MAGNETIZATION PLATEAUS IN N-LEG SPIN LADDERS

In this paper we continue and extend a systematic study of plateaux in magnetization curves of antiferromagnetic Heisenberg spin-1/2 ladders . We first review a bosonic field-theoretical formulation of a single XXZ chain in the presence of a magnetic field, which is then used for an Abelian bosonization analysis of N weakly coupled chains. Predictio ns for the universality classes of the phase transitions at the platea ux boundaries are obtained in addition to a quantization condition for the value of the magnetization on a plateau. These results are comple mented by and checked against strong-coupling expansions. Finally, we analyze the strong-coupling effective Hamiltonian for an odd number N of cylindrically coupled chains numerically. For N = 3 we explicitly o bserve a spin gap with a massive spinon-type fundamental excitation an d obtain indications that this gap probably survives the limit N-->inf inity.