The two-dimensional (2D) frustrated honeycomb Heisenberg antiferromagnet can support several kinds of ground states according to the ratio α defined to be J₂/J₁ where J₁ is nearest-neighbor interaction and J₂ next-nearest-neighbor one. For small and intermediate values of α Néel order, selected by order by disorder, and plaquette resonating valence bond can be formed, respectively. Many theoretical studies agree on the dimer ground state with large value of α. However, the range of α in which this system assumes such ground state is still arguable.

There is a handful of materials in which frustrated honeycomb lattice is realized. Among those is a delafossite-derived compound Cu₅SbO₆ consisting of Cu²⁺ (S = 1/2) arranged in distorted honeycomb layers stacked in c axis. A study of the temperature dependence of magnetic susceptibility of Cu₅SbO₆ shows that the dimer ground state is formed in this magnetic material with a singlet-triplet excitation of ~189 K corresponding to ~16 meV. The crystal structure of Cu₅SbO₆ is displayed in Fig. 1. The unit cell is monoclinic with lattice parameters a = 8.92 Å, b = 5.59 Å, c = 11.8 Å, and β = 103.6°.

In order to get more insight into the nature of dimer ground states in 2D honeycomb magnets, inelastic neutron scattering is beneficial in investigating magnetic excitation of this compound. Inelastic neutron scattering provides an access into the dispersion spectrum of magnetic materials. The dispersion relation can, then, identify the nature of interactions within the layers. Therefore, spin dynamics of Cu₅SbO₆ should be investigated, and magnetic interactions of spin pairs can be extracted. Eventually, the result would be able to justify whether the observation is in agreement with theoretical predictions.