Letters to Nature

Nature 415, 412-416 (24 January 2002) | doi:10.1038/415412a; Received 28 August 2001; Accepted 4 December 2001

Imaging the granular structure of high-Tc superconductivity in underdoped Bi2Sr2CaCu2O8+delta

K. M. Lang1, V. Madhavan1, J. E. Hoffman1, E. W. Hudson1,2,3, H. Eisaki4,5, S. Uchida4 & J. C. Davis1,3

  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. Davis1,3 Correspondence and requests for materials should be addressed to J.C.D. (e-mail: Email: jcdavis@socrates.berkeley.edu).

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 domains2, 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+delta that reveal an apparent segregation of the electronic structure into superconducting domains that are approx3 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 superconductivity7, 8, 9; no Ni resonances were detected in any region where the local energy gap Delta > 50 plusminus 2.5 meV. These observations suggest that underdoped Bi2Sr2CaCu2O8+delta is a mixture of two different short-range electronic orders with the long-range characteristics of a granular superconductor.