Elementary Excitations of Heisenberg Ferrimagnetic Spin Chains

TL;DR

This study numerically investigates elementary excitations in Heisenberg ferrimagnetic spin chains with spins 1 and 1/2, revealing two distinct excitations: a gapless ferromagnetic mode and a gapped mode, using Monte Carlo and exact diagonalization methods.

Contribution

It provides a detailed numerical analysis of elementary excitations in alternating-spin chains, confirming the existence of two distinct excitation modes and elucidating their physical origins.

Findings

Identification of gapless and gapped excitations

Quadratic dispersion of the gapless mode

Relation of excitations to specific spin types

Abstract

We numerically investigate elementary excitations of the Heisenberg alternating-spin chains with two kinds of spins 1 and 1/2 antiferromagnetically coupled to each other. Employing a recently developed efficient Monte Carlo technique as well as an exact diagonalization method, we verify the spin-wave argument that the model exhibits two distinct excitations from the ground state which are gapless and gapped. The gapless branch shows a quadratic dispersion in the small-momentum region, which is of ferromagnetic type. With the intention of elucidating the physical mechanism of both excitations, we make a perturbation approach from the decoupled-dimer limit. The gapless branch is directly related to spin 1's, while the gapped branch originates from cooperation of the two kinds of spins.