Spin triangle chain with ferromagnetic and antiferromagnetic interactions on the transition line

TL;DR

This paper studies a spin-1/2 anisotropic chain of triangles with competing interactions, revealing localized magnons and complex thermodynamic behavior, especially in the strong anisotropy limit.

Contribution

It introduces a detailed analysis of the excitation spectrum and thermodynamics of a spin triangle chain with mixed ferromagnetic and antiferromagnetic interactions, highlighting the impact of anisotropy.

Findings

Existence of dispersionless one-magnon bands at certain interaction ratios

Localized-magnon states lead to macroscopic ground state degeneracy

Multiple peaks in specific heat due to multi-scale spectral structure

Abstract

We investigate spin-$\frac{1}{2}$ anisotropic model of a linear chain of triangles with competing ferro- and antiferromagnetic interactions and ferromagneic Heisenberg interactions between triangles. For a certain ratio between interactions the one-magnon excitation band is dispersionless leading to an existence of localized-magnon states which form macroscopically degenerated ground states. The spectrum of excitations has a specific structure depending on the value of the triangle-triangle interaction. Such specific structure determines the low-temperature thermodynamics and, in particular, the temperature dependence of a specific heat. In the limit of strong anisotropy of interactions the spectrum has a multi-scale structure which consists of subsets rank-ordered on small parameter. Each subset is responsible for the appearance of the peak in the temperature dependence of the specific heat.