Spontaneous plaquette dimerization in the J(1)-J(2) Heisenberg model

We investigate the nonmagnetic phase of the spin-half frustrated Heisenberg antiferromagnet on the square lattice using exact diagonalization (up to 3 6 sites) and quantum Monte Carlo techniques (up to 144 sites). The spin gap and the susceptibilities for the most important crystal symmetry breaking operators are computed. A genuine and somehow unexpected "plaquette resonat ing valence bond," with spontaneously broken translation symmetry and no br oken rotation symmetry, comes out from our numerical simulations as the mos t plausible ground state for J(2)/J(1) similar or equal to 0.5.