Dynamics of a pyrochlore quantum spin liquid NaCaNi₂F₇

The Heinsenberg antiferromagnet on a pyrochlore lattice is a well-known model realizing a classical spin-liquid state at low temperatures [1]. Much less is known about the quantum version of this model. The spin-1 pyrochlore material NaCaNi₂F₇ is well described by a weakly perturbed Heisenberg Hamiltonian [2]. It shows no magnetic order down to extremely low temperatures, making it a prime candidate for a three-dimensional quantum spin liquid. Inelastic neutron scattering has revealed a lack of magnetic Bragg peaks and a broad continuum of excitations incompatible with a conventional magnetically ordered state. We combine analytical theory and numerical approaches to elucidate the nature of this enigmatic spin-liquid state [3]. The three approaches---molecular dynamics simulations, stochastic dynamical theory and linear spin wave theory---reproduce remarkably well the energy and momentum dependence of the experimental inelastic neutron scattering intensity with the exception of the lowest energies. Our study offers the following picture of spin dynamics in this system. This frustrated magnet is slowly moving through a (very large) manifold of degenerate ground states lacking long-range order. This slow motion is driven by medium- and high-energy spin waves with Bose statistics. We discuss two surprising aspects and their implications for quantum spin liquids in general: the complete lack of sharp quasiparticle excitations in momentum space and the success of the linear spin wave theory. Similar conclusions have been reached independently about a S=3/2 "pyrochlore" antiferromagnet MgCr₂O₄ [4].

[1] R. Moessner and J. T. Chalker, Phys. Rev. Lett. 80, 2929 (1998).
[2] K.W. Plumb et al., Nat. Phys. 15, 54 (2019).
[3] S. Zhang et al., Phys. Rev. Lett. 122, 167203 (2019).
[4] X. Bai et al., Phys. Rev. Lett. 122, 097201 (2019).