Low-temperature nonlinear microwave response of incipient ferroelectrics

A review of investigations of microwave dielectric permittivity and dielectric nonlinearity of incipient ferroelectrics (SrTiO3, KTaO3, PbTe) is presented. Physical mechanisms of excess anharmonicity of incipient ferroelectrics in weak biasing fields are discussed. It is shown, that measurements of field dependences of dielectric permittivity e and second harmonic generation efficiency in strong biasing fields make it possible to reduce the influence of hindering factors on the accuracy of determination of anharmonic constants in free energy expansion. Low-temperature dependences of ω2c(T) for incipient ferroelectrics are considered with regard to soft mode stabilization. Phenomenological analysis of available experimental data affirms the conclusion that the character of low-temperature paraelectric stabilization of SrTiO3 is strongly affected by structure phase transition. Arguments are presented in proof of the possibility for metastable polar phase to exist in SrTiO3 at helium temperatures, which brings a key to understanding the reasons of numerous observations of hardly reproducible anomalies in strontium titanate.