Observation of the Charge-Stripe Crystal Phase in an Insulating Cuprate*

High-temperature superconductivity in cuprates arises from carrier doping an antiferromagnetic Mott (AFM) insulator. This carrier doping leads to the formation of electronic liquid crystal phases. The insulating charge-stripe crystal phase is predicted to form when a small density of holes is doped into the charge-transfer insulator state, but this phase is yet to be experimentally observed. We realized the lightly-doped charge transfer insulating state of the cuprate Bi2Sr2CaCu2O8+x by surface annealing. In this insulating state with a charge transfer gap at the order of ~1 eV, we discovered a unidirectional charge-stripe order with a commensurate 4a0 period along the Cu-O-Cu bond. Notably, this insulating charge stripe crystal phase develops before the onset of the pseudogap and the formation of the Fermi surface. Our work provides fresh insight into the microscopic origin of electronic inhomogeneity in high-Tc cuprates.