Enhanced superconductivity in Nd1+xBa2-xCu3O7+δby low oxygen partial pressure annealing

Abstract

Previous work has shown that unlike YBa2 Cu3 O7-δ (Y123), the Nd-Ba-Cu-O system exhibits a solid solution Nd1+xBa2-x Cu3O7+δ (Nd123ss) for 0.04≤ × ≤0.6.1–3) An earlier paper showed that although the superconducting properties decrease nonlinearly for increasing x, Tc can be varied by increasing the annealing temperature without changing the low temperature oxygen soak.2 The changes in microstructure and Tc with increasing x are analogous with Y123 with increasing δ except that the total hole concentration remained constant. Tc was modeled in terms of oxygen disorder resulting from Nd3+ atoms on the Ba sites relocating chain oxygens to anti-chain sites. The variability in Tc as a function of x and processing conditions can be explained by the number of fourfold coordinated coppers on the chain sites. In this paper, the model has been further substantiated by processing in 1% O2. The annealing in a reduced oxygen partial pressure followed by a 450°C oxygen soak resulted in a marked increase in Tc compared to the 100% PO2 anneal. The low PO2 anneal favors pairing of Nd3+ substituting for Ba2+ to conserve oxygen ions, resulting in fewer disrupted fourfold-coordinated coppers thus increasing charge transfer from the planes to the chains.