Excitation Spectrum of the Ising-Heisenberg Chain Ferromagnet

Results are presented and interpreted for the zero- and finite-temperature excitation spectrum of ferromagnetic Ising-Heisenberg chains, as probed by dynamic form factors. Finite-chain calculations demonstrate the occurrence of a thermally induced resonance centered around zero frequency, and a broadening of the magnon and two-magnon bound-state resonances at low $T$. The occurrence of bound states in the associated transitions demonstrates the failure of finite-order magnon perturbation theory.