Defect driven thermalization of magnets with scars and Hilbert space fragmentation*

Clean isolated quantum systems, either thermalize or exhibit athermal behavior for a wide variety of initial conditions as in the case of quantum many body scars. We recently predicted the existence of scars (with a superspin structure) and Hilbert space fragmentation in a family of magnetic models with XXZ-Heisenberg interactions [Lee et al. Phys. Rev. B 101, 241111(R) (2020), Lee et al. Phys. Rev. B 103, 235133 (2021)], which have been realized in recent ultracold-atom experiments [Jepsen et al.,Nature Physics 18, 899–904 (2022)]. In this work, we study the robustness of the long-lived superspin by introducing ``defects" in the initially prepared spin texture - either by driving the system locally, misaligning the local spin orientations or by entangling spins. Using a combination of analyses based on the local density of states of the energy or Floquet spectrum and the effective Floquet Hamiltonian, and employing matrix product based TEBD simulations, we suggest ways of predicting the occurrence of athermal (or prethermal) dynamics. We identify dynamical parameter regimes and protocols that are robust to defects, and make predictions that may be of interest to future experiments.