Saturation of superconductivity in cuprates overdoped with high-pressure oxygen: Phase diagram with YBa ₂ Cu ₃ O _7+δ , δ → 1

High temperature superconductivity (HTSC) typically occurs as a "dome" over a narrow range of doping in its phase diagram. The reaction of YBa₂Cu₃O₇ with Ag₂O₂ at ≥800 °C and 6 GPa inserts oxygen atoms between the Cu1 sites to form tetragonal YBa₂Cu₃O₈ without significant changes to its overall structure or interatomic distances. The superconductivity in YBa₂Cu₃O₈ is essentially unaffected, with the reduction of the transition temperature by ≤2 K and its superconducting fraction by ≤ 15% between the O7 and O8 endpoints of the oxygen stoichiometry and associated carrier density. The dome therefore only pertains to compounds doped by cation substitution or presumably interstitial oxygen. Band structure calculations of the fully ordered endpoints show substantial changes in the density of states at the Fermi level because of its shift to lower energy with increasing oxygen stoichiometry. The coincidence of optimum and saturation HTSC at a carrier:CuO₂ ratio of ≈1/6 implies a direct coupling of HTSC with the lattice such that this value is intrinsic to the superconducting phase. The SC in YBa₂Cu₃O₈ is therefore not only pinned at its optimum values but also separated from the other electronic states so that the additional holes have no measurable effect on the condensate. In addition, the 95 K transition temperature of Sr₂CuO_3.4 that possesses CuO_1.5 ladders instead of CuO₂ planes reveals a second distinct behavior of cuprates overdoped with high-pressure oxygen, demonstrating significant gaps in our understanding of HTSC.