Dynamically dominant excitations of string solutions in the spin-1/2   antiferromagnetic Heisenberg chain in magnetic fields
  Using Bethe-ansatz solutions, we uncover a well-defined continuum in dynamical structure factor $S^{+-}(k,\omega)$ of the spin-1/2 antiferromagnetic Heisenberg chain in magnetic fields. It comes from string solutions which continuously connect the mode of the lowest-energy excitations in the zero-field limit and that of bound states of overturned spins from the ferromagnetic state near the saturation field. We confirm the relevance to real materials through comparisons with experimental results. 
Masanori Kohno