Ni resonances were detected in any region where the local energy gap > 50 2.5 meV. These observations suggest that underdoped Bi₂Sr₂CaCu₂O₈₊ is a mixture of two different short-range electronic orders with the long-range characteristics of a granular superconductor.