Schwinger-boson approach to quantum spin systems: Gaussian fluctuactions   in the "natural" gauge
  We compute the Gaussian-fluctuation corrections to the saddle-point Schwinger-boson results using collective coordinate methods. Concrete application to investigate the frustrated J1-J2 antiferromagnet on the square lattice shows that, unlike the saddle-point predictions, there is a quantum nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by considering the corrections to the spin stiffness on large lattices and extrapolating to the thermodynamic limit, which avoids the infinite-lattice infrared divergencies associated to Bose condensation. The very good agreement of our results with exact numerical values on finite clusters lends support to the calculational scheme employed. 
A. E. Trumper, L. O. Manuel, C. J. Gazza and H. A. Ceccatto (IFIR -   CONICET, Rosario, Argentina)