Doping of a spin-1 chain: integrable model
  An exactly soluble model describing a spin S=1 antiferromagnetic chain doped with mobile S=1/2 carriers is constructed. In its continuum limit the undoped state is described by three gapless Majorana fermions composing the SU(2) triplet. Doping adds to this a scalar charge field and a singlet Majorana fermion with different velocity. We argue that this mode survives when the Haldane gap is added. 
Holger Frahm, Markus P. Pfannm\"uller and A. M. Tsvelik