Nanoscale coherent intergrowthlike defects in a crystal ofLa1.9Ca1.1Cu2O6+δmade superconducting by high-pressure oxygen annealing

Superconductivity with Tc = 53.5 K has been induced in a large La₁.₉Ca₁.₁Cu₂O₆ (La-2126) single crystal by annealing in a high partial-pressure of oxygen at 1200°C. Using transmission electron microscopy (TEM) techniques, we show that a secondary Ca-doped La₂CuO₄ (La-214) phase, not present in the as-grown crystal, appears as a coherent “intergrowth” as a consequence of the annealing. A corresponding secondary superconducting transition near 13 K is evident in the magnetization measurement. In this study, electron energy loss spectroscopy (EELS) reveals a pre-edge peak at the O K edge in the superconducting La-2126 phase, which is absent in the as-grown crystal, confirming the hole-doping by interstitial oxygen.