Magnetic properties of doped Heisenberg chains

The magnetic susceptibility of systems from a class of integrable models fo r doped spin-S Heisenberg chains is calculated in the limit of vanishing ma gnetic field. For small concentrations x(h) of the mobile spin-(S - 1/2) ch arge carriers we find an explicit expression for the contribution of the ga pless mode associated to the magnetic degrees of freedom of these holes to the susceptibility which exhibits a singularity for x(h) --> 0 for sufficie ntly large S. We prove a sum rule for the contributions of the two gapless magnetic modes in the system to the susceptibility which holds for arbitrar y hole concentration. This sum rule complements the one for the low tempera ture specific heat which has been obtained previously. (C) 2000 Elsevier Sc ience B.V. All rights reserved.