# A Nonequilibrium quantum phase transition in strongly coupled spin chains

## TL;DR

This paper investigates nonequilibrium quantum phase transitions in strongly coupled XXZ spin chains, revealing different transport behaviors at and around the critical point through advanced simulations.

## Contribution

It demonstrates the existence of a nonequilibrium quantum phase transition in spin transport properties of XXZ chains at the critical point.

## Key findings

- Ballistic transport below the Heisenberg point
- Exponential decay of transport above the Heisenberg point for low energy states
- Power-law decay of current for high energy states at the critical point

## Abstract

We study spin transport in a boundary driven XXZ spin chain. Driving at the chain boundaries is modeled by two additional spin chains prepared in oppositely polarized states. Emergent behavior, both in the transient dynamics and in the long-time quasi-steady state, is demonstrated. Time-dependent matrix-product-state simulations of the system-bath state show ballistic spin transport below the Heisenberg isotropic point. Indications of exponentially vanishing transport are found above the Heisenberg point for low energy initial states while the current decays asymptotically as a power law for high energy states. Precisely at the critical point, non-ballistic transport is observed. Finally, it is found that the sensitivity of the quasi-stationary state on the initial state of the chain is a good witness of the different transport phases.