Transition temperature and irreversibility line of cobalt-doped single-crystal YBa2Cu3O7- delta : The effect of high-pressure oxygen annealing.

Abstract

We have measured the effect of cobalt on the superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) and the dc irreversibility line (IRL) of single-crystal ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBCO). ${\mathit{T}}_{\mathit{c}}$ of a single crystal annealed in flowing oxygen at atmospheric pressure is markedly lower than that of a polycrystalline sample with the same cobalt concentration and annealing pressure. High-pressure oxygen annealing (HPA) increases ${\mathit{T}}_{\mathit{c}}$ to a value comparable to that of a polycrystalline sample. This increase implies that Co-doped YBCO single crystals low-pressure annealed (LPA) in flowing oxygen are not as fully oxygenated as polycrystalline samples. The IRL was found to have two different functional behaviors, one for low concentrations of cobalt and another for high concentrations. By comparison with the Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems, this change in functional behavior is attributed to a decrease in coupling between Cu-O planes.