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 (ED) and density matrix renormalization group (DMRG) methods. We find that the energy to create a bound state of two-magnons \(\Delta_p\) becomes lower than the energy to create a single spin flip \(\Delta_s\) near \(H_{c2}\). In the entire Kitaev spin liquid \(\Delta_p