Low Temperature Properties of Quantum Antiferromagnetic Chains with Alternating Spins S=1 and 1/2

TL;DR

This paper investigates the low-temperature behavior of alternating S=1 and 1/2 antiferromagnetic spin chains using analytical and quantum Monte Carlo methods, revealing both gapped and gapless excitations and ferrimagnetic properties.

Contribution

It provides a combined analytical and numerical analysis of low-lying excitations in alternating spin chains, highlighting the effectiveness of spin-wave theory for this system.

Findings

Spin-wave theory accurately describes the system's low-energy excitations.

The system exhibits both gapped and gapless modes.

Quantum ferrimagnetic chains show combined ferromagnetic and antiferromagnetic features.

Abstract

We study the low-temperature properties of S=1 and 1/2 alternating spin chains with antiferromagnetic nearest-neighbor exchange couplings using analytical techniques as well as a quantum Monte Carlo method. The spin-wave approach predicts two different low-lying excitations, which are gapped and gapless, respectively. The structure of low-lying levels is also discussed by perturbation theory in the strength of the Ising anisotropy. These analytical findings are compared with the results of quantum Monte Carlo calculations and it turns out that spin-wave theory well describes the present system. We conclude that the quantum ferrimagnetic chain exhibits both ferromagnetic and antiferromagnetic aspects.