超导体Cs_{y} Fe_{2-x} Se_2中铁空位超结构和可能的室温反铁磁有序
Iron vacancy superstructure and possible room temperature antiferromagnetic order in superconducting Cs_{y}Fe_{2-x}Se_2
摘要：
用中子、X射线粉末衍射和单晶同步辐射衍射研究了CsyFe_（2-x）Se_（2-x）的超结构反射，其传播矢量k=[2/5，15，1]，相对于平均晶体结构I_4/mmm（a = 4，c = 15）。传播矢量星对应于由变换A=2a+b，B= -a+2b，C= c给出的5倍大的晶胞。原子结构的解决方案被发现在空间群P42/n和I4/m与铁空位对应的缺铁x = 0.29和Cs化学计量y = 0.83的有序图案。超结构卫星在中子衍射图中更明显，表明它们可能有一些磁性贡献。我们整理了可能的对称性适应的磁位形，发现2μB的Fe有序磁矩的AFM有序性的存在与实验数据并不矛盾。然而，解空间是高度退化的，我们不能选择特定的解。相反，我们提出了可能的磁性配置与Fe磁矩在（ab）平面或沿c轴。超结构在Ts ~ 500 K以上被一级相变破坏。
英文摘要：
Neutron and x-ray powder and single crystal synchrotron diffraction of CsyFe2-xSe2 show the presence of superstructure reflections with propagation vector k=[2/5, 1/5, 1] with respect to the average crystal structure I4/mmm (a = 4, c = 15A). The propagation vector star corresponds to the 5 times bigger unit cell given by transformation A=2a+b, B= -a+2b, C= c. A solution for the atomic structure is found in the space groups P42/n and I4/m with an ordered pattern of iron vacancies corresponding to the iron deficiency x = 0.29 and Cs stoichiometry y = 0.83. The superstructure satellites are more pronounced in the neutron diffraction patterns suggesting that they can have some magnetic contribution. We have sorted out possible symmetry adapted magnetic configurations and found that the presence of AFM ordering with the ordered magnetic moment of Fe with 2μB does not contradict to the experimental data. However, the solutions space is highly degenerate and we cannot choose a specific solution. Instead we propose possible magnetic configurations with the Fe magnetic moments in (ab)-plane or along c-axis. The superstructure is destroyed above Ts ~ 500 K by a first-order-like transition.
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