The Heisenberg spin ladder is studied in the semiclassical limit, via a
mapping to the nonlinear σ model. Different treatments are needed if the
inter-chain coupling K is small, intermediate or large. For intermediate
coupling a single nonlinear σ model is used for the ladder. Its predicts
a spin gap for all nonzero values of K if the sum s+s~ of the spins
of the two chains is an integer, and no gap otherwise. For small K, a better
treatment proceeds by coupling two nonlinear sigma models, one for each chain.
For integer s=s~, the saddle-point approximation predicts a sharp drop
in the gap as K increases from zero. A Monte-Carlo simulation of a spin 1
ladder is presented which supports the analytical results.Comment: 8 pages, RevTeX 3.0, 4 PostScript figure