Cluster meanfield approximation for the optical response of weakly doped   semiconductor quantum wells
  The calculation of the optical properties of doped semiconductor quantum wells is an intricate many-body problem because of the dynamical response of the excess carriers to the photogenerated valence band hole. At low densities, however, where the main effect of the dynamical response is the formation of trions, a simple cluster meanfield approximation can be effectively employed to calculate the optical susceptibility. 
Franz X. Bronold