Excitations from a Bose-Einstein condensate of magnons in coupled spin   ladders
  The weakly coupled quasi-one-dimensional spin ladder compound (CH$_3$)$_2$CHNH$_3$CuCl$_3$ 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. 
V. O. Garlea, A. Zheludev, T. Masuda, L.-P. regnault, E. Ressouche, B.   Brenier, H. Manaka, J.-H. Chung, Y. Qiu, K. Habicht...