We present a theory of phonon-assisted optical two-magnon absorption in two-leg spin-ladders. Based on the strong intra-rung-coupling limit we show that collective excitations of total spin S=0, 1 and 2 exist outside of the two-magnon continuum. It is demonstrated that the singlet collective state has a clear signature in the optical spectrum.
@article{arxiv.cond-mat/9910322,
title = {Optical absorption of spin ladders},
author = {C. Jurecka and W. Brenig},
journal= {arXiv preprint arXiv:cond-mat/9910322},
year = {2009}
}