title: Imaging the granular structure of high-Tc superconductivity in underdoped Bi2Sr2CaCu2O8+delta.
abstract: Granular superconductivity occurs when microscopic superconducting grains are separated by non-superconducting regions; Josephson tunnelling between the grains establishes the macroscopic superconducting state. Although crystals of the copper oxide high-transition-temperature (high-Tc) 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. 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 approximately 3 nm in size (and local energy gap <50 meV), located in an electronically distinct background. We used scattering resonances at Ni impurity atoms as 'markers' for local superconductivity; no Ni resonances were detected in any region where the local energy gap Delta > 50 +/- 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.
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA.
authors.forename: K M
authors.initials: KM
authors.lastname: Lang
authors.forename: V
authors.initials: V
authors.lastname: Madhavan
authors.forename: J E
authors.initials: JE
authors.lastname: Hoffman
authors.forename: E W
authors.initials: EW
authors.lastname: Hudson
authors.forename: H
authors.initials: H
authors.lastname: Eisaki
authors.forename: S
authors.initials: S
authors.lastname: Uchida
authors.forename: J C
authors.initials: JC
authors.lastname: Davis