Evolution of a Novel Ribbon Phase in Optimally Doped Bi₂Sr₂CaCu₂O_8+δ at High Pressure and Its Implication to High- T_C...

One challenge in studying high-temperature superconductivity (HTSC) stems from a lack of direct experimental evidence linking lattice inhomogeneity and superconductivity. Here, we apply synchrotron hard X-ray nanoimaging and small-angle scattering to reveal a novel micron-scaled ribbon phase in optimally doped Bi₂Sr₂CaCu₂O_8+δ (Bi-2212, with δ = 0.1). The morphology of the ribbon-like phase evolves simultaneously with the dome-shaped T_C behavior under pressure. X-ray absorption studies show that the increasing of T_C is associated with oxygen-hole redistribution in the CuO₂ plan, while T_C starts to decrease with pressure when oxygen holes become immobile. Additional X-ray irradiation experiments reveal that nanoscaled short-range ordering of oxygen vacancies could further lower T_C, which indicates that the optimal T_C is affected not only by an optimal morphology of the ribbon phase, but also an optimal distribution of oxygen vacancies. Our studies thereby provide for the first time compelling experimental evidence correlating the T_C with micron to nanoscale inhomogeneity.