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 $Hl{H}_{c1}$ and a partially polarized phase at high fields $Hg{H}_{c2}$. 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 ${\mathrm{\ensuremath{\Delta}}}_{p}$ becomes lower than the energy to create a single spin flip ${\mathrm{\ensuremath{\Delta}}}_{s}$ near ${H}_{c2}$. In the entire Kitaev spin liquid, ${\mathrm{\ensuremath{\Delta}}}_{p}l{\mathrm{\ensuremath{\Delta}}}_{s}$ 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.