Generalization of k.p theory for periodic perturbations
  We extend standard k.p theory to take into account periodic perturbations which are rapidly oscillating with a wavelength of a few lattice constants. Our general formalism allows us to explicitly consider the Bragg reflections due to the perturbation-induced periodicity. As an example we calculate the effective masses in the lowest two conduction bands of spontaneously ordered GaInP_2 as a function of the degree of ordering. Comparison of our results for the lowest conduction band to available experimental data and to first principle calculations shows good agreement. 
R. Stubner, R. Winkler, O. Pankratov