Krylov space dynamics of ergodic and dynamically frozen Floquet systems

TL;DR

This paper introduces a Krylov space approach to analyze the long-time behavior of periodically driven quantum many-body systems, providing a new framework and efficient algorithm to study thermalization, dynamical freezing, and eigenstate properties.

Contribution

It presents a novel Krylov space perspective on Floquet thermalization, enabling efficient numerical evaluation of infinite-time averages and insights into ergodic and frozen phases.

Findings

Successfully applied to a 30-spin Ising model

Resolved the transition between ergodic and frozen phases

Linked long-time behavior to Krylov eigenstate localization

Abstract

In isolated quantum many-body systems periodically driven in time, the asymptotic dynamics at late times can exhibit distinct behavior such as thermalization or dynamical freezing. Understanding the properties of and the convergence towards infinite-time (nonequilibrium) steady states however remains a challenging endeavor. We propose a physically motivated Krylov space perspective on Floquet thermalization which offers a natural framework to study rates of convergence towards steady states and, simultaneously, an efficient numerical algorithm to evaluate infinite-time averages of observables within the diagonal ensemble. The effectiveness of our algorithm is demonstrated by applying it to the periodically driven mixed-field Ising model, reaching system sizes of up to 30 spins. Our method successfully resolves the transition between the ergodic and dynamically frozen phases and provides insight into the nature of the Floquet eigenstates across the phase diagram. Furthermore, we show that the long-time behavior is encoded within the localization properties of the Ritz vectors under the Floquet evolution, providing an accurate diagnostic of ergodicity.