Excitation Spectrum of the Spin-1/2 Ferromagnetic-Antiferromagnetic Alternating Heisenberg Chain:

TL;DR

This paper investigates the excitation spectrum of a spin-1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain, revealing how its features evolve with coupling strength and explaining the spectrum of the spin-1 antiferromagnetic chain.

Contribution

It provides a numerical analysis of the excitation spectrum of the alternating chain, linking it to the known properties of the spin-1 antiferromagnetic Heisenberg chain.

Findings

The spectrum includes a propagating triplet branch and a continuum of triplet waves.

Increasing ferromagnetic coupling causes the triplet branch to merge into the continuum.

The results explain the natural spectrum of the spin-1 antiferromagnetic chain.

Abstract

The natural explanation of the excitation spectrum of the spin-1 antiferromagnetic Heisenberg chain is given from the viewpoint of the spin-1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain. The energy spectrum of the latter is calculated with fixed momentum $k$ by numerical diagonalization of finite size systems. It consists of a branch of propagating triplet pair (triplet wave) and the continuum of multiple triplet waves for weak ferromagnetic coupling. As the ferromagnetic coupling increases, the triplet wave branch is absorbed in the continuum for small $k$, reproducing the characteristics of the spin-1 antiferromagnetic Heisenberg chain.