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

Granular superconductivity occurs when microscopic superconducting grains are separated by non-superconducting regions; Josephson tunnelling between the grains establishes the macroscopic superconducting state(1). 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(2-5). Granular superconductivity arising from tunnelling between such domains would represent a new view of the under-doped 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 similar to3nm in size (and local energy gap <50 meV), located in an electronically distinct background. We used scattering resonances at Ni impurity atoms(6) as 'markers' for local superconductivity(7-9); no Ni resonances were detected in any region where the local energy gap Δ>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.