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We study the spin-1 pyrochlore material NaCaNi₂F₇ [1] with a combination of molecular dynamics simulations, stochastic dynamical theory and linear spin wave theory. The dynamical structure factor from inelastic neutron scattering is well described by a near-ideal Heisenberg Hamiltonian incorporating small anisotropic terms and weak second-neighbor interactions [2]. We find that all three approaches reproduce remarkably well the momentum dependence of the scattering intensity and its energy dependence with the exception of the lowest energies. We find (i) a complete lack of sharp quasiparticle excitations in momentum space over much, if not all, of the energy range; (ii) linear spin-wave theory appears to apply in a regime where it would be expected to fail for a number of reasons. We elucidate what underpins these surprises, and note that basic questions about the nature of quantum spin liquidity in such systems pose themselves as a result [3].

[1] J. W. Krizan and R. J. Cava, Phys. Rev. B 92, 014406 (2015)
[2] K. W. Plumb et al., accepted Nat. Phys. (2018)
[3] S. Zhang, H. J. Changlani, K. W. Plumb, O. Tchernyshyov, R. Moessner, arXiv:1810.09481