Dimensional crossover in quantum antiferromagnets

The dimensional crossover in a spin-S nearest-neighbor Heisenberg antiferromagnet is discussed as it is tuned from a two-dimensional square lattice, of lattice spacing a, towards a spin chain by varying the width L(y) of a semi-infinite strip L(x) x L(y). For integer spins and arbitrary L(y), and for half integer spins with L(y)/a an arbitrary even integer, explicit analytical expressions for the zero temperature correlation length and the spin gap an given. For half integer spins and L(y)/a an odd integer, it is argued that the c = 1 behavior of the SU(2)(1) Wess-Zumino-Witten fixed point is squeezed out as the width L(y) --> infinity; here c is the conformal charge. The results specialized to S = 1/2 are applied to spin-ladder systems.