Magnetic excitations from the hexagonal spin clusters in the S = 1/2 distorted honeycomb lattice antiferromagnet Cu₂(pymca)₃(ClO₄)

Cu₂(pymca)₃(ClO₄) (pymca: pyrimidine-2-carboxylate) consists of a slightly distorted honeycomb lattice of Cu²⁺ spins, which shows no long-range magnetic order down to 0.6 K. A magnetization study revealed 1/3 and 2/3 plateau phases [1], which is not expected for regular honeycomb antiferromagnets. Inelastic neutron scattering experiments were performed using a powder sample to investigate the exchange interactions of this material. The spin excitations from the singlet ground state to the first three triplet states, predicted from the antiferromagnetic hexagonal spin cluster interacting with 3.9 meV, were observed. Using the exact diagonalization mothods, the intercluster coupling was estimated from the excitation peak width to be about 20% of the intracluster interaction, which is consistent with the previously reported value. Our exchange path model explains the anisotropic exchange interactions in the distorted honeycomb plane. Interestingly, Cu₂(pymca)₃(ClO₄) is found to be a rare example for the quantum hexagonal spin cluster system.

[1] A. Okutani et al., J. Phys. Soc. Jpn. 88, 013703 (2019).