Finite-temperature properties of the planar ferromagnetic xxz chain

We present analytic weak-coupling and numerical finite-chain results for the specific heat, the longitudinal zero-field susceptibility, and the longitudinal dynamic form factor of the planar ferromagnetic xxz chain at finite temperatures. It is shown that the "bound state" probed by the dynamic form factor Szz(q,) within the Hartree-Fock approximation has to be interpreted as the first moment of Szz of the states lying above the continuum appearing in the isotropic xy model. © 1983 The American Physical Society.