Oxygen disorder effects in high temperature yttrium-barium-copper-oxygen superconductor.

摘要

A study of the oxygen vacancy disordering effects on the properties of high temperature superconducting {dollar}rm YBasb2Cusb3Osb{lcub}7-delta{rcub}{dollar} system is carried out. The degree of atomic order in the crystals is altered by quenching and annealing the sample at selected temperatures for a certain amount of time. Superconducting transition temperature T{dollar}sb{lcub}rm c{rcub}{dollar} depends on atomic ordering, it decreases after quenching, then increases with low temperature aging. The maximum observed variation of T{dollar}sb{lcub}rm c{rcub}{dollar} (at fixed stoichiometry) is increased to about 27 K. The kinetics of atomic short-range ordering is systematically studied. It is found that the activation energies for oxygen diffusion in the basal plane are oxygen contents dependent. The different kinetics for samples with and without Au impurities is also studied.; In addition to this, a change in the nature of the equilibrium state is observed at temperatures above a concentration dependent threshold temperature T*. We associate T* with the phase boundary between the low temperature phase and the high temperature Ortho I phase. When samples are quenched from temperatures above T* and subsequently annealed at temperatures around 51{dollar}spcirc{dollar}C, two relaxation processes with relaxation time constants differing by about two orders of magnitude are observed. The faster and slower relaxation processes are associated with changes in vacancy short-range disorder and the restoration of long-range chain vacancy order respectively. T* is determined for different oxygen concentrations and compared to theoretical phase diagram calculations.