Excitations from a Bose-Einstein Condensate of Magnons in Coupled Spin Ladders

The weakly coupled quasi-one-dimensional spin ladder compound $({\mathrm{CH}}_3{)}_2{\mathrm{CHNH}}_3{\mathrm{CuCl}}_3$ 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 reference to spin-dimer materials. 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.