Restoring thermalization in long-range quantum magnets with staggered magnetic fields

Quantum systems with strong long-range interactions are thought to robustly resist thermalization because their energy spectra organize into a discrete number of multiplets. We show that this nonthermal behavior is surprisingly fragile to an experimentally ubiquitous perturbation: a staggered magnetic field. Using self-consistent mean-field theory and exact diagonalization, we reveal that the energy spectrum, while composed of discrete subspaces or multiplets, collectively forms a dense spectrum. The equilibration time is independent of system size and depends only on the fluctuations in the initial state. For initial states at low to intermediate energies, the long-time average aligns with the Sz sector-weighted microcanonical ensemble. However, for states in the middle of the spectrum, the long-time average depends on the initial state due to quantum scarlike eigenstates localized at unstable points in classical phase space. Our results can be readily tested on a range of experimental platforms, including Rydberg atoms or optical cavities.