Excited states in Bethe ansatz solvable models and the dressing of spin and charge

A general formalism for the study of excitations above equilibrium in Bethe ansatz solvable models is presented. Nonzero temperature expressions for dressed energy, momentum, spin and charge are obtained, and it is found that the dressed spin and charge are in general momentum dependent. For an electronic model one may only have spin-charge separation at zero temperature where the ground state is half-filled and has zero magnetisation. Finally, the excitations of the Hubbard-Shastry models are examined in detail.