BONN - TH - 99 - 01 , SISSA 10 / 99 / EP , ETH - TH / 99 - 01 , ENSL - Th 02 / 99 Magnetic Properties of Zig - Zag Ladders ∗

We analyze the phase diagram of a system of spin-1/2 Heisenberg antiferromagnetic chains interacting through a zigzag coupling, also called zigzag ladders. Using bosonization techniques we study how a spin-gap or more generally plateaux in mag-netization curves arise in different situations. While for coupled XXZ chains, one has to deal with a recently discovered chiral perturbation, the coupling term which is present for normal ladders is restored by an external magnetic field, dimerization or the presence of charge carriers. We then proceed with a numerical investigation of the phase diagram of two coupled Heisenberg chains in the presence of a magnetic field. Unusual behaviour is found for ferromagnetic coupled antiferromagnetic chains. Finally, for three (and more) legs one can choose different inequivalent types of coupling between the chains. We find that the three-leg ladder can exhibit a spin-gap and/or non-trivial plateaux in the magnetization curve whose appearance strongly depends on the choice of coupling.