Density Response and Electron Pairing

该奖项支持研究和教育，以增强我们对电子如何响应其环境变化之间关系的理解，以及它们形成成对而不是保持相对独立的趋势。固体中的电子配对是固体中几个最有趣和最有用的特性的根源，超导性和不均匀磁性是两个重要的例子。2016年在硫化氢中发现的临界起始温度（Tc）高于200 K （-85 F）的超导性，创造了Tc的新记录，推动富氢材料成为该领域研究的新前沿。这种高温超导性的革命是以需要一个封闭的压力电池来提供诱导超导性的200万个大气压为代价的。为了取得进展，如果要发现高Tc或低压力的例子，理论和模拟必须发挥更大的作用。对几种候选氢化物的现有模拟表明，需要在基本水平上理解金属传导电子与振动质子（氢核）的相互作用。这个项目将利用这个材料平台来解决一个基本问题：为什么有些氢化物是惊人的超导体，而其他看似相似的氢化物却不是。类似的模拟将应用于其他特殊的超导体（即单层硒化铁），其中相关类型的配对是促进超导临界温度显著提高的最佳候选。首席研究员将扩展和应用相关理论，并为研究该问题所需的计算编写计算机代码。随着技术的发展，超导性正在产生更广泛的影响，而不仅仅是增强对基础知识的理解。应用的例子包括医用磁共振成像仪（MRI）中的强超导磁体，用于基本粒子研究的大型强子对撞机环，超灵敏超导量子干涉探测器，使世界上最快的列车悬浮的超导/磁技术，以及通过超导电缆向居民区输电的示范项目。超导材料和性能的进一步发展有望加速创新向公共应用的转变。除了研究之外，该奖项还将支持青年科学家在科学技术研究、开发、教育和管理领域的职业发展道路的培训。60年来，高温超导理论一直处于凝聚态理论的前沿。一种新型高温超导体的发现将这一领域推向了另一个不同的水平。一种方法是重新调整基本的科学范式，即实验引导并刺激理论。对于这些氢化物，理论（主要是阿什克罗夫特的工作）早在高压技术能够测试预测之前就出现了。硫化氢和相关氢化物的电子-声子机制可能被认为是合理理解的，但最基本的量——由移动的质子引起的扰动以及它如何影响费米表面上的电子对，特别是矩阵元素——隐藏在复杂的计算机模拟中，无法提供任何理解，从而无法预测和设计出高温或低压下杰出的超导体。本项目提出通过计算和分析电子在费米表面的原子-原子散射，通过对线性响应的数值评估来补充传统的电子线性响应理论，来完成这一理解。与爱迪生的发现相比，增加的理解将促进合理的材料设计，并通过揭示电子-声子矩阵元素的“基因组”来实现更好的超导体的真正前景。这一进展将适用于其他超导材料的挑战，特别是单层硒化铁，人们普遍怀疑电子-声子耦合增强了潜在的、基于磁的配对机制。此外，硫化氢的非常高的Tc以及电子精细结构是使其与众不同的一个重要特征，这对模拟和基本理解行为（电子响应）提出了新的挑战，超出了这类电子配对的传统理论。该项目的结果预计将影响特定应用所需的具有输运特性的新材料的预测、设计和控制，包括半导体和半金属、热电材料和优化的磁电子系数。除了研究之外，该奖项还将支持青年科学家在科学技术研究、开发、教育和管理领域的职业发展道路的培训。

NONTECHNICAL SUMMARYThis award supports research and education to enhance our understanding of the relationship between how electrons respond to changes in their environment and their tendency to form pairs rather than to remain relatively independent. Pairing of electrons in solids lies at the root of several of the most interesting and useful properties of solids, with superconductivity and inhomogeneous magnetism being two important examples.The 2016 discovery of superconductivity with a critical onset temperature (Tc) above 200 K (-85 F) in trihydrogen sulfide has established a new record for Tc, promoting hydrogen-rich materials as the new frontier in this area of research. This revolution in high-temperature superconductivity comes at the expense of requiring an enclosed pressure cell to provide the two million atmospheres of pressure that induces superconductivity. For progress, theory and simulations must play a much larger role if higher Tc or lower pressure examples are to be discovered. Existing simulations for several candidate hydrides reveal the need to understand at a basic level the interaction of the metallic conduction electrons with a vibrating proton (hydrogen nucleus). This project will use this materials platform to address the fundamental question of why some hydrides are spectacular superconductors, while seemingly similar others are not. Analogous simulations will be applied to other peculiar superconductors (viz. single layers of iron selenide) where a related type of pairing is the best candidate for promoting impressive enhancements of superconducting critical temperatures. The principal investigator will extend and apply relevant theory, and write computer codes for calculations necessary in the study of this problem.By edging its way into technology, superconductivity is achieving broader impacts beyond enhanced fundamental understanding. Examples of applications include strong superconducting magnets in medical magnetic resonance imagers (MRI), the Large Hadron Collider ring used for elementary particle study, ultra-sensitive superconducting quantum interference detectors, superconducting/magnetic technology levitating the world's fastest trains, and demonstration projects for power transmission to residential areas through superconducting cables. Further developments in superconducting materials and properties promise to accelerate the transition of innovations to public use. In addition to research, this award will support the training of young scientists for career paths in science & technology research, development, education, and administration areas.TECHNICAL SUMMARYThe theory of high-temperature superconductivity has been at the forefront of theoretical condensed-matter theory for six decades. The discovery of a new class of high-Tc superconductors moves the field to another, different level. One way it does so is by readjusting the basic scientific paradigm, which has been: experiment leads and stimulates theory. For these hydrides, theory (primarily, Ashcroft's work) appeared well before high-pressure techniques became able to test predictions. The electron-phonon mechanism for hydrogen sulfide and related hydrides might be claimed to be reasonably understood, but the most fundamental quantity -the perturbation caused by a moving proton and how it effects pairing of electrons on the Fermi surface, specifically the matrix element- is hidden inside sophisticated computer simulations without providing any understanding that would enable prediction and design of outstanding superconductors at higher temperature or lower pressure. This project proposes to complete this understanding by calculating and analyzing this scattering atom-by-atom of electrons at the Fermi surface by complementing the conventional theory of electronic linear response with numerical evaluation of the linear response. The added understanding will promote rational material design, versus Edisonian discovery, and a real prospect of even better superconductors by revealing the "genome" of the electron-phonon matrix element. Advances will be applicable to other superconducting materials challenges, especially that of single-layer iron selenide, where it is widely suspected that electron-phonon coupling enhances the underlying, magnetically based, pairing mechanism. Additionally, the very high Tc of hydrogen sulfide together with the electronic fine structure, an important feature that makes it special, presents new challenges in the simulation and basic understanding of behavior (electronic response) that extends beyond the conventional theory of this type of electron pairing. The results of this project are expected to impact the prediction, design, and control of new materials with transport properties required for specific applications, including semiconductors and semimetals, thermoelectric materials, and optimized magnetoelectronic coefficients. In addition to research, this award will support the training of young scientists for career paths in science & technology research, development, education, and administration areas.