Many-Body Localization and Level Repulsion.

Insertion of disorder in thermal interacting quantum systems decreases the amount of level repulsion and can turn them into many body localized phases. In this paper we use the many body picture to perturbatively study the effect of level repulsion in the localized phase. We find that most eigenstates can be described accurately in an approximate way, including many with rare resonances. A classification of the rare resonances shows that most types are exponentially rare and requires exponential fine tuning in an approximate description. The classification confirms that no rare thermal eigenstates can exist in a fully localized phase and we argue that all types of resonances need to become common if a continuous transition into a thermal phase should occur.