Bound states and field-polarized Haldane modes in a quantum spin ladder
  The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modelling of the dynamical response demonstrates our complete quantitative understanding of these states. 
S. Ward, P. Bouillot, C. Kollath, T. Giamarchi, K. P. Schmidt, B.   Normand, K. W. Kr\"amer, D. Biner, R. Bewley, T. Guidi, ...
cond-mat.str-el