Dynamical structure factor of the three-dimensional quantum spin liquid candidate NaCaNi$_2$F$_7$

TL;DR

This study investigates the dynamical properties of the NaCaNi$_2$F$_7$ quantum spin liquid candidate using multiple theoretical approaches, revealing a lack of sharp quasiparticle excitations and challenging existing theoretical expectations.

Contribution

It demonstrates that a near-ideal Heisenberg Hamiltonian accurately describes the dynamical structure factor, and shows linear spin-wave theory's unexpected applicability in this quantum spin liquid.

Findings

No sharp quasiparticle excitations observed in momentum space.

Linear spin-wave theory applies unexpectedly well.

Dynamical structure factor matches experimental data with small anisotropic terms.

Abstract

We study the spin-1 pyrochlore material NaCaNi$_2$F$_7$ 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 with a near-ideal Heisenberg Hamiltonian incorporating small anisotropic terms {and weak second-neighbor interactions}. We find that all three approaches reproduce remarkably well the momentum dependence of the scattering intensity as well as its energy dependence with the exception of the lowest energies. These results are notable in that (i) the data show 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.