New Probes of Strong Interactions in Quantum Matter

通过使用超导导线来消除电网中导电效率低下的问题可以节省数十亿美元。德国埃森的AmpaCity项目就是这样一个例子。然而，实现这一目标的一个主要障碍是如何设计出工作温度越来越接近室温的新型超导体。近年来的一个惊喜是在与优质金属毫无共同之处的材料中发现了高温超导性。因此，理解这些材料的超导性是无法用固态物理的标准构建块来完成的。该奖项支持研究和教育，以发展理论模型来理解这种超导体。PI将重点分析最近的一类新实验，这些实验对铜超导体的基本组成部分提供了前所未有的见解。除了培训和指导研究生外，PI还将继续开展广泛的教育和推广活动，包括在当地中学举办讲座和公开讲座，以提高公众对物理学的认识。技术总结：虽然这种奇怪的金属一直是发展铜离子超导理论的基本障碍，但旨在揭示这种物质状态下的普遍激发的实验直到现在还不存在。新的实验，动量分辨电子能量损失谱（MEELS），测量了两粒子的磁化率，从而获得了构建介电函数和单粒子电子自能所需的所有多体信息。该奖项支持从MEELS数据中提取此类信息的研究和教育。将特别强调与光学测量的比较。光学电导率实验测量的是横向介电响应，而MEELS测量的是纵向介电函数。一个关键的悬而未决的问题是，这两者是否相等。当然，它们在零动量传递的极限下是相等的，因为纵向和横向介电函数之间的任何区别在这个极限下消失了。然而，PI对实验数据的分析表明，情况并非如此。对其他材料，如二氯化钽、钌酸盐，甚至铁镍超导体的同样分析表明，在横向和纵向介电函数的零动量极限之间完全一致。由于这是这两个极限不一致的第一个实例，铜合金代表了一种前所未有的材料。理解这种差异是这个项目的核心。由此产生的关键问题是：这两个极限之间的差异是奇异金属物理学的核心吗？将研究特定的模型以获得两个极限之间的差异，并使用这些模型来检查它们是否同时解释了奇怪金属的关键特征，即t -线性电阻率。除了培训和指导研究生外，PI还将继续开展广泛的教育和推广活动，包括在当地中学举办讲座和公开讲座，以提高公众对物理学的认识。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。

NONTECHNICAL SUMMARYEliminating inefficiencies in electrical conduction in the power grid by the use of superconducting wires could result in the saving of billions of dollars. The AmpaCity project in Essen Germany is one such example. However, a major obstacle to achieving that goal is understanding how to design new superconductors that work increasingly close to room temperature. A surprise in recent years was the discovery of high-temperature superconductivity in materials that have nothing in common with good metals. Consequently, understanding how such materials superconduct cannot be done with the standard building blocks of solid-state physics. This award supports research and education towards developing theoretical models to understand such superconductors. The PI will focus on analyzing a new class of recent experiments that offer unprecedented insight into the fundamental building blocks of the cuprate superconductors.In addition to training and mentoring graduate students, the PI will continue broad educational and outreach activities that include giving lectures to local middle-schools and public lectures to raise public awareness of physics.TECHNICAL SUMMARYWhile the strange metal has always stood as the basic obstacle in developing a theory of superconductivity in the cuprates, experiments designed to reveal the universal excitations that underly this state of matter have been nonexistent until now. The new experiments, momentum-resolved electron-energy loss spectroscopy (MEELS), measure the two-particle susceptibility and hence have all the many-body information needed to construct the dielectric function and also the single-particle electron self-energy. This award supports research and education towards extracting such information from the MEELS data. Special emphasis will be placed on comparison with optical measurements. Optical conductivity experiments measure the transverse dielectric response, while MEELS measures the longitudinal dielectric function. A key unanswered question is whether the two are equal. Certainly, they are expected to be equal in the limit of zero-momentum transfer, as any distinction between longitudinal and transverse dielectric functions disappears in this limit. However, the PI's analysis of the experimental data shows that this is not the case. The same analysis on other materials such as tantalum-diselinide, ruthenates, and even iron-pnictide superconductors reveals complete agreement between zero-momentum limits of the transverse and longitudinal dielectric functions. As this is the first instance where these two limits disagree, the cuprates represent an unprecendeted class of materials. Understanding of this discrepancy is at the heart of this project. The key question that arises is: Does the discrepancy between these two limits stand at the heart of strange-metal physics? Specific models will be studied to obtain a discrepancy between the two limits, and these models will be used to examine if they simultaneously explain the key characteristic of the strange metal, namely T-linear resistivity.In addition to training and mentoring graduate students, the PI will continue broad educational and outreach activities that include giving lectures to local middle-schools and public lectures to raise public awareness of physics.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.