Collective Modes and Electrodynamics of Interacting Spin Liquids

该奖项支持量子自旋液体的理论研究和教育。这些是磁性物质的独特状态，由元素磁矩（自旋）组成，但并不表现出整体的磁性。量子自旋液态中的自旋可以被描绘成一种永恒的舞蹈，总是以这样一种方式改变它们的方向，使得状态的总磁矩为零。确定量子自旋液体的实验可观察特征对于推进基础科学以及可能的新量子技术至关重要。当承载量子自旋液态的磁性材料受到外部磁场作用时，沿外加磁场方向和反外加磁场方向的自旋数量之间会产生有限的不平衡（磁化）。在这种条件下，量子自旋液体中出现了一种新型的动态磁振荡。PI将开发i)对该特征的完整理论描述，以及ii)如何将其用作真实材料实验中有价值的自旋液体检测工具。除了研究之外，该项目还包括对学生进行现代理论技术培训，并开设一门关于“现代材料物理学”的研究生课程。此外，PI将以现代量子磁学为主题开展公开讲座，旨在向公众介绍现代凝聚态物理学迄今取得的惊人进展。该奖项支持对量子自旋液体物理性质的理论研究，量子自旋液体是具有强纠缠自旋但没有静态磁序的磁性物质的独特状态。PI试图理解关键部分磁化量子自旋液体的独特可观测特征，这些特征支持新的横向集体自旋-1模式。这种新颖的集体自旋激励与涌现的规范波动相互作用，使其具有有限的寿命。PI计划发展这种集体模式的完整理论，包括它的色散、寿命和其他光谱特征，并确定实验观测所需的条件。狄拉克自旋液体的集体性质，其中自旋带形成相对论性锥色散，也将被研究。PI还计划将这种方法扩展到二维狄拉克材料石墨烯上，使其受到外部面内磁场的影响。泽曼场诱导的费米表面有望部分屏蔽电子之间的库仑相互作用，并再次促进集体自旋振荡。所有被期望拥有量子自旋液体基态的候选材料都会受到各种降低对称性的扰动。PI将研究这些各向异性之间的竞争和自旋子之间的规范场介导的相互作用。在具有强违反自旋守恒的系统中，如基塔耶夫自旋液体，自旋带结构及其与涌现规范场的相互作用通过光吸收和自旋磁共振表现出来，其理论也将得到发展。除研究外，该项目还包括培养学生的现代理论技术，并开设研究生课程“现代材料物理学”，从强自旋轨道和电子-电子相互作用的统一角度描述拓扑材料。此外，PI还将开发一个题为“自旋子：简史”的公开讲座，该讲座后来将变成一篇流行文章，主题是现代量子磁学。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 SUMMARYThis award supports theoretical research and education in quantum spin liquids. These are unique states of magnetic matter, comprised by elemental magnetic moments (spins), but which, however, do not exhibit overall magnetism. The spins involved in a quantum spin liquid state can be pictured as being in a perpetual dance, always changing their directions in such a way that the overall magnetic moment of the state is zero. Identifying experimentally observable features of quantum spin liquids is essential for advancing both fundamental science as well as possible new quantum technologies.When the magnetic material hosting a quantum spin liquid state is subjected to an external magnetic field, a finite imbalance (magnetization) between numbers of spins pointing along and against the direction of the applied field develops. Under this condition a new type of dynamic magnetic oscillation emerges in the quantum spin liquid. The PI will develop i) a complete theoretical description for this feature, and ii) how it can be used as a valuable spin-liquid detection tool in experiments with real materials.In addition to research, the project involves training students in modern theoretical techniques and developing a graduate course on the "Physics of modern materials". In addition, the PI will develop a public-level lecture on the topic of modern quantum magnetism, aiming to inform the public about the amazing progress made by modern condensed matter physics up to the present time.TECHNICAL SUMMARYThis award supports theoretical investigations of physical properties of quantum spin liquids, unique states of magnetic matter with strongly entangled spins but without static magnetic order. The PI seeks to understand unique observable features of critical partially magnetized quantum spin liquids, which support a new transverse collective spin-1 mode. This novel collective spin excitation interacts with emergent gauge fluctuations that provide it with a finite lifetime. The PI plans to develop the full theory of this collective mode, including its dispersion, lifetime, and other spectral characteristics, and to identify conditions needed for its experimental observation. Collective properties of Dirac spin liquids, in which spinon bands form a relativistic cone dispersion, will be investigated as well. The PI also plans to extend this approach to a two-dimensional Dirac material, graphene, subject to an external in-plane magnetic field. Zeeman-field-induced Fermi surfaces are expected to partially screen the Coulomb interaction between electrons and, again, promote collective spin oscillations.All candidate materials expected to host a quantum spin liquid ground state suffer from various symmetry-lowering perturbations. The PI will investigate the competition between such anisotropies and gauge-field-mediated interactions between spinons. In systems with strongly violated spin conservation, such as the Kitaev spin liquid, spinon band structures and their interactions with emergent gauge fields manifest via optical absorption and spinon magnetic resonance, the theory of which will be also developed.In addition to research, the project involves training students in modern theoretical techniques and developing a graduate course on the "Physics of modern materials", which will describe topological materials from a unified perspective of strong spin-orbit and electron-electron interactions. In addition, the PI will develop a public-level lecture titled "Spinon: a brief history", to be turned later into a popular article, on the topic of modern quantum magnetism.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.