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.