Exploration of the existence of a distinct quasi many-body localized phase: Numerical study of a translationally invariant system in the thermodynamic limit

We consider a quench in an infinite spin ladder describing a system with two species of bosons in the limit of strong interactions. If the heavy bosonic species has infinite mass, the model becomes a spin chain with quenched binary disorder which shows true Anderson localization (AL) or many-body localization (MBL). For finite hopping amplitude ${J}^{\ensuremath{'}}$ of the heavy particles, on the other hand, we find an exponential polarization decay with a relaxation rate which depends monotonically on ${J}^{\ensuremath{'}}$. Furthermore, the entanglement entropy changes from a constant (AL) or logarithmic (MBL) scaling in time $t$ for ${J}^{\ensuremath{'}}=0$ to a subballistic power law, ${S}_{\mathrm{ent}}\ensuremath{\sim}{t}^{\ensuremath{\alpha}}$ with $\ensuremath{\alpha}<1$, for finite ${J}^{\ensuremath{'}}$. We do not find a distinct regime in time where the dynamics for ${J}^{\ensuremath{'}}\ensuremath{\ne}0$ shows the characteristics of an MBL phase. Instead, we discover a time regime with distinct dephasing and entanglement times, leading to dynamics in this regime which is different both from a localized and from a fully ergodic phase.