Time Resolved Probing of Unconventional Orders in Novel Kagome Metals

非技术摘要：尽管近几十年来取得了巨大的进步，但基于硅的传统电子产品很快就会达到极限。规避这一问题的一种方法是使用具有非传统性能的新型材料。在这方面，人们对一类新的金属产生了极大的兴趣，在这种金属中，原子以被称为Kagome晶格的角共享三角形的周期图案排列。这些体系中的电子表现出极不寻常的行为，表现出强烈的排斥力、几何受挫和拓扑性质。当这些材料被冷却时，它们首先进入电子密度被周期性调制的状态(电荷密度波)，在进一步冷却时，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仪来研究这些新型Kagome金属的电荷密度波态。该项目的一个核心部分是对下一代学生进行高级光谱学和新型量子材料方面的培训。这项计划还将有助于K-12科学教育和提高大学一年级的物理教育水平。技术摘要：最近，一类新的超导过渡金属Kagome化合物被发现，它显示出多个有序参数之间复杂的相互作用。在超导相变的上方，这些材料具有电荷密度波序和假定的短程手征磁通相，其中反转和时间反转对称性都被打破。该项目旨在使用超快光学和电子探头阵列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，在其他探测器无法达到的长度尺度上研究了最近的Muon自旋弛豫测量所揭示的时间反转对称破缺。通过将具有相反手性的磁区与圆偏振光对齐，研究了二次谐波产生时扫描隧道测量提出的手征电荷密度波序。最后，用超快电子衍射和时间和角分辨光电子能谱相结合的方法来探索不同电荷密度波之间的竞争或合作，这将直接产生电子结构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，并将阐明非常规订单。该奖项反映了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。

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.