Dynamics of quantum spin chains and multi-fermion excitation continua
  We use the Jordan-Wigner representation to study dynamic quantities for the spin-1/2 XX chain in a transverse magnetic field. We discuss in some detail the properties of the four-fermion excitation continuum which is probed by the dynamic trimer structure factor. 
O. Derzhko, T. Krokhmalskii, J. Stolze, G. M\"uller