二价铕化合物EuAl $_4$的电荷密度波序和多重磁跃迁
Charge-Density-Wave Order and Multiple Magnetic Transitions in Divalent Europium Compound EuAl$_4$
摘要：
利用单晶飞行时间中子劳厄技术研究了具有四元结构的二价铕化合物EuAl $_4$的多重跃迁现象。在低于$T_{\rm CDW}$ = 140 K的电荷密度波（CDW）转变温度30.0 K处，超晶格峰出现在由有序矢量$q_{\rm CDW}$ =（0 0 $δ_c$）描述的核布拉格峰附近，其中$δ_c{\sim}$为0.19。相反，在低于$T_{\rm N1}$ = 15.4 K的13.5 K下，磁有序相中的磁峰出现在$δ_2$ = 0.085的$q_2 = (δ_2 δ_2 0)$处。通过进一步冷却到低于$T_{\rm N3}$ = 12.2 K，磁有序矢量变为$q_1 = (δ_1 0 0)$，在11.5 K时$δ_1$ = 0.17，在4.3 K时略微移动到$δ_1$ = 0.194。在$T_{\rm N2}$ =13.2 K和$T_{\rm N4}$ =10.0 K时，磁布拉格峰没有明显变化。在$T_{\rm CDW}$下方与$q_{\rm CDW}$的结构调制的特征在于在（0 0 $l$）轴上不存在超晶格峰。由于在SrAl $_4$中观察到类似的CDW转变，因此$q_{\rm CDW}$的结构调制可以主要归因于Al离子在四面体$ab$ -平面内的位移。复杂的磁性转变形成鲜明对比，一个简单的共线磁结构在等价的EuGa $_4$。这可能源于两种化合物之间的CDW跃迁的不同电子结构。
英文摘要：
Multiple transition phenomena in divalent Eu compound EuAl$_4$ with the tetragonal structure were investigated via the single-crystal time-of-flight neutron Laue technique. At 30.0 K below a charge-density-wave (CDW) transition temperature of $T_{\rm CDW}$ = 140 K, superlattice peaks emerge near nuclear Bragg peaks described by an ordering vector $q_{\rm CDW}$=(0 0 $δ_c$) with $δ_c{\sim}$0.19. In contrast, magnetic peaks appear at $q_2 = (δ_2 δ_2 0)$ with $δ_2$ = 0.085 in a magnetic-ordered phase at 13.5 K below $T_{\rm N1}$ = 15.4 K. By further cooling to below $T_{\rm N3}$ = 12.2 K, the magnetic ordering vector changes into $q_1 = (δ_1 0 0)$ with $δ_1$ = 0.17 at 11.5 K and slightly shifts to $δ_1$ = 0.194 at 4.3 K. No distinct change in the magnetic Bragg peak was detected at $T_{\rm N2}$=13.2 K and $T_{\rm N4}$=10.0 K. The structural modulation below $T_{\rm CDW}$ with $q_{\rm CDW}$ is characterized by the absence of the superlattice peak in the (0 0 $l$) axis. As a similar CDW transition was observed in SrAl$_4$, the structural modulation with $q_{\rm CDW}$ could be mainly ascribed to the displacement of Al ions within the tetragonal $ab$-plane. Complex magnetic transitions are in stark contrast to a simple collinear magnetic structure in isovalent EuGa$_4$. This could stem from different electronic structures with the CDW transition between two compounds.
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