RCQM/FCMP:Cristian D.Batista: Nonperturbative Semiclassical Spin Dynamics for Ordered Quantum Magnet

Tuesday, Sep./30/2025, 2:00 PM to 3:30 PM (Houston)
Speaker: Cristian D. Batista
Institution: Lincoln Professor of Physics, University of Tennessee

Title: Nonperturbative Semiclassical Spin Dynamics for Ordered Quantum Magnets

Abstract: I will present a nonperturbative semiclassical framework for computing spin dynamics in ordered quantum magnets when interactions dominate over kinetic energy. The talk opens with an overview of the SU(N) coherent-state dynamics, which is a generalization of the Landau–Lifshitz dynamics, and clarify how classical trajectories encode quantum spectra. I will then use FeI2 as a case study in which strong interactions reshape the excitation landscape—producing hybridized dipolar–quadrupolar modes, spontaneous magnon decay, and multi-magnon bound states that hybridize with single magnons and redistribute spectral weight observed by inelastic neutron scattering and terahertz time-domain spectroscopy. The focus is our recent work introducing a genuinely nonperturbative strategy based on exact diagonalization in a carefully truncated Hilbert space, quantitatively benchmarked against matrix-product-state simulations. Applied to the triangular-lattice antiferromagnet at the one-third magnetization plateau, this approach captures two-magnon bound states, resonance-enhanced continua from bound-state/continuum hybridization, and overlapping bound states embedded within the continuum—phenomena inaccessible to standard loop expansions yet quantitatively comparable with experiment. I will conclude by outlining how this toolbox, together with SU(N) semiclassics, provides an end-to-end route—from microscopic model inference to predictive calculations of dynamical response—that we are implementing in the Sunny platform to enable reproducible, high-performance simulations.

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