Ground-state phase diagram of the quantum $J_1-J_2$ model on the square   lattice
  We study the ground-state phase diagram of the quantum $J_1-J_2$ model on the square lattice by means of an entangled-plaquette variational ansatz. In the range $0\le {J_2}/{J_1} \le 1$, we find classical magnetic order of N\'eel and collinear type, for ${J_2}/{J_1}\lesssim 0.5$, and $J_2/J_1 \gtrsim 0.6$ 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. 
Fabio Mezzacapo
cond-mat.str-el