Excitation spectrum of the ferromagnetic Ising-Heisenberg chain at zero field

The excitation spectrum of the ferromagnetic $s=\frac{1}{2}$ Ising-Heisenberg chain as probed by dynamic form factors is studied. Exact results are presented for zero temperature, and the modifications introduced by thermally induced transitions are elucidated by finite-chain calculations. A particularly interesting result is the occurrence of a thermally induced resonance around zero frequency for an Ising-like model. Its origin is traced back to transitions involving magnon bound states. The resonance structure of the dynamic form factors associated with a Heisenberg-like system turns out to be less spectacular, because the contributions of different induced transitions nearly overlap on the frequency scale; so that rather broad resonance structures result. Our findings have an important bearing on related models with $s>\mathrm{}\frac{1}{2}$, such as CsNi${\mathrm{F}}_3$, and on the soliton aspect of the classically treated systems.