Time Resolved Probing of Unconventional Orders in Novel Kagome Metals

摘要：尽管近几十年来取得了巨大的进步，但基于硅的传统电子技术很快就会达到极限。解决这个问题的一种方法是使用具有非常规特性的新材料。在这方面，人们对一类新的金属产生了极大的兴趣，这种金属的原子以角共享三角形的周期性模式排列，称为Kagome晶格。这些系统中的电子以一种非常不寻常的方式表现出强烈的排斥，几何挫折和拓扑性质。当这些材料冷却时，首先它们进入一种电子密度周期性调节的状态（电荷密度波），在进一步冷却后，它们成为超导体。一个关键的挑战是理解这些阶段的机制以及两者之间的关系。该项目的目标是使用不同类型的光学和电子光谱来研究这些新型Kagome金属的电荷密度波状态。该项目的核心部分是培养下一代学生在先进光谱学和新型量子材料方面的能力。该项目还将为K-12科学教育做出贡献，并改善大一阶段的物理教育。技术摘要：近年来，人们发现了一类新的超导过渡金属Kagome化合物，它在多个序参量之间表现出复杂的相互作用。在超导相变之上，这些材料具有电荷密度波序和假定的短程手性通量相，其中反转和时间反转对称性都被打破。该项目旨在使用一组超快光学和电子探针来研究这些系统的高度非常规相图。利用静态和时间分辨Kerr旋转显微镜，在其他探针无法达到的长度尺度上研究了最近的μ子自旋弛豫测量所表明的时间反转对称性破断。利用圆偏振光对准反手性畴，研究了扫描隧道测量所得的手性电荷密度波阶。最后，利用超快电子衍射与时间和角度分辨光谱学相结合的方法来探测不同电荷密度波之间的竞争或合作，这将直接产生电子和晶格结构的动力学。这些实验将提供这些材料中微观相互作用的全面图景，并将阐明非常规秩序。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 Abstract:Despite tremendous advances in the recent decades, conventional electronics based on silicon will soon hit a limit. One way to circumvent this is to use novel materials with unconventional properties. In this respect, there has been a tremendous interest in a new class of metals in which the atoms are arranged in periodic patterns of corner sharing triangles known as the Kagome lattice. The electrons in these systems behave in a highly unusual manner displaying strong repulsion, geometric frustration and topological properties. When these materials are cooled, first they go into a state in which the electron density is periodically modulated (charge density wave) and upon further cooling, they become superconductors. A key challenge is to understand the mechanism of these phases and the relationship between the two. The goal of this project is to use different types of optical and electron based spectroscopies to investigate the charge density wave state of these novel Kagome metals. A central part of this project is the training of next generation of students both in advanced spectroscopies as well as in novel quantum materials. This program will also contribute to K-12 science education and improve the physics education at the freshman level.Technical Abstract:Recently, a new class of superconducting transition-metal Kagome compounds were discovered which display a complex interplay between multiple order parameters. Above the superconducting phase transition, these materials host a charge density wave order and a putative short-ranged chiral flux phase in which both inversion and time-reversal symmetry are broken. This project aims to use an array of ultrafast optical and electron probes to investigate the highly unconventional phase diagram of these systems. With static- and time-resolved Kerr rotation microscopy, the time-reversal symmetry-breaking suggested by recent muon spin relaxation measurements is studied at a length scale inaccessible to other probes. By aligning domains of opposite chirality with circularly polarized light, the chiral charge density wave order proposed by scanning tunneling measurements is investigated with second harmonic generation. Finally, the competition or cooperation between different charge density waves will be probed by a combination of ultrafast electron diffraction and time- and angle-resolved photoemission spectroscopy which will directly yield dynamics of electronic and lattice structure. These experiments will provide a comprehensive picture of the microscopic interactions in these materials and will shed light on to the unconventional orders.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.