SU(N) Quantum Magnets - Experiment, Theory and Open-Source Package

The conventional approach to understanding the physical properties of magnetic insulators typically involves substituting quantum spins with classical dipoles and treating the low-energy excitations as harmonic oscillations around the ground state. The quantum-mechanical characteristics of these spins are usually incorporated perturbatively through the 1/S expansion. To explore new physics beyond this conventional paradigm, significant attention has been directed toward spin-1/2 materials with quasi-low dimensionality in recent decades. In this talk, I will present a series of spectroscopic experiments conducted on a triangular-lattice spin-1 magnet, FeI2. These experiments reveal rich spectra of unconventional excitations, where the traditional semi-classical approximations break down qualitatively. A different approximation, based on SU(N) coherent states, proves to be the correct framework for a quantitative description of this material. In addition to providing a high-fidelity modeling of the excitation spectrum in the zero-temperature limit, this approach also qualitatively captures dynamic spin-correlations at arbitrary finite temperatures. To enhance the accessibility of this method for broader community, I will introduce an open-source package, sunny.jl, that implements SU(N) calculations of dynamic spin structure factors for a generic system. These calculations can be directly compared with experiments such as neutron scattering, making it a valuable addition in the toolkit of quantum magnetic materials research.