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

TL;DR

This paper investigates the excitations and phase transitions in the Kitaev honeycomb model under a [111] magnetic field, revealing two-magnon bound states and predicting observable magnon pairing effects in candidate materials.

Contribution

It introduces the analysis of two-magnon bound states in the Kitaev model and predicts their experimental signatures near phase transitions.

Findings

Two-magnon bound state energy becomes lower than single-magnon energy near critical field.

Both single- and two-magnon gaps vanish at the phase transition point.

Predictions for magnon pairing observable in Raman scattering experiments.

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.