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

We study the ground-state phase diagram of the quantum J(1)-J(2) model on a square lattice by means of an entangled-plaquette variational Ansatz. In the range 0 <= J(2)/J(1) <= 1, we find classical magnetic order of Neel and collinear type, for J(2)/J(1) less than or similar to 0.5 and J(2)/J(1) greater than or similar to 0.6, respectively. For intermediate values of J(2)/J(1) the ground state is a spin liquid (i.e., paramagnetic with no valence-bond crystalline order). Our estimates of the entanglement entropy show that such a spin liquid is topological.