论文标题
结构，声子和轨道自由度fe $ _2 $ mo $ $ _3 $ o $ _8 $
Structure, phonons, and orbital degrees of freedom in Fe$_2$Mo$_3$O$_8$
论文作者
论文摘要
我们报告多效FE $ _2 $ MO $ _3 $ o $ $ _8 $的结构和光谱表征。同步加速器X射线和中子衍射以及热膨胀测量结果显示出$ t _ {\ Mathrm {n}} \ Simeq 60 \，$ K的晶格异常，但在磁性有序状态下没有显示任何对称性降低。晶格参数$ c $表现出一种非单调的行为，其明显的最低为$ 200 \，$ k，这也反映在某些观察到的红外活性光学激发的异常行为中，并且在短距离磁性顺序的发作中相似。红外反射率光谱在$ 100-8000 \，$ cm $^{ - 1} $的频率范围内测得的红外反射率光谱揭示了与密度功能官能计算获得的特征相比，大多数预期的声子模式与特征相比。 $ a_1 $的声音在冷却后显示出整体硬化，而对于某些$ e_1 $模式，观察到非单调行为。这些模式还显示了低于$ t_ \ mathrm {n} $的声子寿命的强烈增加，我们将其与磁序状态下的轨道矩的淬灭方向相关联。在四面体Fe $^{2+} $网站的较高较高$ d $ -d $ d $激发的寿命中也观察到了类似的增加，它们仅在$ t_ \ mathrm {n} $下方清晰可见。
We report on the structural and spectroscopic characterization of the multiferroic Fe$_2$Mo$_3$O$_8$. Synchrotron x-ray and neutron diffraction, as well as thermal expansion measurements reveal a lattice anomaly at $T_{\mathrm{N}}\simeq 60\,$K but do not show any symmetry lowering in the magnetically ordered state. The lattice parameter $c$ exhibits a non-monotonic behavior with a pronounced minimum around $200\,$K, which is also reflected in an anomalous behavior of some of the observed infrared-active optical excitations and parallels the onset of short-range magnetic order. The infrared reflectivity spectra measured between 5 and 300$\,$K in the frequency range of $100-8000\,$cm$^{-1}$ reveal most of the expected phonon modes in comparison with the eigenfrequencies obtained by density-functional calculations. The $A_1$ phonons show an overall hardening upon cooling, whereas a non-monotonic behavior is observed for some of the $E_1$ modes. These modes also show a strongly increased phonon lifetime below $T_\mathrm{N}$, which we associate with the quenched direction of the orbital moment in the magnetically ordered state. A similar increase is observed in the lifetime of the higher-lying $d$-$d$ excitations of the tetrahedral Fe$^{2+}$ site, which become clearly visible below $T_\mathrm{N}$ only.
