Magnetic properties of a spin-1/2 honeycomb lattice antiferromagnet

We present an S =1 /2 antiferromagnetic (AF) honeycomb lattice composed of a verdazyl-based complex [Zn (hfac_{) 2}] [o -Py-V-(4-F_{) 2}] . Ab initio molecular orbital calculations indicate that two AF interactions and a ferromagnetic interaction lead to the formation of a honeycomb lattice. We explain the magnetic susceptibility, magnetization curve, and magnetic specific heat based on the S = 1/2 Heisenberg AF honeycomb lattice using the quantum Monte Carlo method. Further, by considering the distortion effect on the magnetic behavior, we confirm that the dimer-like lattice distortion in the present compound is small enough not to affect the intrinsic two-dimensional properties of the honeycomb lattice. Our numerical study on the distorted honeycomb lattice reveals a quantum phase transition from a disordered dimer phase to an AF ordered phase at a critical distortion ratio.