Properties of magnetic impurities embedded into an anisotropic Heisenberg chain with spin gap

We consider a U(1)-invariant model consisting of the integrable anisotropic easy-axis Heisenberg chain of arbitrary spin S embedding an impurity of spin S'. The host chain has a spin gap for all values of S. The ground state properties and the elementary excitations of the host are studied as a function of the anisotropy and the magnetic field. The impurity is located on a link of the chain and interacts only with both neighboring sites. The coupling of the impurity to the lattice can be tuned by the impurity rapidity p0 (usually playing the role of the Kondo coupling). The impurity model is then integrable as a function of two continuous parameters (the anisotropy and the impurity rapidity) and two discrete variables (the spins S and S'). The Bethe ansatz equations are derived and used to obtain the magnetization of the impurity. The impurity magnetization is non-universal as a function of p0. For small fields the impurity magnetization is determined by the spin gap and the van Hove singularity of the rapidity band. For an overcompensated impurity (S'<S) at intermediate fields there is a crossover to non-Fermi-liquid behavior remnant from the suppressed quantum critical point