Visualizing charge ordering in a charge-transfer insulating cuprate

Although numerous symmetry-broken electronic states have been detected in metallic and superconducting cuprates at relative high carrier doping, there still exists little information about the “parent” state of cuprates after the first few charge carriers are introduced. The bottleneck in large part lies in the difficulty of synthesizing and characterizing bulk cuprates in the very lightly doped insulating regime at low temperatures. We demonstrate that by annealing the surface of optimally-doped superconducting Bi₂Sr₂CaCu₂O_8+x (T_c ~ 91 K) in ultra-high vacuum, we can reduce the oxygen dopant density near the surface and extend the hole doping range to achieve the previously inaccessible charge-transfer insulating state at low temperature (Zhao et al., Nat. Mater. 18, 103 (2019)). At this low doping, we discover a unidirectional charge-stripe order with a commensurate 4a₀ period, the same as the charge ordering wave vector previously seen across the pseudogap phase and the superconducting dome. Our work provides strong evidence that charge ordering in cuprates is unlikely to be related to the pseudogap phase.