Thermalization away from Integrability and the Role of Operator Off-Diagonal Elements

TL;DR

This study examines how breaking integrability in a one-dimensional Heisenberg model affects the rate of thermalization, highlighting the role of operator off-diagonal elements and energy scales in this process.

Contribution

It provides a detailed analysis of the connection between operator off-diagonal elements and thermalization rates in non-integrable quantum systems.

Findings

Thermalization slows near integrability due to high-energy off-diagonal elements.

Stronger next-nearest neighbor interactions accelerate thermalization.

Three regimes of thermalization rates are identified based on integrability breaking strength.

Abstract

We investigate the rate of thermalization of local operators in the one-dimensional anisotropic antiferromagnetic Heisenberg model with next-nearest neighbor interactions that break integrability. This is done by calculating the scaling of the difference of the diagonal and canonical thermal ensemble values as function of system size, and by directly calculating the time evolution of the expectation values of the operators with the Chebyshev polynomial expansion. Spatial and spin symmetry is exploited and the Hamiltonian is divided in subsectors according to their symmetry. The rate of thermalization depends on the proximity to the integrable limit. When integrability is weakly broken thermalization is slow, and becomes faster the stronger the next-nearest neighbor interaction is. Three different regimes for the rate of thermalization with respect to the strength of the integrability breaking parameter are identified. These are shown to be directly connected with the relative strength of the low and higher energy difference off-diagonal operator matrix elements in the symmetry eigenbasis of the Hamiltonian. Close to the integrable limit the off-diagonal matrix elements peak at higher energies and high frequency fluctuations are important and slow down thermalization. Away from the integrable limit a strong low energy peak gradually develops that takes over the higher frequency fluctuations and leads to quicker thermalization.