Mixed-spin systems: coexistence of Haldane gap and antiferromagnetic long range order

TL;DR

This paper models a 2D mixed-spin system demonstrating the coexistence of Haldane gap excitations with antiferromagnetic long-range order, supported by exact ground state solutions and Matrix Product Method analysis.

Contribution

It introduces a 2D mixed-spin model with an exactly specified ground state, showing Haldane gap excitations persist in the ordered phase.

Findings

Haldane gap excitations coexist with magnetic order.

Exact ground state constructed for the model.

Matrix Product Method confirms gap persistence.

Abstract

Recent experiments on the quasi-1D antiferromagnets $R_{2}BaNiO_{5}$ (R = rare earth) have shown the existence of purely 1D Haldane gap excitations propagating on the Ni chains. Below an ordering temperature, the gap excitations survive and coexist with the conventional spin waves in the ordered phase. We construct a model mixed-spin system in 2D for which the ground state can be exactly specified. Using the Matrix Product Method, we show the existence of Haldane gap excitations in the ordered phase. We consider different cases of ordering to study the effect of ordering on the degeneracy of the Haldane gap excitations.