Low-lying excitations of the Kitaev-Heisenberg model on a honeycomb lattice

Some transition-metal compounds on honeycomb lattices such as RuCl3 are regarded as promising candidates for realizing the Kitaev spin liquid (KSL), although these materials undergo phase transitions to the zigzag magnetic states [1]. Since their ground states are considered to be located close to the KSL state, low-lying excitations are difficult to calculate using analytical methods. The S = 1/2 Kitaev-Heisenberg (KH) model on a honeycomb lattice is a minimal model for these materials [2]. We investigate ground state energies and low-lying excitations of the KH model on a honeycomb lattice by using series expansions. We find that dimer series expansions can approach the close vicinity of the Kitaev limit in the zigzag phase, where the Heisenberg interaction is absent, in the lower order expansion than Ising series expansions. When the system approaches the Kitaev limit, the low-lying mode becomes flatter except for the Bragg wave numbers. We discuss the results in relation to the inelastic neutron scattering experiments [3]. [1] K. W. Plumb, et al., Phys. Rev. B 90, 041112(R) (2014). [2] J. Chaloupka, et al., Phys. Rev. Lett. 110, 097204 (2013). [3] A. Banerjee, et al., Nat. Mater. 15, 733 (2016).