Instabilities of spin-1 Kitaev spin liquid phase in presence of single-ion anisotropies

Abstract

We study the spin-one Kitaev model on the honeycomb lattice in the presence of single-ion anisotropies. We consider two types of single ion anisotropies: A ${D}_{111}$ anisotropy which preserves the symmetry between $X, Y$, and $Z$ bonds but violates flux conservation and a ${D}_{100}$ anisotropy that breaks the symmetry between $X, Y$, and $Z$ bonds but preserves flux conservation. We use series expansion methods, degenerate perturbation theory, and exact diagonalization to study these systems. Large positive ${D}_{111}$ anisotropy leads to a simple product ground state with conventional magnonlike excitations, while large negative ${D}_{111}$ leads to a broken symmetry and degenerate ground states. For both signs there is a phase transition at a small $|{D}_{111}|\ensuremath{\approx}0.12$ separating the more conventional phases from the Kitaev spin liquid phase. With large ${D}_{100}$ anisotropy, the ground state is a simple product state, but the model lacks conventional dispersive excitations due to the large number of conservation laws. Large negative ${D}_{100}$ leads to decoupled one-dimensional systems and many degenerate ground states. No evidence of a phase transition is seen in our numerical studies at any finite ${D}_{100}$. Convergence of the series expansion extrapolations all the way to ${D}_{100}=0$ suggests that the nontrivial Kitaev spin liquid is a singular limit of this type of single-ion anisotropy going to zero, which also restores symmetry between the $X, Y$, and $Z$ bonds.