Antiferromagnetic Kitaev interaction in Jeff=1/2 cobalt honeycomb materials Na3Co2SbO6 and Na2Co2TeO6

Finding new materials with antiferromagnetic (AFM) Kitaev interaction is an urgent issue for quantum magnetism research. We conclude that Na3Co2SbO6 and Na2Co2TeO6 are new honeycomb cobalt-based systems with AFM Kitaev interaction by carrying out inelastic neutron scattering experiments and subsequent analysis. The spin-orbit excitons observed at 20-28 meV in both compounds strongly support the idea that Co2+ ions of both compounds have a spin-orbital entangled Jeff=1/2 state. Furthermore, we found that a generalized Kitaev-Heisenberg Hamiltonian can describe the spin-wave excitations of both compounds with additional 3rd nearest-neighbor interaction. Our best-fit parameters show significant AFM Kitaev terms and off-diagonal symmetric anisotropy terms of a similar magnitude in both compounds. We also found a strong magnon-damping effect at the higher energy part of the spin waves, entirely consistent with observations in other Kitaev magnets. Our work suggests Na3Co2SbO6 and Na2Co2TeO6 as rare examples of the AFM Kitaev magnets based on the systematic studies of the spin waves and analysis.