Structural basis for understanding phase transitions in 3-dimensional charge-density-wave compounds

在金属材料中，向电荷密度波（CDW）态的相变通常与电子带的低维特征有关。CDW相图不被理解为没有明显的低维特征的物质的晶体结构和物理性质。本项目利用变温单晶X射线衍射方法研究了三维CDW化合物Lu 2 Ir 3Si 5、NiV 2S 4（Cr 3S 4结构类型）和CuV 2S 4（尖晶石结构类型）的CDW相变。这些实验的一个重要结果将是与CDW相关的周期性晶格畸变（PLD）。PLD提供了关于CDW在晶体结构的晶胞内的位置、关于受抑相互作用的重要性、关于CDW的对称性（1-q或2-q状态）以及关于可能参与CDW状态的稳定的声子的结论性信息。衍射实验也将用于分析进一步的相变（例如，锁定转变的可能发生），并用于确定序参数的温度依赖性。实验信息将用于理论上的考虑，以了解相变的机制，并解释CDW状态内的物理性质的性质。

Phase transitions towards charge-density wave (CDW) states are often associated with the low-dimensional character of the electron bands in metallic materials. The CDW phase diagram is not understood for substances without obvious low-dimensional features of their crystal structures and physical properties. The proposed project is concerned with the study of CDW phase transitions in the 3-dimensional CDW compounds Lu2Ir3Si5, NiV2S4 (Cr3S4 structure type) and CuV2S4 (Spinel structure type) by means of temperature-dependent single-crystal X-ray diffraction. An important result of these experiments will be the periodic lattice distortion (PLD) associated with the CDW. The PLD provides conclusive information about the location of the CDW within the unit cell of the crystal structure, about the importance of frustrated interactions, about the symmetry (1-q or 2-q state) of the CDW, and about the phonons that may participate in the stabilisation of the CDW state. Diffraction experiments will also be used for analysing further phase transitions (for example a possible occurrence of a lock-in transition) and for the determination of the temperature dependence of the order parameter. The experimental information will be used for theoretical considerations towards understanding the mechanism of the phase transitions and explaining the nature of the physical properties within the CDW state.