Collaborative Research: Frustration, glassiness and spin liquids: from dirty to pristine materials

目前，全球大量的实验努力都是为了寻找量子自旋液体（QSLs），这是一种理论上预测的磁性材料状态，其中自旋或原子大小的条形磁铁在尽可能低的温度下表现出类似液体的性质。qsl可以有效地用于创建量子位，“量子位”，因为它们可以存储在多个自旋状态下同时编码的信息，因此可以免受局部噪声的影响。因此，qsl作为未来计算和通信技术的平台具有很大的前景。目前实现它们的障碍是随机定位的杂质和缺陷，它们可以将QSL转化为另一种物质状态，“自旋玻璃”，其中自旋以随机方向冻结，类似于硅原子在窗户玻璃中具有随机位置。本研究调查了杂质和缺陷对几何挫折磁体基本特性的影响，这是正在寻找qsl的最大一类材料。通过系统地控制材料合成过程中的缺陷，并对由此产生的行为进行理论建模，该团队正在为未来基于qsl的设备的工程奠定基础。这项研究的更广泛的影响是开发了减少晶体样品缺陷的合成方法，并为未来的加工工作提供了理论认识。此外，这项研究还有助于培训直接参与该项目的初级研究人员，以及通过教学媒体和开放源代码平台（如NSF的晶体样本档案）上可访问的数据，培训更广泛的社区。技术摘要本研究是实验与理论相结合的研究，系统地研究了淬火无序对几何挫折（GF）材料集体磁性的影响，并确定了获得纯材料的合成障碍。该研究包括合成跨越整个自旋密度范围的材料，从稀自旋到集中无序自旋，再到稀杂质，最后到超纯材料。部分工作使用淬火无序作为GF磁性基本性质的探测。无序GF材料热力学性质的测量旨在揭示GF系统的微观哈密顿量，揭示无序诱导的自旋玻璃跃迁的可能的普适类别，研究这些系统中最近发现的“隐藏能量尺度”的起源，并揭示其中基本激发的本质。工作包括理论计算哈密顿量微观细节的热力学表现，探索GF介质中不同类型无序的相互作用，建立“隐藏能量尺度”和GF磁体热容行为模型，用于预测和解释实验数据。本研究的部分结果可能是获得表现出强量子自旋液体行为的材料，并证明其中的淬火无序无关。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。

Non-technical abstractCurrently, immense global experimental efforts are directed at finding quantum spin liquids (QSLs), a theoretically-predicted state of magnetic materials in which the spins, or atom-size bar-magnets, exhibit liquid-like properties down to the lowest possible temperatures. QSLs can be efficiently used to create quantum bits, “qubits”, as they can store information encoded simultaneously in the states of multiple spins, and hence protected from local noise. Thus, QSLs hold great promise as a platform for future computing and communication technology. A present barrier to their realization is randomly located impurities and defects that can convert a QSL into another state of matter, “spin glass”, in which the spins freeze in random orientations, similar to how silicon atoms possess random positions in window glass. This research investigates the effect of impurities and defects on the fundamental properties of geometrically frustrated magnets, the largest class of materials in which QSLs are being sought. Through systematically controlling defects during material synthesis, and theoretically modeling the resulting behavior, the team is laying the foundations for future engineering of QSL-based devices. The broader impacts of this research are both the development of synthesis methods for reducing defects in crystalline specimens and the theoretical understanding that will inform future processing endeavors. In addition, the research helps train junior researchers directly involved in the project, as well as the broader community through instructional media and data that are accessible on open-source platforms such as the NSF’s Crystal Sample Archive. Technical abstractThis research is a combined experimental and theoretical effort to systematically investigate the effect of quenched disorder on collective magnetism in geometrically frustrated (GF) materials and to determine the synthesis barriers to obtaining pure materials. The research includes synthesis of materials spanning the entire range of spin density from dilute spin, to concentrated disordered spins, to dilute impurities, en route to ultra-pure materials. Part of the work uses quenched disorder as a probe of fundamental properties of GF magnetism. The measurements of the thermodynamic properties of disordered GF materials aim to uncover microscopic Hamiltonians of GF systems, reveal possible universality classes of disorder-induced spin-glass transitions, investigate the origin of the recently discovered “hidden energy scale” in these systems and reveal the nature of elementary excitations in them. The work includes theoretical calculations of the thermodynamic manifestations of the microscopic details of the Hamiltonians, exploring the interplay of different types of disorder in GF media and constructing models for the “hidden energy scale” and the behavior of heat capacity in GF magnets, which is used to make prediction and explain experimental data. Part of this research may result in obtaining materials that exhibit strong quantum-spin-liquid behavior and demonstrating the irrelevance of quenched disorder in them.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.