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DYNAMICAL NONLINEAR RESPONSES AND PHASE TRANSITION IN COMPLEX SYSTEMS

复杂系统中的动态非线性响应和相变

基本信息

批准号：
04640335
负责人：
HAGIWARA Makoto
金额：
$ 1.09万
依托单位：
KYOTO INSTITUTE OF TECHNOLOGY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1992
资助国家：
日本
起止时间：
1992 至 1993
项目状态：
已结题

项目摘要

Successive ordering processes in some complex systems such as graphite intercalation compounds (GIC) or ceramic superconductors have been studied in order to search a universal nature of the characteristic phase transitions introduced by the mesoscopic structure. For the experimental method, nonlinear response (X_2) analyzes are usefully employed in consideration of fluctuations of the mesoscopic clusters at very low frequencies. To this end, a new measuring system, which detects fundamental and highher harmonic responses in a wide frequency range of mHz-120Hz, was constructed.A divergent singularity of X_2 is observed for CoC1_2-GIC at the upper critical point where the intra-cluster long range order appears. This singularity changes from symmetric into antisymmetric when the excitation amplitude is reduced or the frequency is raised enough. Such a behavior indicates that the intra-cluster ferromagnetic critical phenomenon reflects on X_2 when the response of the cluster moment is restrained by the slow relaxation and some pinning field. From the results, some random energy barrier which causes the cluster moments to get into meta stable state can be suggested.As for a series of ceramic YBa_2Cu_4O_8 prepared under several sintering conditions, two step superconducting orderings are observed. The sharp lower critical points are almost independent of the applied field, whereas their locations distinctly depend on the sintering temperature. Considering the facts together with the mesoscopic grain structure, the critical behavior can be taken as a certain thermodynamic phase transition where inter-grain phase coherence is established cooperatively by weak junctions among the grains. The observed X_2 singularity at the critical point might be, therefore, a critical phenomenon of XY model with some randomness.

人们研究了石墨层间化合物（GIC）或陶瓷超导体等复杂系统中的连续有序过程，以寻找介观结构引入的特征相变的普遍本质。对于实验方法，非线性响应（X_2）分析是有用的考虑在很低的频率下的介观团簇的起伏。在此基础上，建立了一个新的测量系统，可以在mHz-120 Hz的宽频率范围内检测基频和高次谐波响应，并在CoC1_2-GIC的上临界点处观察到X_2的发散奇异性，在上临界点处出现了团簇内的长程有序。当激励幅值减小或频率提高到一定程度时，奇异性由对称变为反对称。这表明当团簇磁矩的响应受到慢弛豫和钉扎场的抑制时，团簇内的铁磁临界现象反映在X_2上。结果表明，在不同的烧结条件下制备的YBa_2Cu_4O_8系列陶瓷样品中，观察到了两步超导有序化。尖锐的下临界点几乎与外加电场无关，而它们的位置明显取决于烧结温度。考虑到这些事实以及介观晶粒结构，临界行为可以被视为某种热力学相变，其中晶粒间的相相干通过晶粒之间的弱结协同建立。因此，在临界点处观察到的X_2奇异性可能是XY模型的一种临界现象，具有一定的随机性。