β-Cu2V2O7: A spin-1/2 honeycomb lattice system

We report on band-structure calculations and a microscopic model of the low-dimensional magnet beta-Cu2V2O7. Magnetic properties of this compound can be described by a spin-1/2 anisotropic honeycomb lattice model with the averaged coupling (J) over bar (1)=60-66 K. The low symmetry of the crystal structure leads to two inequivalent couplings J(1) and J(1)' but this weak spatial anisotropy does not affect the essential physics of the honeycomb spin lattice. The structural realization of the honeycomb lattice is highly nontrivial: the leading interactions J(1) and J(1)' run via double bridges of VO4 tetrahedra between spatially separated Cu atoms while the interactions between structural nearest neighbors are negligible. The non-negligible interplane coupling J(perpendicular to) similar or equal to 15 K gives rise to the long-range magnetic ordering at T-N similar or equal to 26 K. Our model simulations improve the fit of the magnetic susceptibility data, compared to the previously assumed spin-chain models. Additionally, the simulated ordering temperature of 27 K is in remarkable agreement with the experiment. Our study evaluates beta-Cu2V2O7 as the best available experimental realization of the spin-1/2 Heisenberg model on the honeycomb lattice. We also provide an instructive comparison of different band-structure codes and computational approaches to the evaluation of exchange couplings in magnetic insulators.