Dynamic Charge-Density Waves and Electronic Anomalies of Inorganic Solids

休斯敦大学的Vassiliy Lubchenko获得了化学系化学理论、模型和计算方法项目的奖项，以支持他对复杂无序系统中电子现象的理论研究。固体结构和性质的惊人多样性奠定了它们在日常应用中的基础，从信息技术到储热再到冶金。然而，制造具有定制特性的材料在某种程度上仍然是一个不断尝试和错误的企业。很难预测原子在特定化合物中的排列方式，这种化合物的导电性或对电磁场的反应有多好。出现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，即使已知，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。与普通液体相比，电子甚至可以在非常低的温度下流动，就像它们在金属中一样。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的光发射的一个令人费解的特征。实验显示，电子明显过剩，本应有助于导电，但奇怪的是，并没有。只有当带正电的原子核有足够的拉力时，电子才会被强迫保持不动。因此，电子被限定在原子核周围，而材料则变成绝缘体或半导体。Lubchenko和他的同事们探索这种局部电子状态来预测非晶合金的性能，这些非晶合金可能用于制造下一代计算机存储器和智能光学，以及其他许多东西。卢布琴科集团教育和推广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的过程。近年来，对具有玻恩-奥本海默振动基态的简单无机化合物的结构和电子谱的理顺研究取得了很大进展。对于一类非常重要的、表现出几乎等效的亚稳态玻恩-奥本海默构型的巨大简并的固体，人们的理解要成功得多。这项研究的微观假设是，多电子激发——比如那些在长波下引起等离子体振荡的电子激发，可以使固体在短波长上不稳定地形成长寿命的带电密度波（CDW）。值得注意的是，这些密度波可以是非周期的。如果与底层原子晶格耦合足够强，非周期CDWs就会形成非晶玻璃半导体。分离不同的非周期低能量电荷模式的畴壁有望容纳特殊的间隙中电子态，这些电子态起着电子活性缺陷的作用。当彼此足够接近时，这些中隙态有助于在迁移率边缘附近的局域态的指数尾，并主导玻璃半导体的电导率。在有趣的情况下，cdw -晶格耦合的强度介于金属和金属间化合物之间，这种不稳定性预计会导致动态无序，因此晶格保持有序，但只是平均有序。如果填充，驻留在动态畴壁上的中隙态可能为高温超导提供一条途径；电子之间不需要额外的有效吸引力。为了验证这一假设，Lubchenko和他的研究小组采用了一种新颖的方法，在这种方法中，人们使用精心选择的空间变化外场来可控地诱导非周期电荷分布，以量化它们的稳定性、简并性和相互转换的动力学。为了在碱金属上测试所提出的方法，他们试图解决几十年来关于费米表面附近光电实验所揭示的电子态明显过剩密度的争议。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。

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.