From spinons to magnons in explicit and spontaneously dimerized antiferromagnetic chains

We reconsider the excitation spectra of a dimerized and Frustrated antiferr omagnetic Heisenberg chain. This model is taken as the simpler example of c ompeting spontaneous and explicit dimerization relevant fur spin-Peierls co mpounds. The bosonized theory is a two-frequency sine-Gordon field theory. We analyze the excitation spectrum by semiclassical methods. The elementary triplet excitation corresponds to an extended magnon whose radius diverges for vanishing dimerization. The internal oscillations of the magnon give r ise to a series of excited states: until another magnon is emitted and a tw o-magnon continuum is reached. We discuss, for weak dimerization, the manne r in which the magnon forms as a result of a spinon-spinon interaction pote ntial. [S0163-1829(99)05213-3].