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# Condensed Matter > Statistical Mechanics

# Title:Off-diagonal matrix elements of local operators in many-body quantum systems

(Submitted on 8 Jul 2014 (v1), last revised 29 Jan 2015 (this version, v2))

Abstract: In the time evolution of isolated quantum systems out of equilibrium, local observables generally relax to a long-time asymptotic value, governed by the expectation values (diagonal matrix elements) of the corresponding operator in the eigenstates of the system. The temporal fluctuations around this value, response to further perturbations, and the relaxation toward this asymptotic value, are all determined by the off-diagonal matrix elements. Motivated by this non-equilibrium role, we present generic statistical properties of off-diagonal matrix elements of local observables in two families of interacting many-body systems with local interactions. Since integrability (or lack thereof) is an important ingredient in the relaxation process, we analyze models that can be continuously tuned to integrability. We show that, for generic non-integrable systems, the distribution of off-diagonal matrix elements is a gaussian centered at zero. As one approaches integrability, the peak around zero becomes sharper, so that the distribution is approximately a combination of two gaussians. We characterize the proximity to integrability through the deviation of this distribution from a gaussian shape. We also determine the scaling dependence on system size of the average magnitude of off-diagonal matrix elements.

## Submission history

From: W. Beugeling [view email]**[v1]**Tue, 8 Jul 2014 11:38:56 UTC (2,391 KB)

**[v2]**Thu, 29 Jan 2015 10:45:55 UTC (2,432 KB)