Anomalous acoustic phonon dispersion in noncommensurate BaMnF4

Abstract

For temperatures slightly less than T₀, the antiferroelectric phase transition temperature, the wavevector dependence of transverse acoustic phonons in BaMnF₄ is anomalously dispersive. This dramatic variation of sound velocity with wavevector can be explained by the presence of a 'phason' branch in the noncommensurate phase below T₀.