Dynamic nonlinear susceptibility in solid dielectrics

Experimental data on the nonlinear dynamic responses of barium titanate (discontinuous phase transition, PT), triglycine sulphate (continuous PT), as well as the prototype relaxor lead magnoniobate, and the weakly doped quantum paraelectric K_0.995Li_0.005TaO₃ are presented. The classic ferro-electrics display all features of the third-order dielectric susceptibility, X₃, as predicted by mean-field theory (including the change in sign of X₃ at the continuous PT). On the other hand, the positive sign of X₃ measured on the relaxor PMN is due to the presence of polar nanoregions and their specific contribution to the dielectric response measured at enhanced electric field.