THERMODYNAMICS OF A SPIN-S' IMPURITY IN A SPIN-S ANTIFERROMAGNETIC HEISENBERG CHAIN

The numerical solution of the Bethe ansatz equations of an integrable SU(2)-invariant generalization of the spin-S antiferromagnetic Heisenb erg chain (Takhtajian-Babujian model) with a spin-S' impurity in zero magnetic field is presented. The entropy, specific heat and susceptibi lity of an impurity of spin S' are obtained numerically as a function of the spin S of the chain. Three situations have to be distinguished: (i) if S' = S the impurity just corresponds to one more site in the c hain; (ii) if S' > S the impurity spin is only partially compensated ( undercompensated) at T = 0, leaving an effective spin of (S' - S); (ii i) if S' < S (overcompensated) the entropy has an essential singularit y at T = h = 0, giving rise to critical behaviour as h and T tend to z ero. These properties are in close analogy with those of the n-channel Kondo problem. The thermodynamics of the two models is compared.