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

****非技术摘要****一种简单的金属，如金或铜，可以想象成一个空盒子，电子在里面自由地弹跳。然而，在某些固体中，电子在高电荷和低电荷交替的区域形成空间波。物质的这种状态被称为“电荷密度波”（CDW）。在这样的固体中，电荷密度波的形成发生在临界温度下，在此温度以上，电子再次自由移动。为什么会发生这种情况？当样品经过临界温度时，这些电子波究竟是如何在空间中形成的？该项目旨在通过对CDW材料进行温度相关的扫描隧道显微镜（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。STM是一种能够以亚原子精度探测材料表面电子的仪器。这些先进的仪器将为这个项目定制，新的STM测量将为我们提供关于不同能量的电子在这些材料中经历CDW转变时的行为的重要信息。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教育，包括电子学、计算机辅助设计、真空技术和低温学。该项目旨在回答固体中电子集体运动的问题，这是现代物理学研究的基本挑战之一。****技术摘要****该项目的目的是利用扫描隧道显微镜（STM）在真实空间中可视化电荷密度波（CDW）相位的起始。在均匀系统的简单二阶相变中，阶参量的振幅在相变温度下趋于零。当存在缺陷或其他空间不均匀性时，情况可能会大不相同。利用可变温度原子分辨率STM，最近的实验表明，在过渡金属二硫族化合物中，纳米级CDW序可以稳定在块体转变温度以上。这些纳米级的CDW片如何转变为批量CDW订单？纳米级贴片的局部电子谱是什么？这些材料的CDW态和正常态的电子能谱有什么不同？在过渡温度以上的缺陷和CDW贴片的散射性质是什么？在这个项目中，最先进的、自制的STM仪器将被用来回答这些问题。研究生和本科生将学习如何建造和操作这些仪器，并将实施新的设计，以提高稳定性和低温效率。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主题问题，而二硫族化合物提供了一个清洁的材料系统，其中可以用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.