Floquet thermalization by power-law induced permutation symmetry breaking

Abstract

Permutation symmetry plays a central role in the understanding of collective quantum dynamics. On the other hand, interactions are rarely uniform in real systems. By introducing power law couplings that algebraically decay with the distance between the spins $r$ as $1/r^α$, we break this symmetry with a non-zero $α$, and probe the emergence of new dynamical behaviors, including thermalization. As we increase $α$, the system interpolates from an infinite range spin system at $α=0$ exhibiting permutation symmetry, to a short range integrable model as $α\rightarrow \infty$ where this permutation symmetry is absent. We focus on the change in the behavior of the system as $α$ is tuned, using dynamical quantities like total angular momentum operator $J^2$ and the von Neumann entropy $S_{N/2}$. Starting from the chaotic limit of the permutation symmetric Hamiltonian at $α=0$, we find that for small $α$, the steady state values of these quantities remain close to the permutation symmetric subspace values corresponding to $α=0$. At intermediate $α$ values, these show signatures of thermalization exhibiting values corresponding to that of random states in full Hilbert space. On the other hand, the large $α$ limit approaches the values corresponding to integrable kicked Ising model. In addition, we also study the dependence of thermalization on the driving period $τ$, with results indicating the onset of thermalization for smaller values of $α$ when $τ$ is large, thereby extending the intermediate range of $α$. We further confirm these results using effective dimension and spectral statistics.