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.