Entanglement and coherence in a spin-s XXZ system under non-uniform fields

TL;DR

This paper explores how non-uniform magnetic fields influence entanglement and coherence in a spin-$s$ XXZ system, revealing control mechanisms for entanglement at various temperatures and identifying critical points.

Contribution

It demonstrates that non-uniform fields can control entanglement in spin-$s$ XXZ systems at any temperature, with a detailed phase diagram and temperature thresholds depending on field differences.

Findings

Entanglement can be controlled for any exchange coupling using non-uniform fields.

The entanglement threshold temperature depends only on the field difference |h1 - h2|.

A separability stripe exists in the (h1,h2) plane for T>0.

Abstract

We investigate entanglement and coherence in an $XXZ$ spin-$s$ pair immersed in a non-uniform transverse magnetic field. The ground state and thermal entanglement phase diagrams are analyzed in detail in both the ferromagnetic and antiferromagnetic cases. It is shown that a non-uniform field enables to control the energy levels and the entanglement of the corresponding eigenstates, making it possible to entangle the system for any value of the exchange couplings, both at zero and finite temperatures. Moreover, the limit temperature for entanglement is shown to depend only on the difference $|h_1-h_2|$ between the fields applied at each spin, leading for $T>0$ to a separability stripe in the $(h_1,h_2)$ field plane such that the system becomes entangled above a threshold value of $|h_1-h_2|$. These results are demonstrated to be rigorously valid for any spin $s$. On the other hand, the relative entropy of coherence in the standard basis, which coincides with the ground state entanglement entropy at $T=0$ for any $s$, becomes non-zero for any value of the fields at $T>0$, decreasing uniformly for sufficiently high $T$. A special critical point arising at $T=0$ for non-uniform fields in the ferromagnetic case is also discussed