The combination of nontrivial band topology and symmetry-breaking phases gives rise to novel quantum states and phenomena such as topological superconductivity, quantum anomalous Hall effect, and axion electrodynamics. Evidence of intertwined charge density wave (CDW) and superconducting order parameters has recently been observed in a novel kagome material ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) that features a ${\mathbb{Z}}_2$ topological invariant in the electronic structure. However, the origin of the CDW and its intricate interplay with the topological state has yet to be determined. Here, using hard-x-ray scattering, we demonstrate a three-dimensional CDW with $2\times 2\times 2$ superstructure in $(\mathrm{Rb},\mathrm{Cs}){\mathrm{V}}_3{\mathrm{Sb}}_5$. Unexpectedly, we find that the CDW fails to induce acoustic phonon anomalies at the CDW wave vector but yields a novel Raman mode that quickly damps into a broad continuum below the CDW transition temperature. Our observations exclude strong electron-phonon-coupling-driven CDW in ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ and support an unconventional CDW that was proposed in the kagome lattice at van Hove filling.

中文翻译：

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Kagome超导体AV3Sb5(A=Rb, Cs)中无声子异常的非常规电荷密度波的观测

非平凡的能带拓扑和对称破缺相的结合产生了新的量子态和现象，例如拓扑超导、量子反常霍尔效应和轴子电动力学。最近在一种新型 Kagome 材料中观察到了交织电荷密度波 (CDW) 和超导有序参数的证据${\mathrm{一种}\mathrm{伏}}_3{锑}_5$ ($\mathrm{一种}=\mathrm{钾}$, Rb, Cs) 具有 ${\mathbb{Z}}_2$电子结构中的拓扑不变量。然而，CDW 的起源及其与拓扑状态的复杂相互作用尚未确定。在这里，使用硬 x 射线散射，我们展示了一个三维 CDW$2\times 2\times 2$ 上层建筑 $(铷,\mathrm{CS}){\mathrm{伏}}_3{锑}_5$. 出乎意料的是，我们发现 CDW 未能在 CDW 波矢量处引起声学声子异常，但会产生一种新颖的拉曼模式，该模式迅速衰减到低于 CDW 转变温度的宽连续谱。我们的观察排除了强电子声子耦合驱动的 CDW${\mathrm{一种}\mathrm{伏}}_3{锑}_5$ 并支持在 van Hove 填充的 Kagome 格子中提出的非常规 CDW。