Optimized RVB states of the 2-d antiferromagnet: Ground state and excitation spectrum

The Gutzwiller projection of the Schwinger-boson mean-field solution of the 2-d spin-1/2 antiferromagnet in a square lattice is shown to produce the optimized, parameter-free RVB ground state. We get $-0.6688J$/site and $0.311$ for the energy and the staggered magnetization. The spectrum of the excited states is found to be linear and gapless near $\bk\cong 0$. Our calculation suggests, upon breaking of the rotational symmetry, $\epsilon_{\bk}\cong 2JZ_{r} \sqrt{1-\gamma_{\bk}^{2}}$ with $Z_{r}\cong 1.23$. To appear in Phys. Lett. A (1993).