Eigenstate thermalization for observables that break Hamiltonian symmetries and its counterpart in interacting integrable systems

TL;DR

This paper investigates the statistical properties of off-diagonal matrix elements of symmetry-breaking observables in quantum spin chains, revealing eigenstate thermalization in chaotic systems and distinct distributions in integrable systems, with detailed analysis of their scaling behaviors.

Contribution

It demonstrates eigenstate thermalization for symmetry-breaking observables in chaotic systems and characterizes their distribution and scaling in integrable models, extending understanding of thermalization beyond symmetric cases.

Findings

Eigenstate thermalization holds in quantum-chaotic models with Gaussian-distributed matrix elements.

In integrable models, matrix elements follow a skewed log-normal-like distribution.

Variance of matrix elements scales as 1/D and exhibits diffusive or ballistic behavior at low frequencies.

Abstract

We study the off-diagonal matrix elements of observables that break the translational symmetry of a spin-chain Hamiltonian, and as such connect energy eigenstates from different total quasimomentum sectors. We consider quantum-chaotic and interacting integrable points of the Hamiltonian, and focus on average energies at the center of the spectrum. In the quantum-chaotic model, we find that there is eigenstate thermalization; specifically, the matrix elements are Gaussian distributed with a variance that is a smooth function of $\omega=E_{\alpha}-E_{\beta}$ ({$E_{\alpha}$} are the eigenenergies) and scales as $1/D$ ($D$ is the Hilbert space dimension). In the interacting integrable model, we find that the matrix elements exhibit a skewed log-normal-like distribution and have a variance that is also a smooth function of $\omega$ that scales as $1/D$. We study in detail the low-frequency behavior of the variance of the matrix elements to unveil the regimes in which it exhibits diffusive or ballistic scaling. We show that in the quantum-chaotic model the behavior of the variance is qualitatively similar for matrix elements that connect eigenstates from the same versus different quasimomentum sectors. We also show that this is not the case in the interacting integrable model for observables whose translationally invariant counterpart does not break integrability if added as a perturbation to the Hamiltonian.