EFFECTIVE ACTIONS FOR SPIN LADDERS

We derive a path-integral expression for the effective action in the c ontinuum limit of an antiferromagnetic Heisenberg spin ladder with an arbitrary number of legs. The map is onto an O(3) nonlinear sigma mode l (NL sigma M) with the addition of a topological term that is effecti ve only for odd-leg ladders and half-odd integer spins. We derive the parameters of the effective NL sigma M and the behavior of the spin ga p for the case of even-leg ladders.