PHASE-DIAGRAM OF THE SPIN-1/2 HEISENBERG-ANTIFERROMAGNET ON A SQUARE LATTICE WITH NEAREST-NEIGHBOR AND NEXT-NEAREST-NEIGHBOR COUPLINGS

We study the phase diagram of the spin-1/2 Heisenberg model on a square lattice with antiferromagnetic couplings between nearest-neighbor and next-nearest-neighbor spins. We have found three phases as a function of the ratio alpha between the couplings: (a) a Neel-ordered state for alpha < alpha-1 = 0.417, (b) a disordered state which we identify as a resonating-valence-bond state for alpha-1 less-than-or-equal-to alpha less-than-or-equal-to alpha-2 = 0.683, and (c) a collinear ordered state for alpha less-than-or-equal-to alpha-2. The phase transition from the Neel to the disordered state is found to be second order, whereas the transition from the collinear to the disordered state is first order.