Towards lattice-gas description of low-temperature properties above the Haldane and cluster-based Haldane ground states of a mixed spin-(1,1/2) Heisenberg octahedral chain

TL;DR

This paper extends a localized-magnon approach to analyze low-temperature magnetic properties of a mixed spin-(1,1/2) Heisenberg octahedral chain, revealing that classical lattice-gas models effectively describe these properties above certain quantum ground states.

Contribution

The study develops a comprehensive localized-magnon theory that applies to less frustrated regions, including Haldane and cluster-based Haldane ground states, validated by numerical methods.

Findings

Low-temperature magnetic properties can be modeled by classical lattice-gas models.

The extended approach accurately describes properties above Haldane phases.

Comparison with numerical methods confirms the theory's validity.

Abstract

The rich ground-state phase diagram of the mixed spin-(1,1/2) Heisenberg octahedral chain was previously elaborated from effective mixed-spin Heisenberg chains, which were derived by employing a local conservation of a total spin on square plaquettes of an octahedral chain. Here we present a comprehensive analysis of the thermodynamic properties of this model. In the highly frustrated parameter region the lowest-energy eigenstates of the mixed-spin Heisenberg octahedral chain belong to flat bands, which allow a precise description of low-temperature magnetic properties within the localized-magnon approach exploiting a classical lattice-gas model of hard-core monomers. The present article provides a more comprehensive version of the localized-magnon approach, which extends the range of its validity down to a less frustrated parameter region involving the Haldane and cluster-based Haldane ground states. A comparison between results of the developed localized-magnon theory and accurate numerical methods such as full exact diagonalization and finite-temperature Lanczos technique convincingly evidence that the low-temperature magnetic properties above the Haldane and the cluster-based Haldane ground states can be extracted from a classical lattice-gas model of hard-core monomers and dimers, which is additionally supplemented by a hard-core particle spanned over the whole lattice representing the gapped Haldane phase.