Thermalization of Dilute Impurities in One-Dimensional Spin Chains

We analyze a crossover between ergodic and nonergodic regimes in an interacting spin chain with a dilute density of impurities, defined as spins with a strong local field. The dilute limit allows us to unravel some finite size effects and propose a mechanism for the delocalization of these impurities in the thermodynamic limit. In particular, we show that impurities always relax by exchanging energy with the rest of the chain. The relaxation rate only weakly depends on the impurity density and decays exponentially, up to logarithmic corrections, with the field strength. We connect the relaxation to fast operator spreading and show that the same mechanism destabilizes the recursive construction of local integrals of motion at any impurity density. In the high field limit, impurities appear to be localized, and the system is nonergodic, over a wide range of system sizes. However, this is a transient effect, and the eventual delocalization can be understood in terms of a flowing localization length.