Quantum selection of order and dynamic properties of the Kitaev-Heisenberg ferromagnet on a triangular lattice

Recent interest in monolayer materials motivated a search for two-dimensional ferromagnets with sizable spin-orbit coupling. Magnetic anisotropy of exchange Hamiltonian, induced by spin-orbit coupling, may not only stabilize long-range order, but also in turn can be a source of frustration and accidental degeneracy, which is the case for the Kitaev-Heisenberg model. Here we present an extensive study of ground state and excitations of ferromagnetic anisotropic-exchange Kitaev-Heisenberg model on a triangular lattice using order-by-disorder and augmented spin-wave theory calculations. It is shown that while bond-dependent terms of the model do not affect the ground state classically, quantum fluctuations select preferred magnetization direction of the ferromagnetic state and significantly alter classical phase diagram. Anisotropic terms of the magnetic Hamiltonian also give rise to magnon-magnon interactions that lead to spontaneous decay and spectral renormalization, which we illustrate using non-linear spin-wave theory.