Ordered structure and its fluctuation of superconductive ceramics as frustrated systems

点击翻译按钮获取中文摘要

This research was executed in order to clarify magnetic critical behavior caused by intergrain superconductive ordering of weakly sintered ceramics, in which intergrain π-junction and o-junction coexist and thus frustrations occur in the grain network. For the experimental study, stoichiometric superconductive oxide material must be sintered at low temperature condition. At first stage of the work, therefore, syuthetic condition of YBa_2Cu_4O_8 (124) by citrate pyrolysis method was examined, and then a reaction tube device for the calcination was developed. Using the synthetic technique with valying sintering time, we successfully prepared a series of pure 124 ceramics which show intergrain ordering at each different critical temperatures.For the obtained ceramic series, nearly the same grain size about 300-500 um and slight morphological change in the sintered structure were observed by SEM experiments, Temperature dependencies of fundamental and harmonic niagnetic responses show systematic change by the sintering condition. The intergrain ordering temperature estimated by divergent peak point of the nonlinear responses changes consistently with the sintering time, indicating that the coupling strength can be controlled by the duration of sintering. Furthermore, dual anomalies of the third harmonic response are found. The results suggest that intergrain ordering occurs first within domain regions but must be re-constructed when an ordered structure over the whole system is formed. On the basis of this understanding, singular divergent of nonlinear susceptibility and its critical exponent of about -2.7 are found for the specimen with the longest sintering time 200 h.