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arXiv 1804.02012cond-mat.str-el
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单晶Na $_{2.7}$ Ru $_4$ O $_9$的对称性破缺和非常规电荷有序

Symmetry breaking and unconventional charge ordering in single crystal Na$_{2.7}$Ru$_4$O$_9$

Arvind Yogi, C. I. Sathish, Hasung Sim, Matthew J. Coak, Y. Noda, Je-Geun Park

摘要:

物质中电荷、自旋和晶格自由度的相互作用通过相变导致各种形式的有序状态。复杂材料的这些现象的一个重要子类是电荷有序(CO),主要由混合价态驱动。结合电阻率($ρ$)、比热、磁化率(\textit{T})和单晶X射线衍射(SC-XRD)的结果,我们发现具有单斜隧道型晶格(空间群$C$ 2/ $m$)的Na $_{2.7}$ Ru $_4$ O $_9$在室温下表现出非常规的CO,同时保留了金属性。随温度变化的SC-XRD结果显示出连续的相变,在低于$T_{\textrm{C2}}$(365 K)的\textbf{q} $_1$ =(0,$\frac{1}{2}$,0)和\textbf{q} $_2$ =(0,$\frac{1}{3}$,$\frac{1}{3}$)处具有超晶格反射,并且仅在$T_{\textrm{C2}}$和$T_{\textrm{C1}}$(630 K)之间的\textbf{q} $_1$ =(0,$\frac{1}{2}$,0)处具有超晶格反射。我们将这些解释为非常规CO形成的证据。它揭示了在$T_{\textrm{C2}}$(冷却)= 345 K和$T_{\textrm{C2}}$(加热)= 365 K时电阻率的一级相变。我们认为,相变的起源是由于本地化的4 $d$ Ru电子。我们的研究结果揭示了一个独特的例子Ru $^{3+}$ /Ru $^{4+}$混合价重\textit{d} $^4$离子。

英文摘要:

The interplay of charge, spin, and lattice degrees of freedom in matter leads to various forms of ordered states through phase transitions. An important subclass of these phenomena of complex materials is charge ordering (CO), mainly driven by mixed-valence states. We discovered by combining the results of electrical resistivity ($ρ$), specific heat, susceptibility $χ$ (\textit{T}), and single crystal x-ray diffraction (SC-XRD) that Na$_{2.7}$Ru$_4$O$_9$ with the monoclinic tunnel type lattice (space group $C$2/$m$) exhibits an unconventional CO at room temperature while retaining metallicity. The temperature-dependent SC-XRD results show successive phase transitions with super-lattice reflections at \textbf{q}$_1$=(0, $\frac{1}{2}$, 0) and \textbf{q}$_2$=(0, $\frac{1}{3}$, $\frac{1}{3}$) below $T_{\textrm{C2}}$ (365 K) and only at \textbf{q}$_1$=(0, $\frac{1}{2}$, 0) between $T_{\textrm{C2}}$ and $T_{\textrm{C1}}$ (630 K). We interpreted these as an evidence for the formation of an unconventional CO. It reveals a strong first-order phase transition in the electrical resistivity at $T_{\textrm{C2}}$ (cooling) = 345 K and $T_{\textrm{C2}}$ (heating) = 365 K. We argue that the origin of the phase transition is due to the localized 4$d$ Ru-electrons. The results of our finding reveal an unique example of Ru$^{3+}$/Ru$^{4+}$ mixed valance heavy \textit{d}$^4$ ions.

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