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The linear, and the second- and third-order nonlinear susceptibilities were measured in order to verify conclusions stemming from the theory of the ferroelectric effect in Rochelle salt proposed by Mitsui (Phys. Rev. 111, 1259 (1958)) and concerning the behavior of the susceptibilities in the vicinity of the phase transition points. Particular attention is focused on the nontrivial 'inverse melting' point. Our experimental results comply with the theoretical predictions. [ABSTRACT FROM AUTHOR]

Peculiarities in the nonlinear dielectric response of the relaxor lead magnoniobate (PMN) are explained within the classic Landau-Ginzburg-Devonshire theory. It turns out that a correction term proportional to the square of the polarization, P02, of polar nanoregions (PNRs) becomes relevant in relaxor ferroelectrics. These PNRs are stabilized by fluctuations of quenched random electric fields. In contrast to regular ferroelectrics with continuous phase transitions, where the third harmonic susceptibility, χ3, is negative at T > Tc, the contribution due to PNRs drives χ3 positive in PMN. The lack of an anomaly in χ3 at the freezing temperature, Tf, does not favor a glassy scenario of relaxors. [ABSTRACT FROM AUTHOR]

The third-order dielectric susceptibility χ3 of a ferroelectric is described within classic Ginzburg-Landau-Devonshire theory. It turns out that a correction term proportional to the square of the polarization, P2, becomes relevant in relaxor ferroelectrics close to, but above the Curie temperature. It is due to the quasi-static polarization of polar nanoregions (PNRs), which are stabilized by fluctuations of quenched random electric fields. In contrast to normal ferroelectrics with continuous phase transitions, where χ3 is negative at T > Tc, the contribution due to PNRs drives χ3 positive in relaxors like strontium barium niobate (SBN). It becomes negative at enhanced driving field amplitudes when activating the flipping dynamics of the polar nanoregions. [ABSTRACT FROM AUTHOR]

Replica-symmetric solution of the transverse tunneling model for proton glass with longitudinal Gaussian random field is employed to obtain an analytic solution of the nonlinear susceptibility χ nl . The results show smearing of the nonlinear susceptibility near the glass transition by the Gaussian random field. Both the cusp anomaly of linear susceptibility and the divergence of nonlinear susceptibility are thus suppressed by random fields of Gaussian distribution. [ABSTRACT FROM AUTHOR]