Imaging the granular structure of high-T_c superconductivity in underdoped Bi₂Sr₂CaCu₂O_8+δ

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^2-5. 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 Bi₂Sr₂CaCu₂O_8+δ 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^7-9; no Ni resonances were detected in any region where the local energy gap Δ 〉 50± 2.5 meV. These observations suggest that underdoped Bi₂Sr₂CaCu₂O_8+δ is a mixture of two different short-range electronic orders with the long-range characteristics of a granular superconductor.