A Note on the Propagation of Excitation in an Idealized Crystal

Abstract

It is shown that, in insulators, overlapping of the electronic wave functions of near-by crystal constituents is not necessary for effective transfer of excitation energy by the "exciton" mechanism. The electromagnetic interaction of the "virtual" dipoles of the transition is sufficient to give rise to such transfer characterized in cubic crystals by a wave number k, and an anisotropic effective mass, given in order of magnitude by the ratio electronic mass/oscillator strength of transition. Implications and limitations of the calculation are discussed.