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 , 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 Bi 2 Sr 2 CaCu 2 O 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 ± 2.5 meV. These observations suggest that underdoped Bi 2 Sr 2 CaCu 2 O 8+δ is a mixture of two different short-range electronic orders with the long-range characteristics of a granular superconductor.