三斜双钙钛矿Ca$_2$CuWO$_6$的准二维磁结构
Quasi two dimensional magnetic structure of the triclinic double perovskite Ca$_2$CuWO$_6$
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中文总结 AI 辅助
本研究通过中子衍射解析三斜双钙钛矿Ca₂CuWO₆的反铁磁基态，引入通用超胞对比其与四方Sr₂CuWO₆的磁结构，阐明对称性破缺对磁畴选择的调控机制。
中文摘要 AI 辅助
我们通过中子粉末衍射解析了三斜双钙钛矿Ca$_2$CuWO$_6$的反铁磁基态，并将其与四方晶系Sr$_2$CuWO$_6$进行直接对比分析。在奈尔温度$T_\text{N} \backsimeq 32$ K以下，Ca$_2$CuWO$_6$的磁布拉格反射可通过其原生$P\ar{1}$晶胞中的公度传播矢量$\boldsymbol{k}=(\ rac{1}{2},\ rac{1}{2},0)$指标化。Rietveld精修结果表明其为共线磁结构：晶体学不等价的Cu1和Cu2格点上的磁矩大小相等、方向相反，1.5 K下每个Cu原子的有序磁矩为$0.66(3)~\u03bc_{\text{B}}$。由于晶体学设置不同、协同Jahn-Teller伸长轴取向有别，直接对比Ca$_2$CuWO$_6$与Sr$_2$CuWO$_6$存在困难。因此我们引入了一个通用的晶体学超胞，用于探究双钙钛矿中从四方到三斜的对称性降低效应：看似不同的磁传播矢量可映射到同一个超胞波矢，揭示出由钨介导的次近邻相互作用稳定的共同磁结构。平均场计算进一步表明，四方对称性维持了Sr$_2$CuWO$_6$中四个磁$\boldsymbol{k}$畴的简并性，而Ca$_2$CuWO$_6$中与对称性相关的交换路径发生三斜劈裂（在Cu2网络中劈裂效应最强），从而选择出单一的$\boldsymbol{k}$畴。这些结果确定了Ca$_2$CuWO$_6$的磁基态，并揭示了对称性破缺如何在不改变Sr基类似物底层磁基元的前提下，选择出特定的有序态。
英文摘要
We present the antiferromagnetic ground state of the triclinic double perovskite Ca$_2$CuWO$_6$solved by neutron powder diffraction, and analyze it in direct comparison with tetragonal Sr$_2$CuWO$_6$. Below $T_\mathrm{N} \simeq 32$ K, the magnetic Bragg reflections of Ca$_2$CuWO$_6$ are indexed by the commensurate propagation vector $\mathbf{k}=(\tfrac{1}{2},\tfrac{1}{2},0)$ in its native $P\bar{1}$ cell. Rietveld refinement yields a collinear structure with equal-magnitude, antiparallel moments on the crystallographically inequivalent Cu1 and Cu2 sites and an ordered moment of $0.66(3)~μ_{\mathrm B}$ per Cu at 1.5 K. A direct comparison with Sr$_2$CuWO$_6$ is obscured by different crystallographic settings and orientations of the cooperative Jahn-Teller elongation axes. Hence, we introduce a common crystallographic supercell through which effects of symmetry lowering from tetragonal to triclinic in the double perovskite are explored: Apparently different magnetic propagation vectors map onto the same supercell wave vector, revealing a common magnetic structure stabilized by tungsten-mediated, second-neighbor interactions. Mean field calculations further show that tetragonal symmetry preserves the degeneracy of four magnetic $\mathbf{k}$-domains in Sr$_2$CuWO$_6$, whereas the triclinic splitting of symmetry-related exchange pathways in Ca$_2$CuWO$_6$, most strongly within the Cu2 network, selects a single $\mathbf{k}$-domain. These results establish the magnetic ground state of Ca$_2$CuWO$_6$ and show how symmetry breaking selects a given ordered state without changing the underlying magnetic motif of the Sr analogue.
