Initial state dependence of the quench dynamics in integrable quantum systems. III. Chaotic states

TL;DR

This paper investigates how the initial state, whether integrable or chaotic, influences the thermalization process in quantum quenches within integrable systems, revealing that chaotic initial states promote ergodic behavior and thermalization.

Contribution

It demonstrates that initial states from nonintegrable (chaotic) Hamiltonians lead to ergodic sampling and thermalization in integrable quenches, contrasting with integrable initial states.

Findings

Chaotic initial states induce ergodic sampling of eigenstates.

Nonintegrable initial states lead to thermal distributions of conserved quantities.

Thermalization is more likely when initial states are from nonintegrable Hamiltonians.

Abstract

We study sudden quantum quenches in which the initial states are selected to be either eigenstates of an integrable Hamiltonian that is nonmappable to a noninteracting one or a nonintegrable Hamiltonian, while the Hamiltonian after the quench is always integrable and mappable to a noninteracting one. By studying weighted energy densities and entropies, we show that quenches starting from nonintegrable (chaotic) eigenstates lead to an "ergodic" sampling of the eigenstates of the final Hamiltonian, while those starting from the integrable eigenstates do not (or at least it is not apparent for the system sizes accessible to us). This goes in parallel with the fact that the distribution of conserved quantities in the initial states is thermal in the nonintegrable cases and nonthermal in the integrable ones, and means that, in general, thermalization occurs in integrable systems when the quench starts form an eigenstate of a nonintegrable Hamiltonian (away from the edges of the spectrum), while it fails (or requires larger system sizes than those studied here to become apparent) for quenches starting at integrable points. We test those conclusions by studying the momentum distribution function of hard-core bosons after a quench.