Formation of magnetic order in the Kitaev-Heisenberg model*

We compute the low-energy excitation spectrum and the dynamical magnetic spin structure factor of the Kitaev-Heisenberg model using a variational approach, that becomes exact at the exactly solvable Kitaev points. This approach reveals the physical origin of the asymmetry in the stability range of Kitaev spin liquid phases around the ferromagnetic and antiferromagnetic Kitaev points. We also show that bound states of fractionalized excitations appear in the proximity of a quantum phase transition between the ferrmagnetic Kitaev spin spin liquid and the magnetically ordered states induced by ferro and antiferromagnetic Heisenberg interactions.