Faraday and Cotton-Mouton effects for acoustic phonons in paramagnetic Rare Earth compounds

A theory is developed for the Faraday effect and Cotton-Mouton effect of acoustic phonons in paramagnetic Rare Earth systems. The effects occur if degenerate transverse acoustic phonons are propagating in the direction of an applied magnetic field and perpendicular to it. The theory is formulated for cubic crystals but a generalization to other crystal symmetries is straightforward. The theoretical findings are applied to TmTe and it is demonstrated that both effects are large enough to be observable.