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，其中低于一定的“转变”温度，存在周期性晶格畸变，并且在整个材料中建立了电子密度的周期性变化。材料的电子特性从材料的原始，未扭曲的状态改变。电荷密度波承载材料已经成为电子器件（如开关、离子场效应晶体管和逻辑电路）的潜在用途。然而，对于1)电荷密度波状态的起源，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，目前还没有完全理解。应变工程是一个新兴的领域，在这个领域中，应变对材料的应用被用来操纵它们的光学、电子和结构特性。在本项目中，对承载电荷密度波态的材料施加控制应变，从而对材料进行压缩、拉伸或剪切，从而改变其性能。扫描隧道显微镜是一种可以在原子尺度上详细描述材料电子和结构变化的技术，它同时被用来详细描述在不同应变和温度条件下电荷密度波状态的演变。该项目的首要目标是对控制这些化合物的物理原理有一个基本的了解。了解如何以可控的方式有效地操纵其属性对于优化其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，他们正在发展在研究、工业和其他领域应用的基本技能。此外，正在开发使用扫描隧道显微镜、原子力显微镜和扫描电子显微镜等实验技术的迷你课程，并将其纳入已建立的以stem为重点的地方推广计划。特别是，这些课程将使大学预科学生接触到尖端的实验技术，为这些学生产生兴趣并从事STEM领域提供独特的体验和机会。技术摘要：电荷密度波（CDW）状态在凝聚态系统中很普遍，它们经常与其他秩序共存。CDW态与量子顺序（如超导性和磁性）相互作用的本质，特别是在纳米尺度上，是复杂的，而且还没有得到很好的理解。此外，CDW国家可能有不同的起源，并不总是确定的起源，这进一步使这种理解复杂化。对CDW载体化合物的研究表明，应变可以改变CDW的一系列性质，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。为了做到这一点，使用温度相关的扫描隧道显微镜来探测电荷密度波状态的纳米级结构和电子特性，它们的形成，它们的操作，以及它们与共存量子秩序的相互作用，在可量化的和不同的应变条件下。为了研究由不同来源（例如，费米表面嵌套与动量依赖的电子-声子耦合）引起的CDW状态的基本差异，研究了来自三个众所周知的，不同的CDW寄主家族的化合物：蓝色青铜，过渡金属二硫族化合物和稀土碲化物。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，为了将科学教育扩展到历史上在科学领域代表性不足的大学预科学生（大学预科女孩和来自当地社区许多低收入家庭的学生），首席研究员正在开发关于“探索材料特性”的迷你课程，该课程利用克拉克的设施向地区学生展示材料研究的基础知识。这些课程将与已建立的、以stem为重点的外展合作项目一起举办，涉及当地的中学生和高中生。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.