SCHWINGER-BOSON APPROACH TO QUANTUM SPIN SYSTEMS - GAUSSIAN FLUCTUATIONS IN THE NATURAL GAUGE

We compute the Gaussian-fluctuation corrections to the saddle-point Sc hwinger-boson results using collective coordinate methods. Concrete ap plication to investigate the frustrated J(1)-J(2) antiferromagnet on t he square lattice shows that, unlike the saddle-point predictions, the re is a quantum nonmagnetic phase for 0.53 less than or similar to J(2 )/J(1) less than or similar to 0.64. This result is obtained by consid ering the corrections to the spin stiffness on large lattices and extr apolating to the thermodynamic limit, which avoids the infinite-lattic e infrared divergencies associated with Bose condensation. The very go od agreement of our results with exact numerical values on finite clus ters lends support to the calculational scheme employed.