Ground-state phase diagram of the quantum J1 - J2 model on the honeycomb lattice

We study the ground-state phase diagram of the quantum J(1) - J(2) model on the honeycomb lattice by means of an entangled-plaquette variational ansatz. Values of energy and relevant order parameters are computed in the range 0 <= J(2)/J(1) <= 1. The system displays classical order for J(2)/J(1) less than or similar to 0.2 (Neel) and for J(2)/J(1) greater than or similar to 0.4 (collinear). In the intermediate region, the ground state is disordered. Our results show that the reduction of the half-filled Hubbard model to the model studied here does not yield accurate predictions.