Nontrivial dependence of dielectric stiffness and SHG on dc bias in   relaxors and dipole glasses
  Dielectric permittivity and Second Harmonic Generation (SHG) studies in the field-cooled mode show a linear dependence of dielectric stiffness (inverse dielectric permittivity) on dc bias in PMN-PT crystals and SHG intensity in KTaO$_{3}$:Li at small Li concentrations. We explain this unusual result in the framework of a theory of transverse, hydrodynamic-type, instability of local polarization. 
S. A. Prosandeev, I. P. Raevski, A. S. Emelyanov, Eugene V. Colla,   J-L. Dellis, M. El Marssi, S. P. Kapphan, and L. Jastrabik