Quantum paramagnetic states in the spin-1/2 distorted honeycomb-lattice Heisenberg antiferromagnet -- application to Cu$_2$(pymca)$_3$(ClO$_4$)

We investigate the ground-state phase diagram of a spin-1/2 honeycomb-lattice antiferromagnetic (AF) Heisenberg model with three exchange interactions, $J_{\rm A}$, $J_{\rm B}$, and $J_{\rm C}$ that is realized in a distorted honeycomb-lattice antiferromagnet ${\rm Cu_2 (pymca)_3 (ClO_4)}$. We remeasured the magnetic susceptibility of its polycrystalline sample with special care, and determined the exchange parameters of this material through the comparison with numerical results based on a quantum Monte Carlo (QMC) method. The QMC method also provides a ground-state phase diagram in the $J_{\rm A}/J_{\rm C}$-$J_{\rm B}/J_{\rm C}$ plane. The phase diagram consists of a small N${\rm \acute{e}}$el phase and a gapped quantum paramagnetic phase surrounding the N${\rm \acute{e}}$el phase. The latter includes six regimes of hexagonal-singlet-type states and dimer-singlet-type states alternatingly without boundaries closing the spin gap. We further calculate the equal-time spin structure factor in each phase using the QMC method. The computed spin dynamics by the exact diagonalization method exhibits continuums near and in the AF phase. Characteristic four energy band structures in the state with strong hexagonal-singlet-type correlations are informative to clarify the ground-state of ${\rm Cu_2 (pymca)_3 (ClO_4)}$ by future neutron scattering measurements.