Anomalous currents in driven XXZ chain with boundary twisting at weak coupling or weak driving

TL;DR

This paper investigates anomalous behaviors in the nonequilibrium steady state of a driven XXZ spin chain at the isotropic point, revealing singularities and current discontinuities influenced by system size and boundary conditions.

Contribution

It analytically demonstrates the existence of anomalies in the NESS of the XXZ chain at $\

Findings

Spin current exhibits singularity at $\

Current discontinuity occurs at $\

Energy current shows twin peak anomaly

Abstract

The spin 1/2 XXZ chain driven out of equilibrium by coupling with boundary reservoirs targeting perpendicular spin orientations in XY plane, is investigated. The existence of an anomaly in the nonequilibrium steady state (NESS) at the isotropic point $\Delta=1$ is demonstrated both in the weak coupling and weak driving limits. The nature of the anomaly is studied analytically by calculating exact NESS for small system sizes, and investigating steady currents. The spin current at the points $\Delta=\pm1$ has a singularity which leads to a current discontinuity when either driving or coupling vanish, and the current of energy develops a twin peak anomaly. The character of singularity is shown to depend qualitatively on whether the system size is even or odd.