Pursuit of Quantum Spin Liquids in Exfoliated Anti-Ferromagnetic Insulators

摘要：导电材料显示出许多迷人的特性，但即使在导电停止时，其非凡的性能也会持续存在。例如，电子的行为可以大致像小条形磁铁，人们可以大致想象这种材料是由磁铁阵列组成的，固定在原地，但每个磁铁都在响应所有其他磁铁而旋转。这通常会导致磁体与其相邻磁体对齐。在极其罕见的情况下，量子力学抑制了这些电子磁体的排列，使它们的运动保持流畅；这种“量子自旋液体”是现代凝聚态研究中最受追捧的现象之一，可能是理解室温超导等潜在突破性技术的关键。最近，有人提出了一种量子自旋液体，这种液体是由层状的氯化钌片制成的大块材料，其中钌原子——每个都被许多氯包围——支持磁性行为。在这里，这种分层材料的独立薄片（低至只有一层厚）被用于使用二维物理领域的技术研究量子自旋液体。为了建立对磁性物理的直觉，在物理入门课程中使用了一个自由旋转棒磁铁的桌面系统作为演示。最后，这项工作通过首席研究员和布朗社会工作学院的教员之间的合作，支持圣路易斯地区非裔美国初高中女孩的拓展项目。技术摘要：量子自旋液体作为一种无法达到有序态的自旋集合，其动力学由量子涨落主导，是磁性量子物质中一个有趣且备受关注的相。二维基塔耶夫量子自旋液体（QSL）最近在层状反铁磁体RuCl3中被观察到。这项工作的重点是将RuCl3薄片剥离成几层和单层系统，通过消除层间相互作用来稳定QSL。在此过程中，几个重要的里程碑将导致研究量子磁性的新方法，特别是关注层状器件中的反铁磁邻近效应。特别是通常的磁性探测技术非常难以应用于微观样品，因此一套新的探针正在开发中，其中RuCl3薄片以及其他感兴趣的反铁磁性材料被放置在石墨烯上，目的是在石墨烯中诱导接近效应。石墨烯的电子特性受到反铁磁交换的影响，可以通过电子输运（量子霍尔效应和非局部输运）、光学（红外磁谱学）和热力学探针（电子可压缩性和磁化）来探索。采用电子方法测量微观薄片的磁化强度，为微观系统的热力学物理研究打开了一个新的窗口。通过对各种异质结构器件的研究，探讨了结构与磁性之间的联系，这些异质结构器件通过改变层的类型、厚度和堆叠顺序来控制层间磁耦合。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: Conducting materials display a host of fascinating properties, but remarkable behavior can persist even when conduction ceases. For example, electrons can behave roughly like small bar magnets, and one may roughly imagine the material as composed of arrays of magnets, fixed in place but each rotating in response to all the others. Usually this results in alignement of the magnets with their neighbors. In extremely rare cases, quantum mechanics inhibits the alignment of these electron magnets so their motion remains fluid; such a "quantum spin liquid" is one of the most highly sought after phenomena in modern condensed matter research and may hold the key to understanding potentially groundbreaking technologies like room temperature superconductivity. Recently, a quantum spin liquid has been proposed in a bulk material made of layered sheets of ruthenium chloride where the ruthenium atoms - each surrounded by many chlorines - support the magnetic behavior. Here, isolated individual flakes of this layered material (down to only one layer thick) are used to study the quantum spin liquid using techniques from the field of 2D physics. To build intuition for the physics of magnetism, a table-top system of freely rotating bar magnets is used as a demonstration in introductory physics courses. Finally, this work supports outreach programs to African-American middle and high school aged girls in the St. Louis area through a collaboration between the principal investigator and faculty in the Brown School of Social Work. Technical Abstract: Quantum spin liquids, as an ensemble of spins frustrated from achieving an ordered state, with dynamics dominated by quantum fluctuations, are an intriguing and highly sought after phase of magnetic quantum matter. The two-dimensional Kitaev quantum spin liquid (QSL) has recently been observed in the layered antiferromagnet RuCl3. This work focuses on exfoliation of RuCl3 flakes to few and single-layer systems seeking to stabilize the QSL by removing interlayer interactions. Several important milestones along the way will lead to new methods of investigating quantum magnetism, focusing in particular on antiferromagnetic proximity effects in layered devices. In particular the usual techniques to explore magnetism are extremely difficult to apply to microscopic samples, so a new set of probes is under development in which flakes of RuCl3, along with other antiferromagnetic materials of interest, are placed on graphene, with the goal of inducing a proximity effect in graphene. The electronic properties of graphene are impacted by the presence of antiferromagnetic exchange and readily explored via electronic transport (quantum Hall effects and non-local transport), optical (infrared magnetospectroscopy), and thermodynamic probes (electronic compressibility and magnetization). An electronic method of measuring magnetization in microscopic flakes is employed, opening a new window on thermodynamic physics of microscopic systems. The link between structure and magnetism is explored through studying a variety of heterostructure devices where the type, thickness, and stacking order of the layers is varied to control interlayer magnetic couplings.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.