超导KxFe2-ySe2（0.7 \leg x \leg 0.8，0.2 \leg y \leg 0.3）中Fe空位有序的崩塌和连续相变
Collapse of the Fe-vacancy order and successive phase transitions in superconducting KxFe2-ySe2 (0.7 \leg x \leg 0.8, 0.2 \leg y \leg 0.3)
摘要：
铁基超导KxFe 2-ySe 2（0.6 { \leg } x { \leg } 1，0.2{ \leg } y { \leg } 0.4）化合物不同于人们熟知的RFe 2 As 2（R = Ba，Sr，Ca）超导体，具有复杂的结构特征和显著的物理性质，如铁空位有序、多超导转变和相分离[1]。最近的实验研究还表明，铁空位有序和反铁磁结构之间存在显着的相互作用[2]。在这里，我们证明了良好表征的超导KxFe 2-ySe 2（0.6 { \leg } x { \leg } 0.8，0.2 { \leg } x { \leg } 0.3）样品包含复杂的微观结构特征，并在低温下经历连续的相变。对几种KxFe_（2-y）Se_2超导体的TEM原位观察表明，在超导转变之上，Fe空位有序发生了显著的崩塌，结果表明，超导相实际上是一种没有Fe空位有序的四方晶系结构。此外，我们在低温下的分析表明，超导体可能采用缺铁的结构与K0.75Fe 2-ySe 2的组成。这些结果不仅对进一步优化现有体系中的超导相结构，而且对理解铁基超导电性的机理具有重要意义。
英文摘要：
The Fe-based superconducting KxFe2-ySe2 (0.6 {\leg} x {\leg} 1, 0.2{\leg} y {\leg} 0.4) compounds, unlike the well-known RFe2As2 (R = Ba, Sr, Ca) superconductors, contain complex structural features and notable physical properties, such as the Fe-vacancy ordering, multi-superconducting transitions and the phase separation[1]. Recent experimental studies also suggested the presence of remarkable interplay among the Fe-vacancy order and the antiferromagnetic structure[2]. Here we demonstrate that the well-characterized superconducting KxFe2-ySe2 (0.6 {\leg} x {\leg} 0.8, 0.2 {\leg} x {\leg} 0.3) samples contain complex microstructure features and undergo successive phase transitions at low temperatures. In-situ TEM observations on a number of the KxFe2-ySe2 superconductors demonstrated the presence of a remarkable collapse of the Fe-vacancy order above the superconducting transition, as a result, the superconducting phase actually adopts a tetragonal structure without the Fe-vacancy ordering. Moreover, our analysis at the low temperatures suggests that the superconductors likely adopt a Fe-deficient structure with composition of K0.75Fe2-ySe2. These results are important not only for the further optimization of superconducting phase in present system but also for understanding the mechanism of the Fe-based superconductivity.
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