ANOMALOUS FINITE-SIZE SPECTRUM IN THE S = 1 2 2-DIMENSIONAL HEISENBERG-MODEL/

We study the low energy spectrum of the nearest neighbor Heisenberg mo del on a square lattice as a function of the total spin S. By quantum Monte Carlo simulation we compute this spectrum for Heisenberg models with local moments s = 1/2, s = 1, and s = 3/2. We conclude that the n onlinear cf model prediction for the low energy spectrum is always ver ified for a large enough system size. However, the crossover to the co rrect scaling regime is particularly slow just for the s = 1/2 Heisenb erg model. The possible detection of this unexpected anomaly with fini te temperature experiments on a = 1/2 isotropic quantum antiferromagne ts is also discussed.