Extent Of Frustration In The Classical Kitaev-Gamma Model Via Bond Anisotropy

In the pseudospin-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-RuCl3. 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 anisotropic bond strengths using Monte Carlo simulations to understand the phases that emerge 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.