Extended nonergodic regime and spin subdiffusion in disordered SU(2)-symmetric Floquet systems

TL;DR

This paper investigates a disordered SU(2)-symmetric Floquet system revealing an extended nonergodic phase with subdiffusive spin transport, despite the absence of many-body localization.

Contribution

It uncovers a nonergodic regime in a disordered SU(2) symmetric Floquet model where traditional localization does not occur, characterized by nonthermal eigenstates and subdiffusive dynamics.

Findings

Level statistics deviate from Wigner-Dyson and Poisson distributions.

Spectral form factor lacks linear growth at early times.

Eigenstates exhibit subthermal entanglement entropy.

Abstract

We explore thermalization and quantum dynamics in a one-dimensional disordered SU(2)-symmetric Floquet model, where a many-body localized phase is prohibited by the non-abelian symmetry. Despite the absence of localization, we find an extended nonergodic regime at strong disorder where the system exhibits nonthermal behaviors. In the strong disorder regime, the level spacing statistics exhibit neither a Wigner-Dyson nor a Poisson distribution, and the spectral form factor does not show a linear-in-time growth at early times characteristic of random matrix theory. The average entanglement entropy of the Floquet eigenstates is subthermal, although violating an area-law scaling with system sizes. We further compute the expectation value of local observables and find strong deviations from the eigenstate thermalization hypothesis. The infinite temperature spin autocorrelation function decays at long times as $t^{-\beta}$ with $\beta < 0.5$, indicating subdiffusive transport at strong disorders.