Two-Magnon Bound States in the Kitaev Model in a [111]-Field

It is now well established that the Kitaev honeycomb model in a magnetic field along the [111]-direction harbors an intermediate gapless quantum spin liquid (QSL) phase sandwiched between a gapped non-abelian QSL at low fields H<Hc₁ and a partially polarized phase at high fields H>Hc₂. Here, we analyze the low field and high field phases and phase transitions in terms of single- and two-magnon excitations using exact diagonalization (ED) and density matrix renormalization group (DMRG) methods. We find that the energy to create a bound state of two-magnons Δ_p becomes lower than the energy to create a single spin flip Δ_s near Hc₂. In the entire Kitaev spin liquid Δ_p<Δ_s and both gaps vanish at Hc₂. We make testable predictions for magnon pairing that could be observable in Raman scattering measurements on Kitaev QSL candidate materials.