ANTIFERROMAGNETIC SPIN LADDERS - CROSSOVER BETWEEN SPIN S=1 2 AND S=1CHAINS/

We study a model of two weakly coupled isotropic spin-1/2 Heisenberg c hains with an antiferromagnetic coupling along the chains (spin ladder ). It is shown that the system always has a spectral gap and the lower lying excitations are triplets, For the case of identical chains the model in the continuous limit is shown to be equivalent to four decoup led noncritical Ising models with the Z(2)xSU(2) symmetry. For this ca se we obtain the exact expressions for asymptotics of spin-spin correl ation functions. It is shown that when the chains have different excha nge integrals J(1) much greater than J(2) the spectrum at low energies is described by the O(3)-nonlinear sigma model. We discuss the topolo gical order parameter related to the gap formation and give a detailed description of the dynamical magnetic susceptibility.