Nonlinear nature of the resonant system

The ferroelectric phase transition of second-order in tgs at 0C = 49° C can be described in the framework of the Landau-theory (e.g. [4]) by the thermodynamical potential 

The only temperature-dependent coefficient is A sample is calculated from Equation 14.2 as 

Inserting 14.3 into 14.1 provides the so-called Duffing equation (e.g. [1]) 

At temperatures above c an oscillation of D2 in a potential with one minimum may be observed. Below c the potential becomes a double-well one. 

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