Bosonic representation of one-dimensional Heisenberg ferrimagnets

TL;DR

This paper develops a bosonic representation framework for analyzing the energy structure and thermodynamics of one-dimensional ferrimagnetic Heisenberg chains, introducing a new modified spin-wave scheme that enhances interpretability.

Contribution

It introduces a novel modified spin-wave approach with a Lagrange multiplier, improving the analysis of quantum ferrimagnetism in one dimension.

Findings

The new scheme accurately describes energy and thermodynamics.

The approach effectively interprets quantum ferrimagnetism.

It extends to other ferrimagnetic and antiferromagnetic systems.

Abstract

The energy structure and the thermodynamics of ferrimagnetic Heisenberg chains of alternating spins S and s are described in terms of the Schwinger bosons and modified spin waves. In the Schwinger representation, we average the local constraints on the bosons and diagonalize the Hamiltonian at the Hartree-Fock level. In the Holstein-Primakoff representation, we optimize the free energy in two different ways introducing an additional constraint on the staggered magnetization. A new modified spin-wave scheme, which employs a Lagrange multiplier keeping the native energy structure free from temperature and thus differs from the original Takahashi Scheme, is particularly stressed as an excellent language to interpret one-dimensional quantum ferrimagnetism. Other types of one-dimensional ferrimagnets and the antiferromagnetic limit S=s are also mentioned.