The weakly coupled quasi-one-dimensional spin ladder compound (CH3)2CHNH3CuCl3 is studied by neutron scattering in magnetic fields exceeding the critical field of Bose-Einstein condensation of magnons. Commensurate long-range order and the associated Goldstone mode are detected and found to be similar to those in a reference 3D quantum magnet. However, for the upper two massive magnon branches the observed behavior is totally different, culminating in a drastic collapse of excitation bandwidth beyond the transition point.
@article{arxiv.cond-mat/0608566,
title = {Excitations from a Bose-Einstein condensate of magnons in coupled spin ladders},
author = {V. O. Garlea and A. Zheludev and T. Masuda and L. -P. regnault and E. Ressouche and B. Brenier and H. Manaka and J. -H. Chung and Y. Qiu and K. Habicht and K. Kiefer and M. Boehm},
journal= {arXiv preprint arXiv:cond-mat/0608566},
year = {2015}
}