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 < H-c1 and a partially polarized phase at high fields H > H-c2. Here, we analyze the low-field and high-field phases and phase transitions in terms of single- and two-magnon excitations using exact diagonalization and density matrix renormalization group methods. We find that the energy to create a bound state of two-magnon Delta(p) becomes lower than the energy to create a single spin flip Delta(s) near H-c2. In the entire Kitaev spin liquid, Delta(p) < Delta(s) and both gaps vanish at H-c2. We make testable predictions for magnon pairing that could be observable in Raman scattering measurements on Kitaev QSL candidate materials.