Quantum paramagnetism and magnetization plateaus in a kagome-honeycomb Heisenberg antiferromagnet

A spin-1/2 Heisenberg model on a honeycomb lattice is investigated by doing triplon analysis and quantum Monte Carlo calculations. This model, inspired by Cu-2(pymca)(3)(ClO4), has three different antiferromagnetic exchange interactions (J(A), J(B), J(C)) on three different sets of nearest-neighbor bonds which form a kagome superlattice. While the model is bipartite and unfrustrated, its quantum phase diagram is found to be dominated by a quantum paramagnetic phase that is best described as a spin-gapped hexagonal-singlet state. The Neel antiferromagnetic order survives only in a small region around J(A) = J(B) = J(C). The magnetization produced by the external magnetic field is found to exhibit plateaus at 1/3 and 2/3 of the saturation value, or at 1/3 alone, or no plateaus. Notably, the plateaus exist only inside a bounded region within the hexagonal-singlet phase. This study provides a clear understanding of the spin-gapped behavior and magnetization plateaus observed in Cu-2(pymca)(3)(ClO4), and also predicts the possible disappearance of 2/3 plateau under pressure.