Anomalous spin excitations in honeycomb Na_2+dNi₂XO₆ (X=Te and Sb) systems with zig-zag magnetic ordering

The honeycomb lattice offers a fertile playground for exploring a wide range of exotic magnetic ground states. In addition to the competition between first, second and third neighbor magnetic exchange interactions, the anisotropic exchange interactions that arise from the spin-orbit coupling can lead to various exotic quantum-disordered states. Here we discuss the results of an elastic and inelastic neutron scattering study of the layered honeycomb systems Na_2+dNi₂XO₆ (X=Te and Sb) [1,2] that were proposed as potential candidates for the realization of S=1 Kitaev model [3]. These compounds are derived from the triangular layered compounds NaNiO₂ by substituting 1/3 of the magnetic sites by non-magnetic Te/Sb cations in an ordered fashion. The diffusive Na atoms effectively control the interlayer magnetic coupling and, therefore, provide a route for tuning the lattice dimensionality. The changes in the static magnetic order and spin-dynamics for samples with different Na content are discussed. We also show that the obtained spin-wave excitation spectra from powder samples cannot be described using a simple Heisenberg spin Hamiltonian. An alternative generalized Kitaev-Heisenberg Hamiltonian model that accounts for bond-dependent anisotropic-exchange interactions is evaluated.

[1] A. Samarakoon, Q. Chen, H. Zhou , V. O. Garlea, Phys Rev B 104, 184415 (2021)

[2] E. A. Zvereva et al, B 92, 144401 (2015)

[3] P. P. Stavropoulos, D. Pereira, and H.-Y. Kee, Phys. Rev. Lett. 123, 037203 (2019)