Absence of Thermalization in Finite Isolated Interacting Floquet Systems

TL;DR

This paper demonstrates that finite, clean, periodically driven quantum systems can exhibit non-thermal behavior due to emergent integrability, challenging the expectation of thermalization in such systems.

Contribution

It reveals non-thermal Floquet eigenstates in clean systems caused by emergent integrability, even without disorder or static interactions.

Findings

Non-thermal Floquet eigenstates exist in clean systems.

Emergent integrability explains non-thermal behavior.

Non-thermal features diminish with increasing system size.

Abstract

Conventional wisdom suggests that the long time behavior of isolated interacting periodically driven (Floquet) systems is a featureless maximal entropy state characterized by an infinite temperature. Efforts to thwart this uninteresting fixed point include adding sufficient disorder to realize a Floquet many-body localized phase or working in a narrow region of drive frequencies to achieve glassy non-thermal behavior at long time. Here we show that in clean systems the Floquet eigenstates can exhibit non-thermal behavior due to finite system size. We consider a one-dimensional system of spinless fermions with nearest-neighbor interactions where the interaction term is driven. Interestingly, even with no static component of the interaction, the quasienergy spectrum contains gaps and a significant fraction of the Floquet eigenstates, at all quasienergies, have non-thermal average doublon densities. We show that this non-thermal behavior arises due to emergent integrability at large interaction strength and discuss how the integrability breaks down with power-law dependence on system size.