Quantum spin ladder with dynamic phonons. Bound states of magnons and phonons

We consider the two-chain Heisenberg ladder with antiferromagnetic interactions. Our approach is based on the description of spin excitations as triplets above a strong-coupling singlet ground state. It is shown that interaction with dynamic phonons drastically changes the dispersion of the elementary triplet excitation (magnon) and the spin structure factor. It is also shown that a new triplet excitation appears in the spectrum: the bound state of a magnon and a phonon.