Extent of frustration in the classical Kitaev-$\Gamma$ model via bond anisotropy

TL;DR

This study investigates how bond anisotropy influences the phase diagram and quantum fluctuations in the classical Kitaev-$\Gamma$ model, revealing complex phases and enhanced quantum effects relevant to honeycomb Mott insulators.

Contribution

It provides the first detailed analysis of the classical $K\Gamma$ model with anisotropic bonds using Monte Carlo simulations and spin wave theory, elucidating the effects on phase structure and quantum fluctuations.

Findings

Various large unit cell phases emerge due to frustration.

Anisotropic bond strength enhances quantum fluctuations in the $\Gamma$-dominant regime.

Intrinsic degeneracy of phases is analyzed based on Hamiltonian symmetry.

Abstract

In the pseudospin-$\frac{1}{2}$ honeycomb Mott insulators with strong spin-orbit coupling, there are two types of bond-dependent exchange interactions, named Kitaev ($K$) and $\Gamma$, leading to strong frustration. While the ground state of the Kitaev model is a quantum spin liquid with fractionalized excitations, the ground state of the $\Gamma$ model remains controversial. In particular, the phase diagram of the $K\Gamma$ model with ferromagnetic $K$ and antiferromagnetic $\Gamma$ interactions has been intensively studied because of its relevance to candidate materials such as $\alpha$-RuCl$_{3}$. Numerical studies also included the effects of tuning the bond strengths, i.e., $z$-bond strength different from the other bonds. However, no clear consensus on the overall phase diagram has been reached yet. Here we study the classical $K\Gamma$ model with the anisotropic bond strength using Monte Carlo simulations to understand the emerging phases out of competition between the two frustrated limits. We also address how the anisotropic bond strength affects the phase diagram and strength of quantum fluctuations. We found various large unit cell phases due to the competing frustrations, and analyzed their intrinsic degeneracy based on the symmetry of the Hamiltonian. Using the linear spin wave theory we showed that the anisotropic bond strength enhances quantum fluctuations in the $\Gamma$-dominant regime where a small reduced moment is observed. The implications of our findings in relation to the quantum model are also discussed.