Anomalous finite size spectrum in the S=1/2 two dimensional Heisenberg model

We study the low energy spectrum of the nearest neighbor Heisenberg model on a square lattice as a function of the total spin S. By quantum Monte Carlo simulation we compute this spectrum for the s=1/2, s=1 and s=3/2 Heisenberg models. We conclude that the nonlinear sigma model prediction for the low energy spectrum is always verified for large enough system size. However the crossover to the correct scaling regime is particularly slow just for the s=1/2 Heisenberg model. The possibility to detect this unexpected anomaly with finite temperature experiments on s=1/2 isotropic quantum antiferromagnets is also discussed.Comment: 4 pages, RevTeX + 5 encapsulated postscript figure

Spin correlations in an isotropic spin-5/2 two-dimensional antiferromagnet

We report a neutron scattering study of the spin correlations for the spin 5/2, two-dimensional antiferromagnet Rb_2MnF_4 in an external magnetic field. Choosing fields near the system's bicritical point, we tune the effective anisotropy in the spin interaction to zero, constructing an ideal S=5/2 Heisenberg system. The correlation length and structure factor amplitude are closely described by the semiclassical theory of Cuccoli et al. over a broad temperature range but show no indication of approaching the low-temperature renormalized classical regime of the quantum non-linear sigma model.Comment: 4 pages, 3 EPS figure

Glueball spectrum and the Pomeron in the Wilson loop approach

Using a nonperturbative method based on asymptotic behaviour of Wilson loops we calculate masses of glueballs and corresponding Regge-trajectories. The only input is string tension fixed by meson Regge slope, while perturbative contributions to spin splittings are defined by standard alpha_s values. The masses of lowest glueball states are in a perfect agreement with lattice results. The leading glueball trajectory which is associated with Pomeron is discussed in details and its mixing with f and f' trajectories is taken into account.Comment: LaTeX2e, 49 pages, 2 figure