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

Abstract

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 ${\Delta }_p<{\Delta }_s$ 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.