The spin-1/2 square-lattice J1-J2 model: the spin-gap issue

We use the coupled cluster method to high orders of approximation in order to calculate the ground-state energy, the ground-state magnetic order parameter, and the spin gap of the spin-1/2 J(1)-J(2) model on the square lattice. We obtain values for the transition points to the magnetically disordered quantum paramagnetic phase of J(2)(c1) = 0.454J(1) and J(2)(c2) = 0.588J(1). The spin gap is zero in the entire parameter region accessible by our approach, i.e. for J(2) <= 0.49J(1) and J(2) > 0.58J(1). This finding is in favor of a gapless spin-liquid ground-state in this parameter regime.