Variational study of the Kitaev-Heisenberg-Gamma model

TL;DR

This paper uses a variational approach to analyze the excitation spectrum and phase transitions in the Kitaev-Heisenberg-Gamma model, revealing bound states and their role in phase stability and transitions.

Contribution

It introduces a novel variational method based on fractionalized excitations to study the Kitaev-Heisenberg-Gamma model, explaining phase stability and transitions.

Findings

Fractionalized excitations form bound states in Kitaev spin liquids.

Bound states appear as sharp modes in the dynamical spin structure factor.

Phase transitions involve condensation of bound states.

Abstract

We compute the low-energy excitation spectrum and the dynamical spin structure factor of the Kitaev-Heisenberg-Gamma model through a variational approach based on the exact fractionalized excitations of the pure Kitaev honeycomb model. This novel approach reveals the physical reason for the asymmetric stability of the Kitaev spin liquid phases around the ferromagnetic and antiferromagnetic Kitaev limits. Moreover, we demonstrate that the fractionalized excitations form bound states in specific regions of each Kitaev spin liquid phase and that certain phase transitions induced by Heisenberg and Gamma interactions are driven by the condensation of such a bound state. Remarkably, this bound state appears as a sharp mode in the dynamical spin structure factor, while its condensation patterns at the appropriate phase transitions provide a simple explanation for the magnetically ordered phases surrounding each Kitaev spin liquid phase.