The Spin-Exchange Dynamical Structure Factor of the S=1/2 Heisenberg   Chain
  We determine the spin-exchange dynamical structure factor of the Heisenberg spin chain, as is measured by indirect Resonant Inelastic X-ray Scattering (RIXS). We find that two-spin RIXS excitations nearly entirely fractionalize into *two spinon* states. These share the same continuum lower bound as single-spin neutron scattering excitations, even if the relevant final states belong to orthogonal symmetry sectors. The RIXS spectral weight is mainly carried by higher-energy excitations, and is beyond the reach of the low-energy effective theories of Luttinger liquid type. 
Antoine Klauser, Jorn Mossel, Jean-Sebastien Caux and Jeroen van den   Brink
cond-mat.str-el cond-mat.stat-mech