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Dynamic Charge-Density Waves and Electronic Anomalies of Inorganic Solids

无机固体的动态电荷密度波和电子异常

基本信息

批准号：
1956389
负责人：
Vassiliy Lubchenko
金额：
$ 45万
依托单位：
University of Houston
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2020
资助国家：
美国
起止时间：
2020-04-01 至 2025-03-31
项目状态：
未结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1956389&HistoricalAwards=false
关键词：
Dynamic Charge Density Waves Electronic

项目摘要

Vassiliy Lubchenko of the University of Houston is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry for theoretical research focused on electronic phenomena in complex, disordered systems. The astonishing diversity of structures and properties of solids underlie their use in everyday applications ranging from information technology to heat storage to metallurgy. Yet making materials with tailored properties is still somewhat of a trial and error enterprise. It is difficult to predict how the atoms will arrange in a specific compound, how well this compound will conduct electricity or respond to an electromagnetic field. The reason for these predictive difficulties is that quantum-mechanical equations governing the motion of electrons in atoms (and molecules and materials) are difficult to solve. Their solution, even if known, are hard to survey. Professor Lubchenko’s research aims to reduce this complexity by treating electrons not as separate particles, but as a fluid. In contrast with ordinary liquids, the electrons can flow even at very low temperatures, as they do in metals. Lubchenko hypothesizes that slow, wavelike motions of this quantum fluid account for a puzzling feature of photoemission in sodium and potassium. Experiments reveal an apparent excess of electrons that should contribute to electric conduction but, mysteriously, do not do so. The electrons can be forced to stay put, but only if there is enough pull from the positively charged nuclei. The electrons thus become localized around the nuclei, while the material becomes an insulator or semiconductor. Lubchenko and coworkers explore this localized-electron regime to predict properties of amorphous alloys that may be used in making the next generation of computer memory and smart optics, among many other things. A major component of the Lubchenko group's educational and outreach activities is direct involvement of high school and undergraduate students in the process of scientific discovery. Much progress has been recently achieved in rationalizing the structures and electronic spectrum of simple inorganic compounds whose Born-Oppenheimer vibrational ground state is unique. There has been much less success understanding the very important class of solids that exhibits a vast degeneracy of nearly equivalent metastable Born-Oppenheimer configurations. The microscopic hypothesis of this research is that multi-electron excitations--such as those giving rise to plasma oscillations at long wavelengths, can make solids unstable toward the formation of long-lived charged-density waves (CDW) on short wavelengths. Notably, these density waves can be aperiodic. If coupled sufficiently strongly to the underlying atomic lattice, the aperiodic CDWs then lead to the formation of glassy amorphous semiconductors. Domain walls separating distinct aperiodic low-energy charge patterns are expected to host special midgap electronic states, which play the role of electronically active defects. When sufficiently close to each other, these midgap states contribute to the exponential tail of localized states near the mobility edge and dominate the electrical conductivity in glassy semiconductors. In the interesting case of where the CDW-lattice coupling is intermediate in strength between that found in metallic and intermetallic compounds, the instability is expected to lead to dynamic disorder so that the lattice remains ordered but only on the average. If filled, the midgap states residing on dynamic domain walls could provide a route to high-temperature superconductivity; no additional effective attraction between electrons is necessary. To test this hypothesis, Lubchenko and his research group implement a novel methodology in which one applies carefully chosen spatially varying external fields to controllably induce aperiodic charge distributions in order to quantify their stability, degeneracy, and kinetics of their mutual interconversion. In testing the proposed methodology on alkali metals, they attempt to resolve a decades old controversy regarding the apparent excess density of electron states near the Fermi surface revealed by photoemission experiments.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.

休斯顿大学的Vassiliy Lubchenko因专注于复杂、无序系统中的电子现象的理论研究而获得化学系化学理论、模型和计算方法项目的支持。固体结构和性质的惊人多样性奠定了它们在从信息技术到储热再到冶金等日常应用中的应用基础。然而，制造具有定制特性的材料在某种程度上仍然是一项反复试验的事业。很难预测原子在特定化合物中将如何排列，这种化合物将如何导电或对电磁场做出反应。这些预测困难的原因是，控制原子(以及分子和材料)中电子运动的量子力学方程很难求解。他们的解决方案，即使知道，也很难调查。卢布琴科教授的研究旨在通过将电子视为流体而不是单独的粒子来降低这种复杂性。与普通液体不同，电子即使在非常低的温度下也可以流动，就像它们在金属中所做的那样。卢布琴科假设，这种量子流体的缓慢波状运动解释了钠和钾中光电子发射的一个令人费解的特征。实验揭示了明显过剩的电子，这些电子本应有助于导电，但奇怪的是，并没有这样做。电子可以被迫留在原地，但前提是正电荷的原子核有足够的拉力。因此，电子集中在原子核周围，而材料则成为绝缘体或半导体。卢布琴科和他的同事们探索了这种局域电子机制，以预测非晶态合金的性质，这些合金可能会用于制造下一代计算机存储器和智能光学等许多东西。卢布琴科小组教育和外联活动的一个主要组成部分是让高中生和本科生直接参与科学发现过程。最近在使简单无机化合物的结构和电子光谱合理化方面取得了很大进展，这些化合物的Born-Oppenheimer振动基态是独一无二的。在理解一类非常重要的固体方面取得的成功要少得多，这类固体表现出了几乎相等的亚稳态Born-Oppenheimer组态的巨大简并性。这项研究的微观假设是，多电子激发--例如那些引起长波长等离子体振荡的激发--可以使固体不稳定地形成短波长的长寿命电荷密度波(CDW)。值得注意的是，这些密度波可能是非周期性的。如果足够强地耦合到下面的原子晶格，非周期性的CDW就会导致玻璃状非晶态半导体的形成。分隔不同的非周期性低能电荷图案的磁畴壁有望容纳特殊的中禁带电子态，这些电子态扮演着电子活性缺陷的角色。当彼此足够接近时，这些中间能隙状态有助于形成迁移率边缘附近的局域态的指数尾巴，并主导玻璃半导体的电导。在有趣的情况下，CDW-晶格耦合的强度介于金属化合物和金属间化合物之间，这种不稳定性预计会导致动态无序，从而使晶格保持有序，但仅在平均水平上。如果填充，驻留在动态域壁上的中间能隙状态可以提供一条通往高温超导的途径；电子之间不需要额外的有效吸引。为了验证这一假设，卢布琴科和他的研究小组实施了一种新的方法，其中一个人应用精心选择的空间变化的外场来可控地诱导非周期电荷分布，以便量化它们的稳定性、简并性和它们相互转化的动力学。在测试碱金属的拟议方法时，他们试图解决数十年来关于光电发射实验揭示的费米表面附近的电子状态表观过剩密度的争议。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。