Optical spectroscopy study of the interplay between Dirac electrons, magnetism, and charge density waves in square-net materials

具有拓扑电子能带结构的方网状材料，如典型的节线状半金属ZrSiS，是目前研究的热点。拓扑Dirac半金属具有线性色散电子能带，这些电子能带在倒易空间中的某些点交叉，并显示出特殊的物理性质。在节线半金属中，交叉点位于一条线或闭合环上，它们可以被视为拥有2D狄拉克电子的石墨烯的3D类似物。在这个项目中，将研究与ZrSiS相关的方网材料LnSbTe，其中Ln位被诱导本征磁性的稀土原子占据。因此，LnSbTe化合物在低温下具有磁性拓扑节线半金属状态。研究还表明，适当的取代和空位浓度会导致LnSbxTe2−x-Delta化合物的结构扭曲和电荷密度波(CDW)的发展，从而实现电子能带工程并产生新的拓扑相。我们将用随温度变化的光学光谱从电荷动力学的角度表征化合物LnSbxTe2−x-Delta中狄拉克电子、磁性和CDW之间的相互作用。此外，我们将通过静水压力下的光谱测量来探索外部压力如何调节这种相互作用，因为压力预计会强烈影响与CDW相关的晶格扭曲，从而也影响电子能带结构。

Square-net materials with topological electronic band structure such as the prototypical nodal-line semimetal ZrSiS are currently intensively investigated. Topological Dirac semimetals possess linearly dispersing electronic bands, which cross at certain points in reciprocal space, and show exceptional physical properties. In nodal-line semimetals the crossing points lie on a line or closed loop, and they can be viewed as 3D analogues of graphene hosting 2D Dirac electrons. Within this project, the ZrSiS-related square-net materials LnSbTe will be studied, where the Ln sites are occupied by lanthanide atoms inducing intrinsic magnetism. Therefore, LnSbTe compounds host a magnetic topological nodal-line semimetallic state at low temperatures. It was furthermore demonstrated that suitable substitution and vacancy concentration cause structural distortion and the development of charge density waves (CDWs) in LnSbxTe2−x-delta compounds, which enables an electronic band engineering and can generate novel topological phases. We will characterize the interplay between Dirac electrons, magnetism, and CDWs in the compounds LnSbxTe2−x-delta in terms of the charge dynamics by temperature-dependent optical spectroscopy. Furthermore, we will explore how external pressure tunes this interplay by optical spectroscopy measurements under hydrostatic pressure, since pressure is expected to strongly affect the CDW-related lattice distortion and hence also the electronic band structure.