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+delta with different nominal 6 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 = 5 root 2a(p), b = root 2a(p) and c = c(p)), Cmmm (a = c(p), b = 3 root 2a(p) and c = 4 root 2a(p)) and C2/m (a = 5 root 2a(p), b = c(p), c = root 26 root 2/2a(p) and beta = 101.3 degrees) 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+delta system are mainly caused by the reordering of apical oxygen. (c) 2007 Elsevier B.V. All rights reserved.