Phenomenological Theory of the Linear and Nonlinear Dielectric Susceptibilities in the Type-II Incommensurate Phase

Abstract

The linear and third-order nonlinear dielectric susceptibilities in the type-II incommensurate phase were investigated on the basis of the Landau-type thermodynamic potential. The expression for the frequency dependence of the third-order susceptibility in the incommensurate phase, for which the thermodynamic potential includes no Lifshitz invariant, was obtained. It was found that the third-order nonlinear susceptibility shows a jump with the change of its sign at the incommensurate transition point even in the second order transition, which is caused by the contribution from the amplitude mode of the modulated structure. We found that the third-order susceptibility reflects the behavior of the amplitude mode in the incommensurate phase. As a result, it has turned out that the measurement of third-order nonlinear dielectric susceptibility becomes a powerful method as a probe for the amplitude mode in the modulated structure.