准二维电荷密度波材料的球差校正电镜观察及相关超导电性研究

Charge density wave (CDW) is a significant physical phenomenon in low-dimensional metallic systems, which appears as periodic modulation of conduction electron density and usually accompanies with lattice distortion caused by structural instability. Comprehensive understanding of the local lattice distortions is crucial to understand the origin of CDW, electron-lattice coupling mechanism and CDW- superconductivity correlation in this kind of materials. Recent progress of Cs-corrected atomic-resolution scanning transmission electron microscopy (STEM) with spatial resolution better than 1? ensure the direct observation of local distortion with Pico-meter precision, and is much useful for determining structural modulations (especially incommensurate modulation) in real space; Recently, we, for the first time, observed atomic displacements in the real space for the incommensurate CDW modulation in LaTe2. It was found the periodic atomic displacements exhibit sinusoidal wave behavior, indicating the CDW in LaTe2 is fully incommensurate with the sub-lattice, and the atomic waves within the layers form certain regular pattern due to the interaction between CDW and sub lattice. In this project, we will further develop experimental techniques and data processing to reveal local structural distortion in a high precision. We will systematically analyze the structural modulation in typical two dimensional CDW systems (layered tritellurides and transition metal dichalcogenides), focusing on the structural features in correlation with incommensurability and CDW instability in these layered systems. Combing the first-principles electronic structure calculation for quantitative analysis of structural alterations, the local structural changes and lattice-charge coupling mechanism in quasi-two-dimensional charge density wave systems will extensively investigated. It is also noted that certain CDW systems could show up clearly completion between CDW and superconductivity. Based on the microstructure and superconductivity studies, we will also carry out the exploration of new superconductors in related materials.

电荷密度波（CDW）是低维材料中的重要结构物理现象，通常伴随有电声耦合引起的周期性结构调制，其中，原子结构的精确解析，电声耦合机制研究及其和超导态的关联是凝聚态物理研究的重要方向。新型球差校正高分辨电镜可以实现皮米精度原子位移测量，使得直接观察局域原子结构变化成为可能，对CDW结构，尤其是无公度调制结构的精确测定有重要促进意义，目前还无相关报道。近期我们利用球差校正STEM技术首次对LaTe2准二维CDW体系进行了实空间原子位移分析，结果显示该体系存在无公度CDW，且不同调制带间存在特定耦合模式。本项目将进一步发展高分辨STEM图像处理及高精度位移提取技术；系统分析层状稀土碲化物和金属硫化物中的典型调制结构，揭示准二维CDW体系本征结构特性；结合理论计算推动CDW不稳定性及电声耦合机制研究；此外，我们将重点关注CDW相关超导体中局域微结构特性，加深对CDW态和超导电性关联的理解。

电荷密度波（CDW）是低维材料中的重要结构物理现象，通常伴随有电声耦合引起的周期性结构调制，其中，原子结构的精确解析，电声耦合机制研究及其和超导态的关联是凝聚态物理研究的重要方向。本研究项目主要研究内容包括：发展了高分辨STEM图像处理及高精度位移提取技术实现，提高了原子皮米精度可靠位移测量精度，利用该技术对LaTe2-δ准二维CDW体系进行了实空间原子位移分析，认为该体系存在振幅调制的完全非公度的CDW，并澄清了该体系中调制波矢q2=1/5(3a*+ b*)的超结构的形成归结为单层Te原子层中的Te空位有序；合作在RETe3和RE2Te5(RE为稀土元素)中插层Pd原子，获得了临界温度为0.1K至3K的一系列新型超导材料；对1T-TaSe2-xTex体系的电荷密度波调制三维结构排布、超导机理、相变机制、各向异性晶格动力学进行了系统分析:三维结构解析结果表明其电荷密度波在三维空间的排布具有c方向分量；1T-TaSe2中激光诱导的超快公度-非公度相变研究表明该过程可以分为三个阶段，公度电荷密度波相被抑制和无公度电荷密度波相生成之间存在2 ps的时间延迟，在非平衡相变过程中观测到两相衍射效率相等的“结构不动点”，揭示非平衡相变的中存在亚稳态的物理机制；结合低温输运性质测量，在飞秒激光诱导下，系统分析了TaS2-xSex体系的量子隐含态，探讨了稳定光诱导隐含态的规律。此外，利用透射扫描电镜，在Bi2(Sr2-xLax)CuO6+d体系材料中直接观察到CuO2层中存在非公度调制与基本晶格协同作用产生的显著孤子线和SrO和BiO层间有规律出现的间隙氧原子，在CsxBi4Te6材料体系中观察到结构通道以及填充原子；并开展了LSMO双层钙钛矿型体系在低温下相分离态的光致研究。