The Nanoscale Effects of Intrinsic and Externally-Applied Strain on Charge Density Wave States

非技术摘要：电荷密度波状态发生在许多材料中，在一定的“转变”温度以下，存在周期性的晶格畸变，并且在整个材料中建立了电子密度的周期性变化。材料的电子性质从材料的原始未扭曲状态改变。电荷密度波托管材料已被瞄准用于电子器件（例如开关、离子场效应晶体管和逻辑电路）中的潜在用途。然而，目前对1）电荷密度波态的起源以及2）这些状态如何与其他技术上重要的材料特性（如超导性或磁性）共存/相互作用的理解并不完整。应变工程是一个新兴的领域，其中应变的应用，材料是用来操纵他们的光学，电子和结构性能。在这个项目中，受控应变被施加到承载电荷密度波状态的材料上，以便压缩、拉伸或剪切材料，从而导致其性质的变化。扫描隧道显微镜，一种技术，可以详细的电子和结构的变化，在原子尺度上的材料，是用来，同时，详细的演变下的电荷密度波状态，无论是，不同的应变和温度条件。该项目的首要目标是对这些化合物的物理学有一个基本的了解。了解如何以受控方式有效地操纵它们的特性，对于优化它们在设备中的性能至关重要。该项目加强了本科生和研究生参与者的教育，他们正在开发研究，工业和其他领域的应用程序的基本技能。此外，使用实验技术，如扫描隧道显微镜，原子力显微镜和扫描电子显微镜，正在开发迷你类，并纳入既定的，以STEM为重点的地方外展计划。特别是，这些课程将使预科学生接触尖端的实验技术，为这些学生提供独特的经验和机会，使他们对STEM领域感兴趣并参与其中。CDW态与量子有序（如超导性和磁性，特别是在纳米尺度上）的相互作用的性质是复杂的，并且没有得到很好的理解。此外，CDW国家可能有不同的，并不总是完善的起源，进一步复杂化这一理解。对CDW宿主化合物的研究表明，应变可以改变一系列CDW性质，包括改变转变温度，改变CDW状态的电子和结构周期性，以及改变相关的电子带隙。这个NSF项目的首要目标是了解1）应变如何在原子尺度上驱动这些CDW变化，以及2）如何使用外部施加的应变以受控的方式操纵这些变化。要做到这一点，依赖于温度的扫描隧道显微镜被用来探测电荷密度波态的纳米级结构和电子特性，它们的形成，它们的操纵，以及它们与共存的量子秩序的相互作用，在可量化的和不同的应变条件下。为了研究应变对CDW态的根本差异，我们研究了三个著名的、不同的CDW宿主家族的化合物：蓝青铜、过渡金属二硫属化物和稀土碲化物。该项目通过研究生和本科生参与项目的各个方面来支持他们的研究教育。此外，为了将科学教育扩大到历来在科学方面代表性不足的大学前学生（大学前女孩和当地社区许多低收入家庭的学生），主要研究员正在开发关于“探测材料特性”的微型课程，利用克拉克设施使该地区的学生接触材料研究的基础知识。这些课程将与涉及当地初中和高中学生的既定的、以STEM为重点的外展合作有关。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical abstract:Charge density wave states occur in numerous materials where, below a certain "transition" temperature, there is a periodic lattice distortion, and a periodic variation in electron density is established across the material. The electronic properties of the material are altered from the material's original, undistorted state. Charge density wave-hosting materials have been targeted for potential use in electronic devices (such as switches, ion field-effect transistors, and logic circuits). However, there is currently an incomplete understanding as to 1) the origin of charge density wave states, as well as 2) how these states coexist/interact with other technologically-important material properties such as superconductivity or magnetism. Strain engineering is an emerging field in which the application of strain to materials is used to manipulate their optical, electronic, and structural properties. In this project, controlled strain is applied to materials hosting charge density wave states, so as to compress, stretch, or shear the materials, leading to changes in their properties. Scanning tunneling microscopy, a technique which can detail electronic and structural changes in a material on the atomic scale, is used to, simultaneously, detail the evolution of charge density wave states under, both, varying strain and temperature conditions. An overarching goal of the project is to develop a fundamental understanding of the physics governing these compounds. An understanding of how to effectively manipulate their properties in a controlled fashion, is essential for optimizing their performance in devices. This project enhances the education of undergraduate- and graduate-student participants who are developing essential skills with applications in research, industry, and beyond. In addition, mini-classes using experimental techniques such as scanning tunneling microscopy, atomic force microscopy, and scanning electron microscopy, are being developed and incorporated into established, STEM-focused local outreach programs. In particular, these classes will expose precollege students to cutting-edge experimental techniques, providing a unique experience and opportunity for these students to become interested and engaged in STEM fieldsTechnical abstract:Charge density wave (CDW) states are prevalent in condensed matter systems where they are often found to coexist with other orders. The nature of the interplay of CDW states with quantum orders, such as superconductivity and magnetism, particularly on the nanoscale, is complex and not well-understood. In addition, CDW states can have differing and not always well-established origins, further complicating this understanding. Studies of CDW-hosting compounds demonstrate that strain can alter an array of CDW properties, including changing the transition temperature, altering electronic and structural periodicities of the CDW state, and changing the associated electronic band gap. The overarching goals of this NSF project are to understand 1) how strain drives these CDW changes on the atomic-scale, and 2) how to manipulate these changes in a controlled fashion using externally-applied strain. To do this, temperature-dependent scanning tunneling microscopy is used to probe the nanoscale structural and electronic properties of charge density wave states, their formation, their manipulation, and their interplay with coexisting quantum orders, under quantifiable and varied strained conditions. In order to study fundamental differences strain has on CDW states arising from differing origins (for example, Fermi surface nesting versus momentum-dependent electron-phonon coupling), compounds from three well-known, distinct, CDW-hosting families are studied: the blue bronzes, transition metal dichalcogenides, and the rare-earth tellurides. This project supports the research education of graduate and undergraduate students through their participation in all aspects of the project. Furthermore, in an effort to extend science education to precollege students who are historically underrepresented in the sciences (precollege girls and students from the many lower-income households in the local community), the principal investigator is developing mini-classes on "probing material properties" which utilizes Clark facilities to expose area students to the basics of materials research. These classes will be held in connection with established, STEM-focused outreach collaborations involving local area middle and high school students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.