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 $\mathrm{xxz}$ chain at finite temperatures. It is shown that the "bound state" probed by the dynamic form factor $S_{\mathrm{zz}}(q,\omega )$ within the Hartree-Fock approximation has to be interpreted as the first moment of $S_{\mathrm{zz}}$ of the states lying above the continuum appearing in the isotropic $\mathrm{xy}$ model.

• Received 29 November 1982

DOI:https://doi.org/10.1103/PhysRevB.27.6770