Exploring The Topological Magnetic Excitation Spectrum Of S=half MOFs With Inelastic Neutron Scattering

金属有机框架（MOFs）是研究界的一个热门话题，涉及一系列学科，包括固态化学，凝聚态物理和材料科学。在我的项目中，kagomé MOFs提供了一条有价值的途径来综合实现二维磁性模型，同时由于避免原子位置无序的能力，它仍然优于纯无机系统。在我们的kagomé系统中，磁性来自形成kagomé晶格角的Cu 2+离子，使我们的系统S=一半。当整个层彼此平行排列时，这样的系统是铁磁（FM）的。反铁磁性（AFM）通常是当力矩彼此反平行排列时，尽管由于磁阻挫，kagomé网的结构使得这很困难。以S=1/2 kagomé AFM为例，在基态下磁受挫的材料被称为量子自旋液体（QSL）。平面内FM或AFM相互作用可以为系统提供特征拓扑磁激发谱，从而产生有趣的性质，例如磁振子霍尔效应和手征边缘模式。然而，该光谱可能受到面间相互作用的影响，因此它对我们处理MOF特别有用，因为改变连接体可能影响这些面间相互作用的性质和强度以及存在。目前，这一领域的研究仍处于“蓝天”阶段，尽管人们认为这些系统可用于量子计算中的电子和自旋电子器件，以实现低能量数据存储。虽然第一个S= half kagomé FM已经在过去的十年中合成，充分了解拓扑磁激发谱尚未实现，仍然是我目前研究的目标之一。QSL特别难以合成，迄今为止还没有实验实现这种类型的系统。克拉克小组已经合成了一个S=半kagomé AFM MOF，我们认为这是一个很好的QSL候选者，尽管需要几种互补的技术来明确地确定这一点，这将在适当的时候形成我的研究的一部分。这些技术包括X射线和中子衍射、μ子光谱学、非弹性中子散射和磁力测量，迄今为止，我研究的主要内容包括分析在国际设施ISIS进行的非弹性中子散射测量的初步数据集。INS可以揭示材料的磁性能带结构信息。这类似于电子能带结构如何影响材料的性质。例如，电子能带结构中的带隙产生具有绝缘性质的材料，因为电子不能跨越该距离跳跃以传导由这些电子携带的电荷。此外，相互之间的磁性相互作用的相对大小和性质（FM或AFM）可以影响该光谱的外观，因此可以告诉我们所显示的磁性的性质。这种分析需要构建一个Python代码来将我们的数据拟合到给定的模型中，并使用称为SpinW的MATLAB库与模拟进行比较。我还将直接参与这些系统的合成，以便在这些国际设施上进行束流时间实验。

Metal-Organic Frameworks (MOFs) are a hot topic in the research community across a range of disciplines including solid state chemistry, condensed matter physics and materials science. In my project, kagomé MOFs offer a valuable pathway to synthetically realise two-dimensional magnetic models whilst simultaneously remaining advantageous over purely inorganic systems due to the ability to avoid the atomic site disorder. In our kagomé systems, the magnetism arises from Cu2+ ions forming the corners of the kagomé lattice, making our systems S=half. Such systems are ferromagnetic (FM) when entire layers align parallel with one another. Antiferromagnetism (AFM) is typically when moments align antiparallel to one another, although the structure of the kagomé net renders this difficult due to magnetic frustration. Materials that are magnetically frustrated in their ground state, with S=1/2 kagomé AFMs as an example, are known as quantum spin liquids (QSLs). The in-plane FM or AFM interactions can provide the system with a characteristic topological magnetic excitation spectrum, giving rise to interesting properties such as the magnon Hall effect and chiral edge modes. However, this spectrum can be affected by interplanar interactions and therefore it is particularly useful for us to deal with MOFs as varying the linker can influence the nature and strength and presence of these interplanar interactions. Currently, research in this area remains in its 'blue skies' phase, although it is thought that these systems can be used for electronic and spintronic devices in quantum computing for low energy data storage. Although the first S= half kagomé FM has been synthesised in the past decade, fully understanding the topological magnetic excitation spectrum is yet to be achieved and remains one of the aims of my current research. QSLs are particularly elusive to synthesise, and no system of this type has been experimentally realised to date. The Clark group has synthesised a S=half kagomé AFM MOF which we believe is a good QSL candidate, although several complimentary techniques are required to unambiguously determine this, which will form part of my research in due course. Such techniques include x-ray and neutron diffraction, muon spectroscopy, inelastic neutron scattering (INS) and magnetometry measurements.The major component of my research so far has included analysing preliminary data sets from INS measurements taken at the international facility, ISIS. INS can reveal information about the magnetic band structure of a material. This is analogous to how electronic band structure can influence the properties of a material. For example, a band gap in the electronic band structure gives rise to a material having insulating properties as electrons cannot hop across this distance to conduct the charge carried by these electrons. Furthermore, the relative size and nature (FM or AFM) of the magnetic interactions with one another can impact the appearance of this spectrum and therefore tell us about the nature of the exhibited magnetism. This analysis has required constructing a Python code to fit our data to a given model and comparing to simulations using a MATLAB library known as SpinW. I will also be directly involved in the synthesis of these systems to take them on beamtime experiments at these international facilities.