Relaxation of imbalance in a disordered XX model with on-site dephasing

TL;DR

This paper analyzes how disorder and dephasing affect the relaxation dynamics of imbalance in a disordered XX spin chain, revealing multiple timescales, the false appearance of stretched exponential decay, and a universal power-law decay at long times.

Contribution

It provides an analytical prediction of multiple relaxation timescales and clarifies the nature of decay regimes in disordered XX models with dephasing, supported by large-system numerics.

Findings

Multiple timescales in imbalance relaxation.

Finite-time regime mimics stretched exponential decay.

Asymptotic decay follows a universal power law.

Abstract

The relaxation of observables to their non-equilibrium steady states in a disordered XX chain subjected to dephasing at every site has been intensely studied in recent years. We comprehensively analyze the relaxation of staggered magnetization, i.e., imbalance, in such a system, starting from the N\'eel initial state. We analytically predict emergence of several timescales in the system and extract results which match with large-system numerics without any extra fitting parameter until a universal timescale. An often reported stretched exponential decay is just one of the regimes which holds in a finite window of time and is therefore in fact not a true stretched exponential decay. Subsequently, the asymptotic decay of imbalance is governed by a power law irrespective of the disorder. We show that this emerges from the continuum limit of the low magnitude eigenspectrum of the Liouvillian. However, for finite systems, due to discreteness of the spectrum, the final phase of relaxation is governed by the relevant smallest Liouvillian gap.