铁基超导体的微结构和结构相变研究

Microstructure and structure phase transition in iron-based superconductors

铁基超导体表现出丰富的结构和物理性质,在多个典型体系中存在着结构相变和多重有序态之间的关联与竞争.例如,母相化合物LaFeAsO在150K附近发生从四方相(P4/nmm)到正交相(Cmma)的结构相变,同时其电输运性质和磁化率也表现出明显的反常行为.微结构分析发现,在CaFe2As2样品中,存在准周期调制结构,而且在低温区,122体系存在丰富的孪晶畴结构,这种孪晶结构是结构相变的直接结果.另外,在超导材料KxFe2-ySe2(0.7≤x≤0.8,0.2≤y≤0.4)中,Fe空位有序态和结构相分离是理解其结构,也是理解其磁性和输运性质的关键问题,特别是沿[130]晶体带轴方向的5倍超结构与系统的超导电性存在密切关系.

A rich variety of structural and physical properties have been observed in the Fe-based superconductors. Structural phase transitions and the competition of multi-ordered states commonly appear in the typical Fe-based systems. For instance, LaFeAsO undergoes a structural transition at about 150K from tetragonal （P4/nmm） to the orthorhombic （Cmma） phase in association with visible anomalies in its magnetization. Structural studies on CaFe2As2 have revealed the presence of a pseudo-periodic modulation and remarkable twinning domains at low temperatures. This kind of domain results directly from the tetragonal-to-orthorhombic transition. On the other hand, the KxFe2-ySe2 （0.7≤x≤0.8, 0.2≤y≤0.4） materials often contain Fe-vacancies in the FeSe layers; it is believed that the ordered behavior of these vacancies play a critical role in our understanding of the microstructure and physical properties of this layered superconducting system.