TEM and EELS studies of Sr2CuO3+δ (nominal δ = 0.1–0.4): Effect of apical oxygen ordering on TC of cuprate superconductors

In this paper, transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) techniques are utilized to study the as prepared superconducting samples of Sr2CuO3+δ with different nominal δ to reveal the origin of superconductivity. The 60, 68 and 75 K superconductivities observed in these samples are revealed to arise, respectively, from the Pmmm   (a=52ap, b=2ap and c = cp), Cmmm (a = cp, b=32ap and c=42ap) and C2/m   (a=52ap, b = cp, c=262/2ap and β = 101.3°) modulated phases. These superconducting modulated phases are suggested to be formed by the ordering of apical oxygen, and each of them is associated with a distinct type of the ordering. Oxygen K absorption edge measured by EELS indicates that these modulated phases have a similar hole doping level. Our experimental results suggest that the superconductivity differences in the Sr2CuO3+δ system are mainly caused by the reordering of apical oxygen.