CAREER: Investigation of Spatial and Temporal Scales of Cooperative Spin Clusters in Doped and Undoped Geometrically Frustrated Magnets

***技术摘要***本提案是利用磁与空间和时间分辨光学技术相结合的方法对几何受挫磁系统的基本方面进行研究。受挫的一个重要结果是形成自旋“保护体”的能力——自旋簇与系统的其余部分解耦。将研究的具体问题是：(a)化学无序的影响：孤立自旋保护体的形成归因于晶体结构中空位相关缺陷周围的成核。将研究化学失序（掺杂）引起的有意晶格畸变对失序的影响。(b)动力学：失序磁体中的自旋团簇在广泛的时间尺度上表现出波动，随温度和失序而变化。他们的集合动力学将被研究到皮秒以下，以跟踪受挫行为的发生和发展。(c)相关长度：随着受挫改变能量景观和自旋相关长度，保护体的簇大小发生了实质性变化。本文将对这些保护层的形成和演化进行研究。这项研究将结合不同领域（光学、磁学和低温物理）的流行技术，结果将导致对复杂协同磁体中新的自旋相关现象及其影响因素的理解。***挫折是指系统无法达到一个独特的基态。它在从蛋白质折叠到纳米团簇形成的各种系统中发挥着重要作用。一组有趣的系统是具有独特晶格对称性的磁性化合物，其中磁自旋在几何上受到挫折。该建议将通过结合磁性和高分辨率光学技术对其基本方面的研究，为几何挫折磁系统的研究带来新的视角。将特别强调测量与挫折的发生和发展有关的空间和时间尺度。光学探针的使用将允许首次直接测量受挫系统中的自旋动力学，并且能够访问任何其他现有技术无法实现的时间和长度尺度。该结果将导致对在受挫系统中观察到的复杂集体行为的一般见解，这应该适用于各种领域，如量子计算和信息处理。由于加州大学默塞德分校位于中央山谷的中心位置，为吸引和留住未被充分代表的科学和技术学生提供了一种手段，因此该提案将通过组建一个专注于科学和工程的女性小组，加强少数群体，特别是女性在前沿研究中的参与。它也将导致一个“发现导向”的实验课程的发展，特别强调凝聚态和纳米科学的主题，为本科和研究生物理课程。

*** Technical Abstract ***This proposal is an investigation of the fundamental aspects of geometrically frustrated magnetic systems using a combination of magnetic and spatially and temporally-resolved optical techniques. An important consequence of frustration is the ability to form spin 'protectorates'-spin clusters that are decoupled from the rest of the system. The specific issues that will be investigated are: (a) Effect of chemical disorder: The formation of isolated spin protectorates is attributed to nucleation around vacancy-related defects in the crystal structure. The effects of intentional lattice distortion induced by chemical disorder (doping) on frustration will be investigated, (b) Dynamics: Spin clusters in frustrated magnets exhibit fluctuations over a broad range of time scales, varying with temperature and disorder. Their ensemble dynamics will be investigated down to picoseconds to follow the onset and development of frustrated behavior, and (c) Correlation length: The cluster size of the protectorates changes substantially as frustration alters the energy landscape and the spin correlation lengths. The formation and evolution of these protectorates will be studied. This research will combine techniques prevalent in distinct fields (optics, magnetism, and low temperature physics) and the results will lead to the understanding of novel spin related phenomena, and the factors that influence them, in complicated cooperative magnets. *** Non-technical Abstract ***Frustration is the inability of a system to reach a unique ground state. It plays an important role in a wide variety of systems, ranging from protein folding to the formation of nanoclusters. An intriguing set of such systems is magnetic compounds with unique lattice symmetries where the magnetic spins are geometrically frustrated. This proposal will bring a new perspective to the study of geometrically frustrated magnetic systems via the investigation of their fundamental aspects using a combination of magnetic and high-resolution optical techniques. Particular emphasis will be given to the measurement of spatial and temporal scales relevant to the onset and development of frustration. The use of optical probes will allow the first direct measurement of spin dynamics in frustrated systems, and will be able to access time and length scales unattainable by any other existing techniques. The results will lead to general insights into the complicated collective behavior observed in frustrated systems that should be applicable to a variety of fields, such as quantum computing and information processing. Since the location of UC Merced at the heart of the Central Valley offers a means to attract and retain underrepresented students to science and technology, this proposal will enhance participation of minority groups, particularly women, in cutting-edge research through the formation of a focused Women in Science and Engineering group. It will also lead to the development of a 'discovery-oriented' laboratory course with special emphasis on topics of condensed matter and nanoscience for both undergraduate and graduate physics curriculum.