The Uniaxial Stress Effect on Normal-Incommensurate Phase Transition in K₂SeO₄ -- Observations of the Amplitude Mode by Raman Scattering --

The incommensurate phase transition of K 2 SeO 4 was studied under various uniaxial compressive stresses. The frequency and damping constant of the amplitude mode were measured by means of the Raman scattering as functions of applied stress X 3 at various temperatures. These results were analyzed by use of the Landau-type free-energy written in terms of the order parameter Q and strain component u 3 . The stress dependence of the amplitude mode frequency obeys well the relation ν amp = A [ X 3 0 ( T )- X 3 ] 1/2 , where X 3 0 ( T ) is the transition stress at a temperature T . The normal-incommensurate phase transition point T 2 decreases linearly with the increase of X 3 (d T 2 /d X 3 =-16 K/kbar).