# Spin-1/2 XXZ chain coupled to two Lindblad baths: Constructing nonequilibrium steady states from equilibrium correlation functions

## TL;DR

This paper shows that for the spin-1/2 XXZ chain, the nonequilibrium steady state under weak driving can be constructed from equilibrium correlation functions, bridging closed and open system approaches.

## Contribution

It introduces a method to derive nonequilibrium steady states from equilibrium correlations, enabling analysis of larger systems and clarifying discrepancies in transport coefficient extraction.

## Key findings

- Nonequilibrium steady states can be constructed from equilibrium correlations.
- The approach allows studying larger systems than traditional methods.
- Potential pitfalls exist in extracting transport coefficients from finite systems.

## Abstract

State-of-the-art approaches to extract transport coefficients of many-body quantum systems broadly fall into two categories: (i) they target the linear-response regime in terms of equilibrium correlation functions of the closed system; or (ii) they consider an open-system situation typically modeled by a Lindblad equation, where a nonequilibrium steady state emerges from driving the system at its boundaries. While quantitative agreement between (i) and (ii) has been found for selected model and parameter choices, also disagreement has been pointed out in the literature. Studying magnetization transport in the spin-1/2 XXZ chain, we here demonstrate that at weak driving, the nonequilibrium steady state in an open system, including its buildup in time, can remarkably be constructed just on the basis of correlation functions in the closed system. We numerically illustrate this direct correspondence of closed-system and open-system dynamics, and show that it allows the treatment of comparatively large open systems, usually only accessible to matrix product state simulations. We also point out potential pitfalls when extracting transport coefficients from nonequilibrium steady states in finite systems.