Ordered structure and its fluctuation of superconductive ceramics as frustrated systems

这项研究是为了阐明弱烧结陶瓷的晶间超导有序化所引起的磁临界行为。在弱烧结陶瓷中，晶间π结和o结共存，从而在颗粒网络中发生阻碍。在实验研究中，化学计量比超导氧化物材料必须在低温条件下烧结。为此，首先考察了柠檬酸盐热分解法合成YBa2Cu4O8(124)的工艺条件，并研制了焙烧反应管装置。采用不同烧结时间的合成工艺，成功地制备了一系列在不同临界温度下呈现晶间有序化的纯124陶瓷。扫描电子显微镜观察到，样品的晶粒度基本相同，约为300~500微米，烧成结构略有形态变化，基波和谐磁响应的温度依赖关系随烧成条件呈现系统的变化。由非线性响应的发散峰点估计的晶间有序化温度随烧结时间的变化一致，表明耦合强度可由烧结时间控制。此外，还发现了三次谐波响应的对偶异常。结果表明，晶间有序首先发生在磁区内，但当整个系统形成有序结构时，必须重建晶间有序。在此基础上，发现了烧结时间最长为200h的样品的非线性极化率的奇异性发散及其临界指数约为-2.7。

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.