Nanoscale coherent intergrowthlike defects in a crystal of La1.9 Ca1.1 Cu2 O6+δ made 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 techniques, we show that a secondary Ca-doped La2CuO4 (La-214) phase, not present in the as-grown crystal, appears as a coherent intergrowthlike defect as a consequence of the annealing. A corresponding secondary superconducting transition near 13 K is evident in the magnetization measurement. Electron energy-loss spectroscopy 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.