三维拓扑绝缘体Bi2Se3、Bi2Te3和Sb2Te3体系压力诱导结构相变的高压X射线吸收谱研究

Three-dimensional topological insulators (3D-TIs) Bi2Se3、Bi2Te3 and Sb2Te3 are new quantum matter discovered in recent years, and their surfaces exhibit the feature metallic surface states topologically protected against scattering by time-reversal symmetry due to topologically nontrivial band structure. Under high pressure, some new pressure-induced features , such as superconductivity, semiconductor-metal phase transition and electronic topological transition, are gradually revealed in this 3D-TIs. Their pressure-induced structural phase transitions have also attracted extensive attention. This project intends, using the high-pressure X-ray absorption spectroscopy (XAFS) and X-ray diffraction (XRD) measurements, combined with multiple scattering ab initio calculation and first principle calculation, etc., to investigate systematically the pressure-induced structural phase transition and related problems. The high-pressure XAFS spectroscopy, combined with high-pressure XRD, will be performed on 3D-TIs to explore the pressure-induced crystal structural evolutions, phase structure under high pressure, disordered array of atoms in the crystal lattice, pressure-induced charge transfer and electronic topological transition. We will clarify the argument about pressure-induced structural phase transition of 3D-TIs. The results will provide a new perspective to understand deeply the crystal structure under high pressure and the relationship with the abundant physical phenomena induced by high pressure in 3D-TIs.

三维拓扑绝缘体Bi2Se3、Bi2Te3和Sb2Te3是近些年来发现的一种全新的量子物质态。由于其能带结构拓扑性质与传统意义下绝缘体的不同，表现出受时间反演对称性保护的表面金属态等特殊性质。而在高压下，该体系也表现出诸如压力诱导超导电性、半导体-金属转变和电子拓扑转变等物理现象，与此相关的高压下结构及其变化规律这一基本问题也得到了应有的关注。本项目拟采用高压XAFS谱和高压XRD等实验手段，结合XANES谱多重散射从头计算和第一性原理计算等理论工具，系统研究三维拓扑绝缘体Bi2Se3等体系高压诱导结构相变等问题。从高压XAFS短程序结构视角分析该体系高压诱导结构相变的路径、高压相结构、混合占位、高压相电荷转移和压力诱导电子拓扑转变的局域结构机制等，与高压XRD等提供的长程序结构信息结合，明确该体系高压诱导结构相变若干争议，为理解该体系高压诱导出现的丰富物理现象提供晶体结构基础。

三维拓扑绝缘体Bi2Se3、Bi2Te3和Sb2Te3是近些年来发现的一种全新的量子物质态。在高压下，该体系也表现出诸如压力诱导超导电性、半导体-金属转变和电子拓扑转变等丰富的物理现象，与此相关的高压下结构及其变化规律仍存在一些争议和盲区。本项目实施的主要内容是采用高压XAFS谱、高压XRD和高压拉曼等实验手段，结合XANES谱多重散射从头计算和第一性原理计算等，系统研究了三维拓扑绝缘体Bi2Se3等体系高压诱导结构相变等问题，明确该体系高压诱导结构相变若干争议。此外，还将研究内容拓展到了拓扑半金属和超硬材料等体系，以及高压XAFS谱测量方法学研究等。本项目所获得的主要成果如下：.1. 研究了Bi2Se3、Bi2Te3在35 GPa低压段存在着同构相变问题。首次揭示了在35 GPa附近存在同构相变且有电荷重分布和结构扭曲并伴随着结构无序度的增大等现象。揭示了同构相变与与电子拓扑相变（Lifshitz相变）相关机理。.2. 研究了Bi2Te3中无序替位合金形成过程中局部结构的变化。首次用小波变换EXAFS明确指出Bi2Te3是替位合金的第一个直接实验证据。.3. 研究了Bi2Se3在静水和非静水压力下高达42GPa的结构相变序列和高压相结构。证实了Bi2Se3在高压相具有I4/mmm结构，而不是Im-3m和C2/m相。明确了准静水压条件下R-3m→C2/m→C2/c→I4/mm m相和非静水压条件下R-3m、C2/m→I4/mm m相的结构相变序列。.4. 还拓展研究了1T单分子层叠加的MoS2的三个预测体结构及拓扑性质，及超硬材料WB4的三维硼网络承受压缩变形的机理。.5. 研究高压XAFS规避来自金刚石压砧的衍射峰的方法学。利用毛细管半透镜和纳米金刚石压砧的结合，获得了完全抑制衍射峰的高压XAFS谱，彻底解决了这一国际难题。