Charge density waves (CDWs), periodic modulations of the charge density, are among the most abundant and non-trivial ordered phases in condensed matter. Here we demonstrate the occurrence of multi-valley charge density waves (MV-CDW) on the semimetal Sb(111), a phenomenon previously observed at semiconductor surface inversion layers. The topological nature of the pocket surface states in Sb(111) ensures perfect nesting conditions giving rise to sharp peaks in helium atom scattering (HAS) diffraction spectra. The peculiar temperature dependence permits to distinctly associate the diffraction peaks with surface electron and hole-pocket states due to the non-trivial surface electronic band structure known from recent experimental data and present ab-initio calculations. Inelastic HAS spectra, besides providing the surface phonon dispersion curves in agreement with density functional perturbation theory calculations, reveal two additional dispersion curves of elementary excitations in the gap well below Rayleigh waves. They are attributed to collective excitations of the quasi-commensurate MV-CDWs associated with the surface electron M-pocket states at the Fermi level. These findings suggest that HAS can be regarded as a choice spectroscopy for the investigation of surface electronic excitations in the THz domain, opening up a new window for the search of collective phases at surfaces.

电荷密度波（CDW）是电荷密度的周期性调制，是凝聚态物质中最丰富且最重要的有序相之一。在这里，我们演示了半金属Sb（111）上多谷电荷密度波（MV-CDW）的出现，这是先前在半导体表面反型层处观察到的现象。Sb（111）中的袋状表面态的拓扑性质可确保理想的嵌套条件，从而在氦原子散射（HAS）衍射光谱中产生尖锐的峰。由于最近的实验数据和从头算起的计算中已知的非平凡的表面电子能带结构，奇特的温度依赖性使得衍射峰与表面电子和空穴能态有明显的联系。非弹性HAS光谱，除了提供与密度泛函微扰理论计算一致的表面声子色散曲线外，还揭示了在瑞利波以下的带隙中基本激发的另外两条色散曲线。它们归因于与费米能级上的表面电子M口袋态相关的准相称MV-CDW的集体激发。这些发现表明，HAS可被视为研究THz域中表面电子激发的一种选择光谱学，从而为寻找表面集体相开辟了新窗口。它们归因于与费米能级上的表面电子M口袋态相关的准相称MV-CDW的集体激发。这些发现表明，HAS可被视为研究THz域中表面电子激发的一种选择光谱学，从而为寻找表面集体相开辟了新窗口。它们归因于与费米能级上的表面电子M口袋态相关的准相称MV-CDW的集体激发。这些发现表明，HAS可被视为研究THz域中表面电子激发的一种选择光谱学，从而为寻找表面集体相开辟了新窗口。