Energy separation of single-particle and continuum states in a S=1/2   weakly-coupled chains antiferromagnet
  Inelastic neutron scattering is used to study transverse-polarized magnetic excitations in the quasi-one-dimensional S=1/2 antiferromagnet BaCu_2Si_2O_7, where the saturation value for the N\'eel order parameter is $m_0=0.12 \mu_{\rm B}$ per spin. At low energies the spectrum is totally dominated by resolution-limited spin wave-like excitations. An excitation continuum sets in above a well-defined threshold frequency. Experimental results are discussed in the context of current theories for weakly-interacting quantum half-integer spin chains. 
A. Zheludev, M. Kenzelmann, S. Raymond, E. Ressouche, T. Masuda, K.   Kakurai, S. Maslov, I. Tsukada, K. Uchinokura, A. Wildes