Soft modes below the incommensurate transition in K 2SeO 4

Abstract

A coordinated series of Raman and Brillouin scattering measurements were carried out in K 2SeO 4 with emphasis on temperatures near and below the incommensurate structural phase transition ( T i =129 K). Below the lock-in phase transition ( T c =99 K) Raman scattering from both the amplitude mode and the phase mode analog was observed. However, only the amplitude mode could be followed into the incommensurate phase. Details of the amplitude mode frequency and damping are reported for temperatures considerably closer to T i than previously possible. Our results do not confirm the diverging linewidth extrapolated from earlier Raman work. We also report here the first Brillouin scattering results in K 2SeO 4. Although substancial interaction effects between the acoustic phonons and the soft amplitude mode are evident near T i , the coupled mode lineshapes can be well described without the introduction of a relaxing self energy for the soft mode. Finally, limits on the characteristics of the phason are drawn from our failure to observe it directly in the incommensurate phase.