Phenomenological Description of the Acoustic Phonon Branch of K₂SeO₄

Abstract

A phenomenological approach is used in investigating dispersion of the acoustic phonon branch of K₂SeO₄ in the extended zone scheme. The dispersion is expressed through coefficients of a simple thermodynamic potential. According to symmetry of the crystal (D_2h = mmm) the Lifshitz-type invariant exists in the potential. This invariant is responsible for the linear and dynamical interaction of the acoustic and optical branches. The soft optical branch lowering with decreasing temperature forces to lower the acoustic branch which becomes zero at some value of the wave vector.

Keywords:

• Phenomenological approach
• acoustic branch
• K₂SeO₄

Acknowledgments

This work was carried out under the Research Program for Special Promotion of the Venture Business Laboratory, Yamaguchi University. One author (D.G.S.) gratefully acknowledges the financial support of the Russian Foundation for Basic Research (Grant No. 03-02-16104).