CAREER: Visualizing the Formation of the Charge Density Wave Phase at the Atomic Scale

*非技术抽象*一种简单的金属，如金或铜，可以想象为一个空盒子，里面的电子自由反弹。然而，在一些固体中，电子在空间中形成波，具有交替的高电荷和低电荷区域。这种物质状态被称为“电荷密度波”(CDW)。在这种固体中，电荷密度波的形成发生在一个临界温度，超过这个温度，电子可以再次自由移动。这一切为什么要发生？当样品经过临界温度时，这些电子波到底是如何在太空中形成的？该项目旨在通过对CDW材料进行依赖于温度的扫描隧道显微镜(STM)测量来直接可视化临界温度下电荷密度波的开始，从而回答这个问题。扫描隧道显微镜是一种仪器，我们可以用它来探测材料表面的电子，精度达到亚原子。这些先进的仪器将为这个项目量身定做，新的STM测量将为我们提供重要信息，了解不同能量的电子在这些材料中经历CDW转变时的行为。该项目将支持本科生和研究生进行STM实验所需的先进技术的教育，包括电子学、计算机辅助设计、真空技术和低温技术。这个项目试图回答有关固体中电子的集体运动的问题，这是现代物理学研究中的基本挑战之一。*技术摘要*本项目的目的是使用扫描隧道显微镜(STM)可视化真实空间中电荷密度波(CDW)相的开始。在均匀体系的简单二阶相变中，序参数的幅值在相变温度处趋于零。当存在缺陷或其他空间不均匀时，情况可能会有很大的不同。最近的实验表明，利用变温原子分辨率扫描隧道显微镜，过渡金属二卤化物中的纳米级CDW有序可以稳定在体相变温度以上。这些纳米级的CDW补丁是如何转变为块状CDW顺序的？纳米级补丁中的局部电子光谱是什么？在这些材料中，CDW态和正常态之间的电子光谱差异是什么？在转变温度以上的缺陷和CDW贴片的散射是什么性质？在这个项目中，将使用最先进的自制扫描隧道显微镜来回答这些问题。研究生和本科生将学习如何建造和操作这些仪器，并将实施提高稳定性和低温效率的新设计。空间有序集体电子相的性质是许多现代材料中出现的一个热门问题，二盐基化物提供了一种清洁的材料系统，在该系统中，可以以原子空间精度和毫伏能量分辨率测量这种相的开始。

****NON-TECHNICAL ABSTRACT****A simple metal such as gold or copper can be imagined as an empty box with electrons bouncing around freely inside. In some solids, however, the electrons form waves in space with alternating regions of higher and lower charge. This state of matter is known as a "charge density wave" (CDW). In such solids, the formation of the charge density wave happens at a critical temperature, above which the electrons are once again free to move around. Why does this happen? How exactly do these waves of electrons form in space as the sample goes through the critical temperature? This project aims to answer this question by performing temperature-dependent scanning tunneling microscopy (STM) measurements of CDW materials to directly visualize the onset of charge density waves at the critical temperature. An STM is an instrument with which we can probe the electrons at the surface of a material with sub-atomic precision. These advanced instruments will be custom-built for this project, and the new STM measurements will give us vital information on how electrons with different energies behave in these materials as they go through the CDW transition. This project will support the education of undergraduate and graduate students in the advanced technologies required to perform STM experiments including electronics, computer-aided design, vacuum technology and cryogenics. This project seeks to answer questions about the collective motion of electrons in solids, one of the fundamental challenges in modern physics research.****TECHNICAL ABSTRACT****The aim of this project is to visualize the onset of the charge density wave (CDW) phase in real space using scanning tunneling microscopy (STM). In a simple second-order phase transition in a uniform system, the amplitude of the order parameter goes to zero at the phase transition temperature. When defects or other spatial inhomogeneity is present, the situation can be dramatically different.Using variable-temperature atomic resolution STM, recent experiments have shown that nanoscale CDW order can be stabilized above the bulk transition temperature in the transition metal dichalcogenides. How do these nanoscale patches of CDW transition to bulk CDW order? What is the local electronic spectrum in a nanoscale patch? What is the electronic spectroscopic difference between the CDW state and the normal state in these materials? What is the nature of scattering from defects and CDW patches above the transition temperature? During this project, state of the art, homebuilt STM instruments will be used to answer these questions. Graduate and undergraduate students will learn how to build and operate these instruments, and new designs for improved stability and cryogenic efficiency will be implemented. The nature of spatially ordered collective electronic phases is a topical question that arises in many modern materials, and the dichalcogenides present a clean material system where the onset of such a phase can be measured with atomic spatial precision and millivolt energy resolution.