Observation of Optical Phonons Bound to Neutral Donors

We have observed new "local-mode lattice vibrations" in GaP, both in Raman scattering and as cooperating phonons in optical transitions. The modes are best qualitatively described as a particle—the LO phonon —bound to a neutral donor, which provides an effective attractive interaction for the phonon. In this analogy, bound states having various angular momenta around the donor can be expected, and both $S$ and $P$ states have been identified experimentally.