Superconductivity and phase separation of La2CuO4+δ single crystals

Lanthanum cuprate( La2CuO4) is an antiferromagnetic insulator, which becomes superconducting after introducing interstitial oxygen by high-pressure, chemical or electrochemical oxidation We report here the flux growth of superconducting single crystal in an ambient atmosphere, which the morphology is remarkably different from that of insulating crystal. The behavior of superconductivity is studied by AC-susceptibility measurement. A phase separation makes a strong effect on the superconductivity. The phase separation mainly occurs at the temperature range of 270 to 190 K and the separation is reversible in the solid state. Superconducting transition temperature (Tc) depends on the temperature, the time and the cooling process. Te reaches an equilibrium value after several hour treatment at constant temperature, which indicates the completion of the phase separation It increases to a higher value level only by 30 minute cooling from 260 to 230 K, which suggests the stripe nucleation and growth. A mechanism of the stripe formation is discussed based on the staging phase diagram of La2CuO4+delta. Parallel intergrowth of the oxygen-rich phase and the oxygen-poor phase takes place alternatively in the miscibility gap, in result of self-assembling a composite structure in the duplex single crystal.