Eigenvectors of the [111] longitudinal phonons in diamond

In analogy to an earlier work carried out on silicon and germanium we have determined experimentally the eigenvector phase of the longitudinal phonons along [111] in diamond. We provide an analysis showing that this phase can be attributed to the eigenvector exchange resulting from coupling between the acoustic and optic branches. The qualitative difference in the behaviour of the phase angle between diamond and the other group-IV elements can then be related to the underlying bare mode frequencies rather than to the coupling term itself.