Collective Modes and Electrodynamics of Interacting Spin Liquids

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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.